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Electronic and Inertial Effects of Methylation on Excited-State Hydrogen Transfer
Pratip Chakraborty, Rafael C. Couto, and Nanna H. List*

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Cite This: J. Phys. Chem. A 2026, 130, 1090-1103

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ABSTRACT: Excited-state intramolecular hydrogen transfer (ESIHT) is among the fastest chemical reactions and is a key design element in photoprotective molecules and functional chromophores. Despite the apparent simplicity of the symmetric HO-CC-CO ESIHT prototype, its multifunctional nature enables competing nonradiative decay channels, including CC torsional motion. Here, we compare malonaldehyde (MA), the minimal motif, with its methylated derivative acetylacetone (AcAc) to investigate how electronic and inertial effects of methylation shape the ultrafast dynamics initiated on S2(ππ*). XMS- CASPT2 nonadiabatic dynamics on the singlet manifold reveal bond-length alternation that
drives the wavepacket toward the H-transfer intersection seam rather than undergoing torsional motion directly out of the Franck- Condon region. Methylation destabilizes the S1(nπ*) state, reducing the S2/S1-energy gap and enhancing the asymmetry of the H- transfer intersection seam. As a result, S2/S1-decay precedes H-transfer, which mostly takes place only after the population arrives on S1. Moreover, the methyl groups in AcAc introduce an inertial mismatch between the central methine hydrogen and the terminal methyl groups, which gives rise to two distinct behaviors on S1: (i) an early ballistic rise in ground-state population within ∼75 fs via twist-pyramidalized geometries akin to the behavior of α,β-enones and (ii) a slower repopulation through torsional motion, with the majority of the population remaining near the planar S1-minimum. In contrast, MA displays no ballistic channel. Our results for AcAc are consistent with recent time-resolved photoelectron spectroscopy, confirming the ultrafast S2-lifetime. We propose extending such experiments into the X-ray regime, where the evolution of the oxygen 1s binding energies offers direct, site-specific sensitivity to the H-transfer-mediated motion governing the early decay.

© 2026 The Authors. Published by American Chemical Society

1.		INTRODUCTION
Excited-state intramolecular hydrogen transfer (ESIHT) is among the fastest known chemical reactions, typically unfolding within tens of femtoseconds.1-5 It plays a key role in diverse biological processes1,2 and serves as a photofunctional unit in a range of light-driven technologies.6-9 Despite its apparent structural simplicity, the HO-CC-CO enolone motif underlying symmetric ESIHT supports competing nonradiative decay channels, including H-transfer and CC torsional motion. The latter pathway is well-established in α,β-enones (CC-CO), where excitation of the S2(ππ*) state weakens the double bond and promotes ultrafast torsion-mediated decay.10-12
Chemical substitution provides a powerful knob to tune photochemical reactivity.13-22 Substituents influence dynamics through two mechanisms:23 (i) electronic effects, which modify potential-energy surfaces (PESs) by stabilizing or destabilizing regions of particular electronic character, and (ii) inertial effects, which alter specific nuclear modes, changing the direction and velocity of the nuclear wavepacket. Methylation is an important example with relevance also in biological settings (see e.g., refs 24,25). While methyl groups are traditionally considered inertial substituents,16,17 they also exert weak electron-donating effects that can stabilize/ destabilize charge-polarized configurations depending on their position. Methyl substitution has, for example, been shown to significantly affect the excited-state dynamics of unsaturated hydrocarbons, allenes, carbonyls,12,18,26 but its role in enolone systems has not been systematically investigated with high-level nonadiabatic dynamics.
Malonaldehyde (MA) is formally the smallest (symmetric) β-diketone, but in the gas phase it predominantly exists as the enolone tautomer stabilized by electronic conjugation and intramolecular hydrogen bonding (Figure 1).27-35 This renders MA the minimal HO-CC-CO system and hence a prototype for ESIHT.36 Acetylacetone (AcAc) retains the enolone tautomer,37 but the methyl groups lower symmetry and introduce both electronic and inertial perturbations. Both molecules share a basic electronic structure with a dark S1(nπ*) state lying below the bright S2(ππ*) state.38,39 However, while gas-phase MA is unstable at room temperature and thus experimentally challenging,40 AcAc is stable and has been the subject of extensive time-resolved spectroscopic studies.
Received: October 30, 2025
Revised: January 16, 2026
Accepted: January 21, 2026
Published: January 27, 2026

Figure 1. Molecular structures of the enolone tautomer of malonaldehyde (MA) and acetylacetone (AcAc).

CASSCF-based nonadiabatic dynamics simulations have reported a competition between H-transfer and torsion in MA and AcAc. For MA, the wavepacket was found to decay predominantly via a higher-lying H-transfer intersection, with subsequent relaxation on S1 proceeding through torsional motion.41-43 For AcAc, H-transfer was also found to be the dominant S2/S1-decay pathway, with torsion accounting for ∼25% of the population transfer.44 Dosľ ić and co-workers later compared MA and AcAc dynamics at the CASSCF and ADC(2) levels,45,46 focusing mainly on the fate of the S1 wavepacket and differences in triplet quantum yields. These studies provided important first comparative insights, yet further investigation is warranted to overcome the limitations of CASSCF and ADC(2) for excited-state dynamics in carbonyl-containing systems: CASSCF neglects dynamical electron correlation, leading to blue-shifted excitation energies and overestimated S2/S1-gaps, while ADC(2) is prone to artificial S1/S0-crossings that mediate premature S1(nπ*)- decay.47 Very recently, XMS-CASPT2 dynamics were reported for AcAc alone,48 offering a more reliable description of the electronic structure but leaving the role of methylation unresolved.
The early dynamics of AcAc in the gas phase following photoexcitation (near 260 nm) to the S2(ππ*) have also been probed experimentally by a variety of techniques, including ultrafast electron diffraction,49,50 femtosecond pump-probe photoionization with electron and ion detection,51 photoelectron spectroscopy,46,48,52 and transient carbon K-edge X- ray absorption.53 Recent experiments indicate that AcAc undergoes ultrafast internal conversion to the S1(nπ*) state within the first 20 fs, followed by intersystem crossing to T1 with a time constant of ∼1.5 ps. Longer time scale dynamics (tens to a few hundreds of picoseconds) have been assigned to the relaxation of the T1 state and intersystem crossing to the ground state. Detailed characterization of the photoproducts (after photoexcitation at 248, 266, or 280 nm) suggests both Norrish type-I pathways from the T1 state and phototautomerization followed by dissociation from the vibrationally hot ground state.54,55 Despite these advances, the role of methylation in shaping ultrafast dynamics in enolones, particularly the competition between the photochemistry of different functional units, remains unresolved, and no high- level comparative dynamics of MA and AcAc has yet been reported.
To address this gap, we report trajectory surface hopping simulations of MA and AcAc at the XMS-CASPT2 level, disentangling the electronic and inertial effects of methylation and establishing how these factors modulate the earliest internal conversion dynamics. To capture the influence of low- frequency methyl rotations and possible H-tunneling on the ground state, initial conditions for the nonadiabatic dynamics simulations were sampled from quantum-thermostat ab initio molecular dynamics56 rather than from a harmonic Wigner distribution. We find that S2/S1-decay in both systems proceeds exclusively via H-transfer-mediated dynamics, with no evidence for the torsion-mediated pathway. While methylation leaves the initial decay largely unchanged, it imparts a more pronounced inertial influence on the ensuing S1 dynamics, leading to different extents of ground-state recovery.
2.		COMPUTATIONAL DETAILS
This section outlines the computational setup used in this work. An overview of the key parameters for initial-condition generation, electronic-structure method, and nonadiabatic dynamics simulations is provided in Section S1.
2.1.		Electronic-Structure Level and Benchmark
We described the electronic structures of MA and AcAc using extended multistate complete active-space second-order perturbation theory (XMS-CASPT2)57-59 based on a state- averaged complete active-space self-consistent field reference, as implemented in BAGEL 1.2.2.60,61 Initial-condition sampling was handled separately (see below). For the singlet manifold, we included the three lowest states (with equal weights) in the state-averaging (SA), whereas a two-state average was used for the triplet manifold. To serve as a reference for benchmark, we computed critical points and their energies along interpolated paths with a larger active space comprised of 14 electrons in 12 orbitals (see Figure S2 of the Supporting Information for an illustration of the orbitals) using the multistate multireference (MSMR)62 contraction scheme with frozen core and a real shift of 0.3 Eh (no IPEA shift). This electronic-structure level is denoted as SA3(SA2 for triplets)- XMS(Re=0.3)-CASPT2(14,12) in the following. Based on a favorable agreement with this reference, we selected a smaller active space of 10 electrons in 8 orbitals with a single-state single-reference (SSSR)62 contraction scheme as a costeffective alternative for the production calculations (UV photoabsorption and nonadiabatic dynamics simulations). Specifically, we used SA3-XMS(Im=0.3)-CASPT2(10,8) with a frozen core and an imaginary level shift of 0.3 Eh (no IPEA shift). All calculations used the cc-pVDZ basis set together with the cc-pVDZ-jkfit density-fitting basis. Benchmark results are provided in Section S2 together with a discussion of activespace stability in Section S3.
2.2.		Initial-Condition Sampling
The methyl groups in AcAc complicate initial-condition (IC) sampling using a harmonic Wigner distribution. In particular, the low-frequency torsional mode associated with ketonic methyl rotation is poorly described by a linear approximation, leading to artificially long C-H bonds and potential activespace instabilities. While excluding such modes has been used when they are deemed dynamically irrelevant,22,63 this may not be appropriate here, as methyl orientation influences which regions of the intersection seam are accessed (see below). Moreover, the shallow barriers for methyl rotation give rise to multiple distinct ground-state minima that are not captured by Wigner sampling around the global staggered minimum. To address these limitations, we employed quantum-thermostat ab initio molecular dynamics (QT-AIMD64,65) to generate ICs, as recently proposed by Prlj et al.56,66
QT-AIMD was performed at the density-functional theory67,68 level with the B3LYP69-72 exchange-correlation functional using the D3-BJ73,74 dispersion correction and the 6-31G(d,p)75-77 basis set. The dynamics were initiated from the corresponding optimized geometries (AcAc with methyl groups in a staggered conformation), with velocities drawn from a Boltzmann distribution. We used a time step of ∼0.48 fs (20 au) and QT parameters (GLE drift and diffusion matrices A and C) from the GLE4MD database,78 corresponding to a temperature of 298.15 K, Ns = 6, and ℏωmax/kBT = 20 (strongcoupling regime). For this temperature, ωmax = 4114.5 cm-1, which exceeds the highest-frequency normal mode of both MA and AcAc. We determined the equilibration time of the trajectories by monitoring the convergence of the average kinetic energy.56 From the thermalized trajectories, we extracted 1890 and 1784 samples for MA and AcAc, respectively. Snapshots were sampled in ∼120 fs intervals (lowest frequency ∼283 cm-1) and ∼710 fs intervals (lowest- frequency methyl rotation is ∼48 cm-1) for AcAc. All QT- AIMD simulations were carried out using the ABIN code79 interfaced with the TeraChem program80-83 for the electronicstructure calculations.
We also generated 5000 initial conditions for both molecules from a thermal harmonic Wigner distribution at 298.15 K using B3LYP(D3-BJ)/6-31G(d,p)-optimized geometries and normal modes. A comparison of the results of the two IC sampling methods and the advantages of QT-AIMD sampling for AcAc is provided in Section S4.
2.3.		Nonadiabatic Molecular Dynamics
ICs for the nonadiabatic dynamics simulations were selected to mimic 266 nm (4.661 eV) photoexcitation of the first absorption band of AcAc (experimental maximum at 4.716 eV in the gas phase39). Vertical excitation energies and oscillator strengths were computed for all QT-AIMD sampled geometries at the SA3-XMS(Im=0.3)-CASPT2(10,8)/cc- pVDZ (SSSR) level. To simulate the absorption spectrum, the resulting stick spectra were convolved with a Gaussian line shape (FWHM = 0.24 eV). For AcAc, the spectrum was uniformly shifted by +0.096 eV to align the first absorption maximum with the experimental data (Figure S11). The same broadening and energy shift were applied for MA. To approximate the finite bandwidth of a typical pump pulse, initial conditions were selected from a 0.1 eV window around the pump energy (i.e., 4.661 ± 0.05 eV after applying the uniform shift). Within this window, 265 ICs were selected for AcAc, with 250 and 15 ICs having S2 and S1 as the bright state, respectively. For MA, 256 ICs were selected, of which 255 had S2 and one had S1 as the bright state.
The nonadiabatic dynamics simulations were carried out using SHARC 3.0 (Surface Hopping including ARbitrary Couplings)84 interfaced with BAGEL 1.2.2.60,61 We performed trajectory surface hopping simulations on SA3-XMS(Im=0.3)- CASPT2(10,8)/cc-pVDZ full-dimensional potential-energy surfaces (PESs) calculated in the molecular Coulomb Hamiltonian (MCH) representation for a duration of 200 fs. The fewest-switches surface hopping (FSSH)85 algorithm was used to take into account nonadiabatic events among the S2, S1, and S0 states. We employed an energy-based decoherence correction using the recommended α = 0.1 Eh value.86 Given the short simulation time, we did not apply any corrections to zero-point leakage. The velocity Verlet algorithm was employed to integrate the nuclear equations of motion with a time step of 0.5 fs, while the electronic wave function was propagated with a time step of 0.02 fs. To conserve the total energy after a successful hop, the kinetic energy was adjusted by rescaling the momentum along the nonadiabatic coupling vector, while the momentum direction was left unaltered when a frustrated hop was encountered. Energy conservation was monitored for all trajectories. The reader is referred to Section S3 for details on active-space stability, discarded trajectories, and an analysis of their influence on populations and geometric distributions.
2.4.		X-ray Photoelectron Spectroscopy Calculations
To assess whether oxygen K-edge X-ray photoelectron spectroscopy (XPS) can resolve signatures of H-transfer, we computed the oxygen 1s core-electron binding energies along selected paths and representative trajectories using the extended multistate restricted active-space second-order perturbation theory (XMS-RASPT2).87 The five lowest doublet states were included in the state-averaging of the core-ionized states, considering every 1 fs along the selected trajectories. The core-ionized states at each oxygen K-edge were computed separately and subsequently combined in the final spectrum. This approach was used due to active-space instabilities when the hydrogen was transferred between the two oxygens, causing the absence of one of the signals. Accordingly, one oxygen 1s-orbital was placed in the RAS1 space (with a one-hole constraint), while the RAS2 space contained the orbitals corresponding to the valence CAS- (14,12), i.e., XMS-RASPT2(11,1,0;1,12,0) using the notation (n, l, m i, j, k), where n is the number of active electrons, l is the maximum numbers of holes in RAS1, and m is the maximum number of electrons allowed in RAS3, while i, j, k are the number of active orbitals in RAS1, RAS2, and RAS3, respectively. This larger valence space was needed for the computation of core-ionized states, as instabilities were otherwise observed at distorted geometries. The cc-pVDZ basis set was employed together with an imaginary shift of 0.3 Eh to avoid intruder states (no IPEA shift). The highly excited- state procedure88 was used to target the oxygen K-edge, and Cholesky decomposition of the two-electron repulsion integrals was employed.89 While Dyson norms are often used to approximate photoionization intensities, the similar shapes of the localized 1s-core orbitals can render pre-edge XPS intensities at a given K-edge largely comparable.90 While assuming stoichiometric ratios is not always valid in XPS,91 both experiment and theory support that this is a reasonable approximation in AcAc: the steady-state oxygen K-edge XPS spectrum shows ketonic and enolic features of similar intensities,92,93 and a restricted active-space state interaction (RASSI94) calculation at the Franck-Condon (FC) point confirms a near-unity intensity ratio of 0.93 (based on Dyson norms of 0.56 and 0.60, respectively). For this reason, we assumed uniform intensities for the two oxygen sites in our time-resolved XPS (TRXPS) analysis. All XMS-RASPT2 calculations were performed using OpenMolcas 24.06.95-97
3.		RESULTS AND DISCUSSION
We first analyze static potential-energy trends along interpolated paths in AcAc and assess the effects of methylation by comparison with MA. We then examine the time-resolved FSSH dynamics and the resulting oxygen K-edge TRXPS signatures of H-transfer. The key reaction coordinates are bond-length alternation (BLA), H-transfer (HT), chelate-ring contraction/expansion, as quantified by the sum-of-angles (SOA) coordinate, pyramidalization of the central methine carbon (PyrC), and CC torsion. The definitions of these geometric parameters can be found in Section S1.
3.1.		Signposts on the Potential-Energy Landscapes
Figure 2 presents the XMS-CASPT2(14,12) potential-energy curves of the lowest singlet and triplet states of AcAc,

Figure 2. Ground- and valence-excited potential-energy curves for AcAc along geodesic interpolated pathways between minima and MECIs, indicating possible deactivation pathways upon S2(ππ*) photoexcitation. The x-axis is given in mass-weighted distance 1 Å (· amu1/2/tick), considering all atoms except methyl hydrogens. This choice avoids ambiguity arising from different methyl orientations and emphasizes the heavy-atom framework governing the decay pathways. The effect of methyl orientation is discussed in Figure 3. The electronic character along the curves is marked in yellow: ππ* and blue: nπ*, as gauged by the nature of the dominant configuration. The torsional displacement proceeds near an avoided crossing with a concomitant interchange of the characters of S2 and S1. The asterisk indicates the transition state on S1. Critical points were obtained at the SA3(or SA2)-XMS(Re=0.3)-CASPT2(14,12)/cc-pVDZ (MSMR) level of theory with corresponding geometries visualized in Figure S5.
computed along geodesic interpolated pathways98 connecting minima and minimum-energy conical intersections (MECIs). Geometric parameters and relative energies of all critical points are compiled in Table S2, and a brief comparison to experimental and past theoretical results at the FC point can be found in Section S2.
It should be noted that the ground-state PES of AcAc features several shallow minima due to low barriers to intramolecular H-transfer and methyl rotation.99-106 This flatness makes the determination of the true ground-state vibrational wave function challenging and still an open question (see brief discussion in Section S4.2). When the zero-point energy is neglected, the staggered conformer is consistently predicted to be the global S0-minimum. This is also the case at the present XMS-CASPT2 level of theory.
Following photoexcitation to the bright S2(ππ*) state, two distinct deactivation channels leading to S1 become accessible, as indicated by the curly arrows in Figure 2. Along the ESIHT pathway (dotted arrow), AcAc can funnel from S2 to S1 via a planar H-transfer intersection (HTI) seam. From there, the molecule can relax toward a planar, ring-expanded S1- minimum. Returning to the ground state from this geometry requires overcoming a torsional barrier of 0.25-0.65 eV (the lower value is the true barrier, whereas the upper bound is from the interpolated path, assuming the dynamics to be sufficiently fast that other degrees of freedom do not have time to relax) due to an avoided crossing with the S2 state. Alternatively, and as suggested by some earlier studies,46,48,53,55 the system can undergo intersystem crossing (ISC) enabled by the T1(ππ*) and T2(nπ*) states being almost degenerate with the S1 state near its minimum and El-Sayed allowed.107 While the energy profile of the T2(nπ*) state displays strong geometry-dependence (approximately following the singlet nπ* diabat), the T1(ππ*) surface remains relatively flat over the explored configuration space with its minimum at a CC twisted geometry (T1-min in Figure 2). We note that the ordering and character of the triplets near the S1-minimum are method-dependent46,48 and that even small methodological differences can shift their relative energies and the corresponding triplet intersection seam relative to S1.
The second pathway (dashed arrow) involves torsional motion, as facilitated by the weakening of the CC bond in the S2(ππ*) state. This leads to a twisted S2/S1-MECI (S2/S1- Tw), located ∼0.1 eV below the minimum of the HTI seam. Torsional motion significantly destabilizes the ground state, bringing the three lowest singlet states in energetic proximity (nearby S1/S0-Tw). As a result, torsion may mediate efficient internal conversion from S2 to S0 via S1, as indicated by some previous CASSCF-based studies.41,42,44 The interpolated energy profile suggests that torsional motion proceeds over a small barrier and is accompanied by a change in the character of the electronic state. The corresponding picture for MA is overall similar to that of AcAc, as summarized in Figure S3 and Table S4 (see also Figure 2 in ref 108). Since the HTI seam lies geometrically closer and downhill from the FC region, it is likely the primary pathway for nonradiative decay to S1. However, high-level nonadiabatic dynamics incorporating inertial effects are necessary to map out the potential pathway competition. Before reaching that point, we focus on how methylation modifies the static trends.
3.1.1.		Methylation Promotes Early Access to the HTI Seam via S1-Destabilization. Figure 3 compares the XMS- CASPT2(14,12) energy profiles of the singlet states in MA (dashed lines) and AcAc (solid lines) along the interpolated H-transfer path, which connects the FC region to the symmetric HTI geometry and back. Key structural changes along this path, including BLA and chelate-ring contraction (SOA), are shown alongside the energy profiles.
The energies of the bright S2(ππ*) state are nearly identical for MA and AcAc. On the other hand, the S2/S1-energy gap is ∼0.3-0.4 eV smaller in AcAc. This difference is caused by the weak electron-donating character of the methyl groups, which destabilize the S1(nπ*) state accordingly (compare green dashed and solid lines). This electronic effect places AcAc as an intermediate case between MA and canonical ESIHT systems, such as methyl salicylate,109,110 for which the S1 state is the bright ππ* state. The extent of the methyl-induced destabilization depends on the orientation of the methyl groups: the AcAc profile in Figure 3 corresponds to the eclipsed-up conformer (diamond markers indicate the energies for the ground-state staggered conformer), which shows the largest electronic effect due to increased C-H hyper- conjugation.111 The size of the S2/S1-gap also depends on the theory level (here, considering variations in the XMS- CASPT2 setup). With SA3-XMS-CASPT2(14,12), the energy gaps are ∼0.9 eV for MA and ∼0.5 eV for AcAc. The latter value is consistent with available experimental UV-absorption data,39 which is about 0.2 eV smaller than the gap reported in electron energy loss spectroscopy.112 The gaps decrease by ∼0.1 eV with the reduced (10,8) active space employed in our dynamics simulations, mainly due to an increase in the S1 energy. Accordingly, our dynamic setup may slightly over-

Figure 3. Energy profiles along the H-transfer pathway (linear interpolation from the FC region through the symmetric S2/S1-HTI to the H-transferred FC region) together with key geometric parameters as obtained at the SA3-XMS(Re=0.3)-CASPT2(14,12)/ cc-pVDZ (MSMR) level. (a) S0, S1, and S2 energies for MA (dashed) and AcAc (solid) along the H-transfer coordinate. Energies are reported relative to their respective S0-minima, and the broken y-axis is used for magnification. The H-transfer pathway is characterized by (b) BLA equalization and (c) chelate-ring contraction, as quantified by the SOA coordinate. In the case of AcAc, the eclipsed-up (Eup) methyl-group conformations have been used. As a result, the S0- energy at HT = -0.6 Å is not exactly zero. The Eup conformation exerts the strongest electron-donating effects of the methyl group, and hence displays the smallest S2/S1-gap in the FC region. For AcAc, the energies at the staggered S0-minimum (global minimum) are indicated by diamonds.
estimate the rate of S2/S1-decay. Including five states in the state-averaging would reduce the gap by another ∼0.1 eV and would therefore offer a poorer description near the FC region (see further discussion on the active space in Section S3).
Besides their effect on the FC region, the methyl groups also influence the HTI intersection seam. As shown in Figure 3, methylation extends the accessible configurational space of the seam and introduces pronounced asymmetry along the H- transfer coordinate. In AcAc, the seam is flat, and the MECI is substantially displaced from the C2v-symmetric HTI geometry (HT = ±0.35 Å vs 0 Å). As a result, the symmetric HTI acts as a shallow transition structure on the seam, connecting two equivalent asymmetric HTIs (compare asym-HTI-Eup and sym-HTI-Eup in Table S2). In MA, the same qualitative picture holds; however, the degree of asymmetry is reduced. Although the asymmetric structure is also slightly more stable in MA, it is geometrically closer to the symmetric counterpart (HT = ±0.23 Å vs 0 Å, see Table S4). Taken together, this static analysis suggests that methylation enhances the asymmetry of the HTI seam through the destabilization of the S1(nπ*) state, shifting the low-energy access to the seam away from the symmetric geometry and potentially facilitating earlier access to S1 along the H-transfer coordinate.
3.2.		Ultrafast Internal Conversion Dynamics
Having established the methylation effects on the potentialenergy landscape, we now turn to the dynamically accessed internal conversion pathways and inertial implications of methylation; we performed decoherence-corrected FSSH simulations initiated on the bright ππ* state of both MA and AcAc. Initial conditions were sampled using QT-AIMD and selected within a ±0.05 eV window around 4.661 eV to mimic a 266 nm pump pulse targeting the red side of the lowest absorption band (see Figure S11).
The resulting population dynamics during the first 200 fs following photoexcitation are shown in Figure 4. Although MA

Figure 4. Adiabatic population dynamics following photoexcitation to the bright (ππ*) state in (a) MA and (b) AcAc. Shaded regions represent one bootstrap standard deviation obtained from 2000 samples. For the ICs, 199/200 for MA and 188/203 for AcAc had S2 as the bright state, while S1 was the bright state for the remaining ICs.

exhibits a ∼0.3 eV larger S2/S1-energy gap in the FC region than AcAc, this translates into only a slightly slower ultrafast S2/S1-decay (by a few femtoseconds). For both molecules, most of the S2 population transfers to S1 within 20 fs. A small fraction (∼5-10%) is transiently trapped on S2, leading to a population plateau in the 20-60 fs time window (see also Figure S18). The repopulation of the ground state (or the growth and decay of S1) is more distinct in the two systems. For AcAc, the repopulation occurs in two stages: an initial (∼15%) faster repopulation within 25-75 fs, followed by a slower transfer reaching a S0 population of ∼25% (∼20% when all trajectories are included; see discussion in Section S3) at the end of the 200 fs simulations. For MA, the initial transfer stage is essentially absent and only the slower growth remains, resulting in about half the ground-state repopulation within the 200 fs simulation time compared to AcAc. The underlying mechanisms and differences between the two molecules will be explored next.
3.2.1.		Evolution of the Wavepacket toward the HTI Seam. To understand the structural motion mediating the ultrafast internal conversion, we examined the progress of the nuclear density along key coordinates. Figure 5 summarizes the results for BLA, HT, and SOA, while the projection onto the PyrC and torsional coordinates can be found in Figure S19. The line-outs (right plots) highlight the nuclear distribution in the FC region (black lines, as given by the IC distribution), on S2 (from the FC region until surface hop to S1, orange lines),

Figure 5. Time evolution of the nuclear density along the (a) BLA, (b) HT, and (c) SOA coordinates within 200 fs after photoexcitation for (left column) MA and (right column) AcAc. The adiabatic population traces from Figure 4 have been overlaid (no explicit y-axis but spans the range from 0 to 1) to highlight the change in population transfer (orange: S2; green: S1; black: S0). The one-dimensional distributions on the right show how the nuclear distribution changes from the FC region and up to 5 fs after population transfer to S1: black lines represent the IC distribution; orange lines represent the integrated S2 density; and green lines represent the S1-density integrated over the initial 5 fs immediately following the S2/S1-hop. The reduced nuclear densities were generated by Gaussian convolution along the respective geometric dimensions (standard deviations for BLA: 0.04 Å; HT: 0.1 Å; SOA: 2°).

and on S1 immediately following population transfer (defined by the behavior 5 fs after a surface hop, green lines). Furthermore, Figure 6 shows the distributions of geometric parameters reflecting the S2/S1- and S1/S0-nonadiabatic transfer events (i.e., hopping geometries).
Upon photoexcitation to the ππ* state, the nuclear wavepacket proceeds coherently along the BLA coordinate for ∼2 periods (period time of ∼30 fs). The nuclear distribution on S2 shifts from the initially negative BLA value characterizing the FC region to BLA equalization. This brings the wavepacket toward the HTI seam and mediates the observed ultrafast population transfer to S1. The behavior along the HT coordinate shows that although the distribution slightly shifts toward reduced HT values on S2, it remains bimodal. As such, population transfer occurs predominantly near the asymmetric part of the HTI seam. As expected from the static picture, the asymmetry is somewhat reduced in MA. However, the inertia acquired on S2 along the BLA coordinate persists immediately after population transfer and leads to a transient near-unimodal S1 distribution along the H-transfer coordinate, which indicates motion of the hydrogen. The ultrafast dynamics is also characterized by a significant but slower contraction of the H-chelate ring (quantified by a ∼10° reduction of the SOA coordinate) as reminiscent of the progress toward the HTI seam. This early contraction is dominated by the ∠C(CO) keto (and less enol) bending mode (Figure S20) and its oscillatory motion persists on S1.
Until now, we have considered only in-plane modes. In fact, there is no substantial coherent out-of-plane motion during the initial 25 fs in any of the systems (see Figure S19). In particular, the twisted S2/S1-intersection seam is not accessed in the XMS-CASPT2 dynamics. By contrast, earlier CASSCF- based studies42,44,45 reported a ∼50 fs decay and a branching between the HTI (∼75-80%) and torsional (∼20-25%) pathways (ref 46 only reported CO stretch-mediated transfer). This discrepancy between the branching ratios likely stems from two main factors: (i) use of different electronicstructure methods. In particular, the S2/S1-energy gap near the FC region is considerably smaller at the XMS-CASPT2 level (SA3-XMS(Im=0.3)-CASPT2(10,8)/cc-pVDZ (SSSR): 0.4 eV) than at the CASSCF level (SA3-CASSCF(10,8)/cc- pVDZ: 1.2 eV). As discussed further in Section S2, this gap is highly method-dependent, with XMS-CASPT2 giving values at the lower end of the range, and (ii) differences in IC sampling methodology. Specifically, our simulations employ QT-AIMD sampling combined with energy-based filtering corresponding to a 266 nm pump pulse, whereas the CASSCF- based simulations sampled from a harmonic Wigner distribution of the vibrational ground state without applying selection criteria. The 266 nm pump lies on the red side of the absorption spectrum and therefore selects ICs with smaller S2/ S1-energy gaps compared to sampling across the entire absorption band (see also Figure S11). Together, these factors may promote access to the HTI region and accelerate population transfer in our XMS-CASPT2 dynamics.
3.2.2.		S1-Trapping or Rapid S0-Recovery via Twist- Pyramidalized Seam. We now turn to the structural dynamics on S1 to better understand the early faster groundstate repopulation stage observed in AcAca feature that is absent for MA. Figure 6 compares the location of the S2/S1-

Figure 6. Differentiating the S2/S1-nonadiabatic transfer events (i.e., surface hops) from the early and late S1/S0 transfer events in the space spanned by PyrC and torsional modes for (a) MA and (b) AcAc. The S2/S1 transfer is mediated by in-plane motion near the HTI region, whereas the S1/S0 transfer is mediated by out-of-plane modes. In AcAc, the early S1/S0-hops clearly show a preference to be more centrally pyramidalized, with the late hops becoming increasingly twisted. Red, dark-green, and lime-green solid circles represent the S2/ S1-hops, the early S1/S0-hops (<75 fs), and the late S1/S0-hops (≥75 fs), respectively. The golden markers represent the locations of the MECIs. The structures for the dynamically important symmetric and asymmetric S2/S1-HTI, S1/S0-Tw, and S1/S0-CIpyrC intersection points for AcAc are shown on the right. Bond lengths in Å and (dihedral) angles are in degrees.

and subsequent S1/S0-nonadiabatic transfer events along the CC torsional and PyrC coordinates (Figure S21 shows the corresponding projections along the BLA/HT coordinates). Overall, the divisions between the two types of intersection seams are very similar for both molecules. The S2/S1-hopping geometries (red solid circles) cluster around planar geometries, consistent with the HTI seam (S2/S1-HTI-MECIs). On the other hand, the S1/S0-hops (green solid circles) involve substantial displacements along both the torsional and PyrC coordinates. As discussed above, the S0-repopulation in AcAc displays two types of growth: a ballistic rise occurring within 75 fs and a subsequent slower growth. Separating the S1/S0- hops by this 75 fs temporal threshold reveals that the early transfers (dark green) in AcAc are preferentially more pyramidalized, whereas the few later ones (light green) tend toward a larger degree of twisting. As expected from the lack of a fast S0-repopulation component, such a demarcation in the S1/S0-transfer events is largely missing in MA. We therefore turned to AcAc to uncover the structural basis of its ballistic ground-state recovery.
The faster component in AcAc arises from a smaller portion (∼15%) of the nuclear density that is initially guided toward a higher-lying, nonstationary part of the twisted S1/S0- intersection seam that features substantial pyramidalization of the central C atom while retaining the intramolecular H-bond.
This part of the seam is also characterized by significant contraction of the H-chelate ring (SOA) mediated mostly by ∠C(CO). A MECI search starting from a representative hop geometry from the dynamics (and terminated early, upon locating the seam) suggests that this twist-pyramidalized region (denoted S1/S0-CIPyrC, Figure 6) of the seam is almost isoenergetic with the S1-minimum and thus located ∼0.5 eV above the twisted minimum on the S1/S0-intersection seam (S1/S0-Tw). Beyond this early decay, reaching the twisted S1/ S0-intersection is a rarer event due to a torsional barrier (transition state located ∼0.25 eV (∼0.35 eV for MA) above the planar S1-minimum), where the S1 state gradually acquires ππ* character as shown in Figure 2. Accordingly, the majority of the population remains (transiently) trapped near the planar, ring-expanded S1-minimum throughout the duration of our simulations. As expected from the two distinct behaviors of AcAc, the electronic character of S1 along these pathways differs: the portion that undergoes ballistic ground-state recovery retains ππ* character upon reaching S1, whereas the trapped population near the S1-minimum gains nπ* character. These two behaviors are exemplified by two representative trajectories in Figure 7. Such differences in S1-valence character are expected to yield distinct fingerprints in oxygen K-edge X- ray absorption spectroscopy, providing a potential means for experimental discrimination.108,113
A closer examination of the chelate-ring contraction (SOA in Figure 5c) reveals the origin of the differing ground-state repopulation behaviors in MA and AcAc. The methyl groups in AcAc have two key effects. First, they slow specific vibrational modes, particularly the keto ∠C(CO) bending mode. As a result, the full chelate-ring contraction is delayed, occurring at around 40 fs in AcAc, compared to 20 fs in MA. Accordingly, the contraction unfolds primarily on the S1 surface in AcAc, rather than proceeding more closely in sync with the BLA motion. Second, the added mass on the terminal carbon atoms introduces an inertial mismatch that specifically activates the methine hydrogen out-of-plane motion (PyrC). Together, these effects allow AcAc to access a higher-lying twist- pyramidalized part of the S1/S0-intersection seam, promoting early ground-state repopulation. In contrast, this pathway is basically unexplored in MA. Instead, the excess energy in MA is funneled into larger-amplitude in-plane ring breathing (i.e., SOA) motion around the S1-minimum (Figure 5c), which is generally more vigorous due to the fewer degrees of freedom. The slower structural envelope of the SOA coordinate is dominated by ∠CCC and ∠C(C-O) bending, while the faster oscillations arise mainly from ∠C(CO) bending (Figure S20).
In summary, the valence dynamics of MA and AcAc display a shared initial response to S2(ππ*) photoexcitation, driven by BLA motion that steers both systems toward the HTI region of the S2/S1-intersection seam. In other words, the absence of methyl groups alone does not promote early CC torsional motion at the XMS-CASPT2 level of theory. Rather, methyl- induced inertial effects shape subsequent ground-state recovery: AcAc exhibits both an early ballistic decay via a higher-lying twist-pyramidalized region of the S1/S0-intersec- tion seam and a slower component resembling that of MA, where excess energy is primarily redirected into in-plane ring opening. These differences are evident by 200 fs, where the ground-state population of AcAc reaches ∼20-25% compared to ∼10% for MA.

Figure 7. Electronic character of the singlet valence states in AcAc along two representative trajectories that proceed via (a, b) S1-trapping and (c, d) ballistic ground-state recovery. The colored markers show the leading electronic character. Time evolution of the key geometric parameters. Evolution of (a, c) key geometric parameters and (b, d) electronic character of the adiabatic states. The black outline on the markers indicates the active state. The transparency of the markers is proportional to the weight of the dominant contribution to the electronic wave function. Gray solid lines highlight the time point with HT = 0 Å, while the dashed and dotted lines indicate S2/S1- and S1/S0-hops, respectively.

This mechanistic picture contrasts with that of the smallest α,β-enones, acrolein, and its methylated derivatives, which share the CC-CO moiety.10,12 In enones,31 the absence of the enol group blue-shifts the S2(ππ*) state by ∼2 eV and red-shifts the S1(nπ*) state by ∼0.5 eV, enlarging the S2/S1- energy gap at the Franck-Condon point and leading to substantially higher excess kinetic energy once reaching S1. Dynamically, the S2/S1-deactivation occurs within ∼50 fs through a twist-pyramidalized part of the intersection seam.12 From there, ground-state recovery follows two comparably important pathways that resemble those in the enolones. Before significant planarization, part of the wavepacket accesses a nearby higher-lying twist-pyramidalized region of the S1/S0-intersection seam, leading to efficient, nearly concerted S1/S0-decay. The remainder relaxes toward the planar S1-minimum from which S0-repopulation occurs on a 0.9-3 ps time scale via torsional motion. Unlike in MA and AcAc, where a ∼0.3-0.4 eV barrier separates the S1-minimum and the lower-lying S1/S0-Tw MECI, the corresponding intersection seam in the enones is sloped and located 0.40.7 eV above the S1-minimum (with formyl methylation stabilizing the twisted S1/S0-MECI). Access to this seam is nonetheless facilitated by the high excess energy of the vibrationally hot S1 state, rendering this pathway an important ground-state recovery mechanism in enones.
Accordingly, the key mechanistic distinction between the enolones and enones following S2(ππ*) excitation lies in the departure route from the FC point. MA and AcAc decay through the HTI seam, whereas acrolein and its derivatives proceed via the twist-pyramidalized S2 pathway that enables a more direct and efficient return to the ground state through the geometrically proximal, higher-lying part of the S1/S0- intersection seam. In the enolones, the smaller excess kinetic energy is instead funneled predominantly into in-plane chelatering expansion, limiting access to the twisted S1/S0-intersection seam by intramolecular vibrational redistribution into the torsional mode.
3.2.3.		Comparison with Previous AcAc TRPES Studies. Our results for AcAc are broadly consistent with recent time- resolved photoelectron spectroscopy (TRPES). Squibb et al. observed a TRPES signal of the S2 state in AcAc but lacked the time resolution to determine its lifetime.46 Recently, Severino et al. confirmed this feature with sub-20 fs TRPES and directly resolved the ultrafast S2/S1-decay with a time constant of 23 ± 2 fs.48 In addition, they reported oscillations in the TRPES signal, which, based on accompanying XMS-CASPT2 dynamics (SA5/SA2 for singlets/triplets), were assigned to vibrational motion associated with deformations toward the ring- expanded S1-minimum, indicative of an ESIHT-mediated mechanism. Severino et al. also identified a distinct signature of the triplet state with a rise time of 1.6 ± 0.1 ps in line with a previous carbon K-edge TRXAS study (1.5 ± 0.2 ps).53 Their XMS-CASPT2 simulations predicted ∼45% internal conversion to S0, ∼39% population remaining near the S1- minimum, and ∼16% intersystem crossing by 700 fs. It should be noted that the TRPES signature of the triplet state becomes distinct only after relaxation toward the twisted T1-minimum, while ISC may occur earlier near the planar S1-minimum, where the S1, T1, and T2 states are nearly degenerate. Accordingly, the reported decay (∼1.5 ± 0.1 ps) of the https://doi.org/10.1021/acs.jpca.5c07439 J. Phys. Chem. A 2026, 130, 1090-1103
TRPES signal associated with the S1-minimum likely represents an upper bound to the true S1-lifetime.
Three subtle differences nevertheless emerge when comparing our results with the XMS-CASPT2 simulations by Severino et al.48 First, they did not explicitly report the ballistic groundstate recovery component observed in our simulations, characterized by pyramidalization of the methine C atom and intermediate CC torsion. Second, their theory level, employing state-averaging over five states, suggests a lower torsional barrier on S1 (∼0.18 eV, Table S6 in ref 48) than we obtain with three-state-averaging (∼0.26 eV). This difference might account for the less pronounced S0-repopulation found in our work (compared to ∼30% at 200 fs in the Severino work). Third, our simulations were restricted to the singlet manifold and therefore exclude ISC. The higher torsional barriers on S1 found in this work suggest a longer S1-lifetime and potentially a branching ratio shifted toward a higher triplet yield. Quantifying these aspects and their sensitivity to methodological details, including IC generation, electronicstructure level, and nonadiabatic dynamics method, will be an important target for future studies.
While TRPES has established the ultrafast time scale of the S2 state and revealed spectral remnants on S1 consistent with an ESIHT mechanism, it does not directly resolve the specific motion that mediates S2/S1-internal conversion. Below, we consider the extension to K-edge time-resolved oxygen X-ray photoelectron spectroscopy (TRXPS) as a means to probe the evolving local structure at the two oxygen atoms.
3.3.		Time-Resolved X-ray Photoelectron Spectroscopic Fingerprints
The localized nature and chemical sensitivity of X-ray spectroscopic probes make them well-suited to tracking ultrafast dynamics at the two oxygen sites. While we have previously proposed TRXAS at the oxygen K-edge as an electronic probe of valence-state character and competing pathways in MA,108 we here explore the complementary potential of TRXPS. Specifically, we report oxygen K-edge XPS fingerprints along interpolated paths and representative pathways of the earliest decay in AcAc observed in our XMS-CASPT2 dynamics. This pathway-based approach highlights how TRXPS could serve as a probe of symmetric ESIHT in future experiments. Compared with TRXAS, TRXPS does not encode valence-orbital character. While this might seem like a limitation, it enhances its role as a structural probe: TRXAS intensities depend sensitively on the interaction- weighted overlap between the 1s-orbital and the singly occupied molecular orbital, whereas in TRXPS, the nearidentical shapes of the core orbitals leave the pre-edge photoionization cross sections largely comparable (see discussion above). As a result, TRXPS emphasizes chemical shifts instead of valence-orbital character.
As a baseline, Figure 8 compares the experimental static XPS spectrum of AcAc to the simulated counterpart. The two preedge features can be explained by a chemical-shift argument within a Cs ground-state description: the ketonic O pre-edge feature is red-shifted by ∼1.4 eV relative to the enolic feature due to increased electronic screening. This pre-edge splitting is smaller by ∼1 eV than the pre-edge gap in the static XAS spectrum of AcAc (∼2.4 eV).114 An alternative interpretation proposed by Feyer et al. invokes H-tunneling in the C2v-ground state combined with keto- or enol-localized vibrational wave functions in the core-ionized states,92 underscoring that the

Figure 8. Comparison of the Experimental and Computed Static XPS Spectra of AcAc. Ionization energies were computed for each initial condition, and the spectra were convolved with a Gaussian line shape of full-width-half-maximum of 1.0 eV, broader than the O 1s core-hole lifetime due to the limited sampling. We assumed unity cross sections for all transitions. Only the two pre-edge features from each oxygen were included in the simulated XPS spectrum, which was uniformly shifted by -2.110 eV to align with the first experimental pre-edge peak. The ionization energies were obtained at the RASPT2 level of theory. The experimental gas-phase spectrum was digitized from ref 92 using WebPlotDigitizer.115 The experimental pre-edge maxima are located at 537.5 and 538.9 eV, while the shoulder at ∼540 eV was assigned to residual water in the vacuum.

correct ground-state vibrational description remains an open question. We will leave this aspect for future work and instead turn to transient oxygen K-edge XPS signatures along the two decay pathways.
Figure 9 shows the transient oxygen K-edge signatures associated with the dynamics, represented in two complementary ways: (a,b) simplified interpolated pathways and (c,d) representative trajectories from Figure 7. H-transfer is marked by coalescence of the two oxygen pre-edge features, reflecting the near-equivalence of the O atoms at approximate C2v- symmetric geometries. By contrast, twist-pyramidalization produces a substantial blue-shift (∼2-3 eV) of both preedge features, reflecting the reduced O 1s core-hole screening that accompanies the breaking of the π-conjugation. As is evident from the representative trajectories, this merging is expected irrespective of which valence state is populated. In TRXAS, the signals are convolved with large intensity differences: on S2, the two pre-edge features are of comparable intensity (keto:enol ratio of 1:2-4), whereas on S1, the keto pre-edge is about an order of magnitude stronger than the enol (keto:enol ratio ∼10:1).108 Consequently, the weaker feature becomes masked, and the spectrum effectively appears as a single peak. In contrast, the comparable pre-edge intensities in TRXPS render the coalescence during H-transfer discernible.
Together, TRXPS and TRXAS therefore provide complementary windows into the dynamics: TRXPS can serve as a direct structural probe of the H-transfer, while TRXAS, through its valence-orbital sensitivity, could allow for probing the lifetime of the S1(nπ*) state.

4.		
Figure 9. Simulated oxygen K-edge ionization energies along the decay pathways of AcAc. Picture emerging along geodesic interpolated pathways: (a) H-transfer mediated pathway with signatures of intersystem crossing to T1 (ESIHT pathway in Figure 2); and (b) S2/S1-internal conversion through the asymmetric HTI seam followed by ballistic decay to the ground state through a nonstationary twist-pyramidalized region of the S1/S0- intersection seam. This path is an idealized representation of the dynamically accessed ballistic ground-state recovery pathway rather than the torsional pathway in Figure 2. The valence electronic state is indicated by colored arrows, and geometries for the end points of each interpolated segment are highlighted in the boxes. The TRXPS map along the two representative trajectories in Figure 7 and is analogous to the interpolated paths: (c) decay near the symmetric part of the HTI seam and trapping on S1. H-transfer occurs at ∼6 fs, and (d) ballistic ground-state recovery with H-transfer occurring at ∼26 fs. Stick spectra for the two pre-edge features were obtained at the RASPT2 level, convolved with a Gaussian line shape of fwhm 1.0 eV and uniformly shifted by -2.110 eV.

CONCLUSION AND OUTLOOK
We have investigated the earliest ultrafast decay in the enolone HO-CC-CO motif with XMS-CASPT2-based non- adiabatic dynamics simulations, focusing on how methylation modifies the potential-energy landscape and the ensuing dynamics by comparing MA and AcAc. While methyl groups are often thought of as inertial substituents, their weakly electron-donating character also modulates PESs of charge- polarized regions of configurational space, such as relative energetics of nπ* and ππ* states.
Our simulations show that both molecules exclusively undergo H-transfer-mediated S2/S1-internal conversion driven by BLA that guides the nuclear wavepacket to the HTI seam on a sub-20 fs time scale. The population decay occurs predominantly near the asymmetric parts of the HTI seam, while actual H-motion takes place mainly on S1, followed by chelate-ring contraction and subsequent expansion. While methylation leaves the fundamental S2/S1-decay pathway unchanged, it subtly reshapes the ensuing motion on S1. Its weak electronic effects extend the asymmetry of the HTI seam, and the added mass slows the chelate-ring contraction and introduces dynamical asymmetry between the central C-H and terminal C-CH3 groups. This imbalance promotes early out-of-plane motion, facilitating ballistic access to a higher- lying twist-pyramidalized region of the S1/S0-intersection seam within ∼100 fs. Consequently, AcAc exhibits both this early ground-state recovery followed by a slower torsional component similar to that in MA, resulting in somewhat different S0-populations after 200 fs (∼20-25% for AcAc and ∼10% for MA).
In comparison, α,β-enones, sharing the CC-CO motif, relax directly via a twist-pyramidalized pathway, which provides an efficient and almost concerted route to the ground state.10-12 In the enolones, the intramolecular H-bond introduces and redirects the wavepacket through the HTI seam and instead transiently traps a larger part of the population near the planar S1-minimum. Excess energy is mainly dissipated into the in-plane modes rather than driving torsion-mediated S1/S0-internal conversion. This mechanistic difference explains the markedly lower ground-state recovery in MA and AcAc compared with the enones.
While our considerations were restricted to the singlet manifold, the proximity of the T1(ππ*) state near the S1(nπ*)- minimum indicates that ISC should play a role.107 This picture is consistent with a recent TRPES study of AcAc, which resolved the ultrashort lifetime of the S2 state, vibrational remnants on S1 induced by the S2/S1-decay, and the emergence of a triplet fingerprint on the ps-time scale.48 In this work, we identified oxygen K-edge TRXPS as a potential experimental route to directly track the ultrafast BLA and H- motion governing S2/S1-decay by leveraging its sensitivity to the local chemical shifts at the enolic and ketonic O atoms. Remaining open questions concern the S1-lifetime and the pstime scale branching ratio between triplet formation and ground-state recovery. The corresponding oxygen K-edge TRXAS observable allows monitoring nπ* populations through their intense pre-edge features,108,113 and hence holds potential to determine the S1-lifetime. We hope that our theoretical work will inspire further experiments combining complementary modalities to fully map the ultrafast dynamics in these prototype systems.
■		ASSOCIATED CONTENT
Data Availability Statement
10.5281/zenodo.17210367: xyz files for critical points at the XMS-CASPT2 levels of theory. Quantum-thermostat generated initial conditions (positions and momenta) used in the FSSH simulations.
sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.5c07439.
Electronic-structure theory validation, critical point analysis, definitions of geometric parameters, details on initial condition sampling, and additional dynamics analyses (PDF)
■		AUTHOR INFORMATION
Corresponding Author
Nanna H. List - Department of Chemistry, KTH Royal Institute of Technology, Stockholm, SE 10044, Sweden;
School of Chemistry, University of Birmingham, Birmingham B15 2TT, United Kingdom; orcid.org/0000-0002-0246- 3995; Email: nalist@kth.se, n.h.list@bham.ac.uk
Authors
Pratip Chakraborty - Department of Chemistry, KTH Royal Institute of Technology, Stockholm, SE 10044, Sweden;
Present Address: School of Chemistry, Pharmacy and Pharmacology, University of East Anglia, Norwich NR4 7TJ, U.K;	orcid.org/0000-0002-0248-6193
Rafael C. Couto - Department of Chemistry, KTH Royal
Institute of Technology, Stockholm, SE 10044, Sweden;
orcid.org/0000-0003-4020-0923
Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpca.5c07439
Author Contributions
P.C.: formal analysis (equal), investigation (lead), methodology (equal), resources (supporting), validation (equal), visualization (equal), writingoriginal draft (equal), and writingreview and editing (supporting). R.C.C.: investigation (supporting), visualization (supporting), writingoriginal draft (supporting), and writingreview and editing (supporting). N.H.L.: conceptualization (lead), data curation (lead), formal analysis (equal), funding acquisition (lead), investigation (supporting), methodology (equal), project administration (lead), resources (lead), supervision (lead), validation (equal), visualization (equal), writingoriginal draft (equal), and writingreview and editing (lead).
Notes
The authors declare no competing financial interest.
■		ACKNOWLEDGMENTS
This work was supported by start-up funds from the School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, the Swedish Research Council (Grant no. 2022-02871), and the Swedish e-Science Research Center (SeRC). Computations were enabled by supercomputing resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) and the Swedish National Infrastructure for Computing (SNIC) at NSC/PDC, partially funded by the Swedish Research Council through grant agreement nos. 2022-06725 and 2018-05973. We also acknowledge the use of supercomputing resource Berzelius provided by the NSC at Linköping University and the Knut and Alice Wallenberg Foundation, as well as the EuroHPC Joint Undertaking for access to Discoverer at SofiaTech, Bulgaria.
■		REFERENCES
(1)		Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-coupled electron transfer. Chem. Rev. 2012, 112, 4016-4093.
(2)		Chattoraj, M.; King, B. A.; Bublitz, G. U.; Boxer, S. G. Ultra-fast excited state dynamics in green fluorescent protein: multiple states and proton transfer. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 83628367.
(3)		Lochbrunner, S.; Wurzer, A. J.; Riedle, E. Ultrafast excited-state proton transfer and subsequent coherent skeletal motion of 2-(2’- hydroxyphenyl)benzothiazole. J. Chem. Phys. 2000, 112, 1069910702.
(4)		Lochbrunner, S.; Schultz, T.; Schmitt, M.; Shaffer, J. P.; Zgierski, M. Z.; Stolow, A. Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy. J. Chem. Phys. 2001, 114, 2519-2522.
(5)		Herek, J. L.; Pedersen, S.; Banares, L.; Zewail, A. H. Femtosecond real-time probing of reactions. IX. Hydrogen-atom transfer. J. Chem. Phys. 1992, 97, 9046-9061.
(6)		Tseng, H.-W.; Liu, J.-Q.; Chen, Y.-A.; Chao, C.-M.; Liu, K.-M.; Chen, C.-L.; Lin, T.-C.; Hung, C.-H.; Chou, Y.-L.; Lin, T.-C.; Wang, T.-L.; Chou, P.-T. Harnessing Excited-State Intramolecular ProtonTransfer Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules. J. Phys. Chem. Lett. 2015, 6, 1477-1486.
(7)		Zhao, J.; Ji, S.; Chen, Y.; Guo, H.; Yang, P. Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials. Phys. Chem. Chem. Phys. 2012, 14, 8803-8817.
(8)		Sedgwick, A. C.; Wu, L.; Han, H.-H.; Bull, S. D.; He, X.-P.; James, T. D.; Sessler, J. L.; Tang, B. Z.; Tian, H.; Yoon, J. Excited- state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents. Chem. Soc. Rev. 2018, 47, 8842-8880.
(9)		Kwon, J. E.; Park, S. Y. Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process. Adv. Mater. 2011, 23, 3615-3642.
(10)		Lee, A. M. D.; Coe, J.; Ullrich, S.; Ho, M.-L.; Lee, S.-J.; Cheng, B.-M.; Zgierski, M.; Chen, I.; Martinez, T.; Stolow, A. Substituent effects on dynamics at conical intersections: α, β-enones. J. Phys. Chem. A 2007, 111, 11948-11960.
(11)		Hohenstein, E. G.; Yu, J. K.; Bannwarth, C.; List, N. H.; Paul, A. C.; Folkestad, S. D.; Koch, H.; Martínez, T. J. Predictions of Preedge Features in Time-Resolved Near-Edge X-ray Absorption Fine Structure Spectroscopy from Hole-Hole Tamm-Dancoff-Approxi- mated Density Functional Theory. J. Chem. Theory Comput. 2021, 17, 7120-7133.
(12)		Chakraborty, P.; Couto, R. C.; List, N. H. Deciphering Methylation Effects on S2 (ππ*) Internal Conversion in the Simplest Linear α, β-Unsaturated Carbonyl. J. Phys. Chem. A 2023, 127, 53605373.
(13)		Olivucci, M.; Bernardi, F.; Ottani, S.; Robb, M. A. Substituent Effects in Buta-1,3-diene Photochemistry: A CAS-SCF Study of 2,3- Dimethylbutadiene and 2-Cyanobutadiene Excited-State Reaction Paths. J. Am. Chem. Soc. 1994, 116, 2034-2048.
(14)		MacDonell, R. J.; Schalk, O.; Geng, T.; Thomas, R. D.; Feifel, R.; Hansson, T.; Schuurman, M. S. Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation. J. Chem. Phys. 2016, 145, No. 114306.
(15)		MacDonell, R. J.; Schuurman, M. S. Site-selective isomerization of cyano-substituted butadienes: Chemical control of nonadiabatic dynamics. J. Phys. Chem. A 2019, 123, 4693-4701.
(16)		Neville, S. P.; Wang, Y.; Boguslavskiy, A. E.; Stolow, A.; Schuurman, M. S. Substituent effects on dynamics at conical intersections: Allene and methyl allenes. J. Chem. Phys. 2016, 144, No. 014305, DOI: 10.1063/1.4938561.
(17)		Schalk, O.; Boguslavskiy, A. E.; Stolow, A. Substituent effects on dynamics at conical intersections: Cyclopentadienes. J. Phys. Chem. A 2010, 114, 4058-4064.
(18)		Wu, G.; Boguslavskiy, A. E.; Schalk, O.; Schuurman, M. S.; Stolow, A. Ultrafast non-adiabatic dynamics of methyl substituted ethylenes: The π3s Rydberg state. J. Chem. Phys. 2011, 135, No. 164309, DOI: 10.1063/1.3652966.
(19)		MacDonell, R. J.; Schuurman, M. S. Substituent effects on the nonadiabatic dynamics of ethylene: π-donors and π-acceptors. Chem. Phys. 2018, 515, 360-368.
(20)		Herperger, K. R.; Röder, A.; MacDonell, R. J.; Boguslavskiy, A. E.; Skov, A. B.; Stolow, A.; Schuurman, M. S. Directing excited state dynamics via chemical substitution: A systematic study of π-donors and π-acceptors at a carbon-carbon double bond. J. Chem. Phys. 2020, 153, No. 244307.
(21)		MacDonell, R. J.; Corrales, M. E.; Boguslavskiy, A. E.; Bañares, L.; Stolow, A.; Schuurman, M. S. Substituent effects on nonadiabatic excited state dynamics: Inertial, steric, and electronic effects in methylated butadienes. J. Chem. Phys. 2020, 152, No. 084308.
(22)		List, N. H.; Jones, C. M.; Martínez, T. J. Chemical control of excited-state reactivity of the anionic green fluorescent protein chromophore. Commun. Chem. 2024, 7, No. 25.
(23)		Schuurman, M. S.; Stolow, A. Dynamics at conical intersections. Annu. Rev. Phys. Chem. 2018, 69, 427-450.
(24)		Malis,̌ M.; Loquais, Y.; Gloaguen, E.; Jouvet, C.; Brenner, V.; Mons, M.; Ljubic,́ I.; Dosľ ic,́ N. Non-radiative relaxation of UV photoexcited phenylalanine residues: probing the role of conical intersections by chemical substitution. Phys. Chem. Chem. Phys. 2014, 16, 2285-2288.
(25)		Crespo-Hernández, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Ultrafast Excited-State Dynamics in Nucleic Acids. Chem. Rev. 2004, 104, 1977-2020.
(26)		MacDonell, R. J.; Corrales, M. E.; Boguslavskiy, A. E.; Bañares, L.; Stolow, A.; Schuurman, M. S. Substituent effects on nonadiabatic excited state dynamics: Inertial, steric, and electronic effects in methylated butadienes. J. Chem. Phys. 2020, 152, No. 084308, DOI: 10.1063/1.5139446.
(27)		Baughcum, S. L.; Smith, Z.; Wilson, E. B.; Duerst, R. W. Microwave spectroscopic study of malonaldehyde. 3. Vibrationrotation interaction and one-dimensional model for proton tunneling. J. Am. Chem. Soc. 1984, 106, 2260-2265.
(28)		Rowe, W. F. J.; Duerst, R. W.; Wilson, E. B. The intramolecular hydrogen bond in malonaldehyde. J. Am. Chem. Soc. 1976, 98, 40214023.
(29)		Irving, R. J.; Wadsö, I.; Ballhausen, C.; Ragnarsson, U.; Rasmussen, S.; Sunde, E.; Sørensen, N. A. Enthalpy of Vaporization or Organic Compounds at 25 degrees CV Acetylacetone. Acta Chem. Scand. 1970, 24, 589-592.
(30)		Grosch, A. A.; van der Lubbe, S. C. C.; Guerra, C. F. Nature of Intramolecular Resonance Assisted Hydrogen Bonding in Malonalde- hyde and Its Saturated Analogue. J. Phys. Chem. A 2018, 122, 18131820.
(31)		Dannenberg, J. J.; Rios, R. Theoretical study of the enolic forms of acetylacetone. How strong is the hydrogen bond? J. Phys. Chem. A 1994, 98, 6714-6718.
(32)		Nagy, P. I.; Alagona, G.; Ghio, C. Theoretical Investigation of Tautomeric Equilibria for Isonicotinic Acid, 4-Pyridone, and Acetylacetone in Vacuo and in Solution. J. Chem. Theory Comput. 2007, 3, 1249-1266.
(33)		Belova, N. V.; Oberhammer, H.; Trang, N. H.; Girichev, G. V. Tautomeric Properties and Gas-Phase Structure of Acetylacetone. J. Org. Chem. 2014, 79, 5412-5419.
(34)		Howard, D. L.; Kjaergaard, H. G.; Huang, J.; Meuwly, M. Infrared and Near-Infrared Spectroscopy of Acetylacetone and Hexafluoroacetylacetone. J. Phys. Chem. A 2015, 119, 7980-7990.
(35)		Tsukahara, T.; Nagaoka, K.; Morikawa, K.; Mawatari, K.; Kitamori, T. Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces. J. Phys. Chem. B 2015, 119, 14750-14755.
(36)		Scheiner, S. Theoretical Studies of Excited State Proton Transfer in Small Model Systems. J. Phys. Chem. A 2000, 104, 58985909.
(37)		Conant, J. B.; Thompson, A. F. J. The Free Energy of Enolization in the Gaseous Phase of Substituted Acetoacetic Esters. J. Am. Chem. Soc. 1932, 54, 4039-4047.
(38)		Seliskar, C. J.; Hoffmann, R. E. 11B1(nπ*) ← 11A1 transition of malondialdehyde. J. Am. Chem. Soc. 1977, 99, 7072-7073.
(39)		Nakanishi, H.; Morita, H.; Nagakura, S. Electronic Structures and Spectra of the Keto and Enol Forms of Acetylacetone. Bull. Chem. Soc. Jpn. 1977, 50, 2255-2261.
(40)		Baughcum, S. L.; Duerst, R. W.; Rowe, W. F.; Smith, Z.; Wilson, E. B. Microwave spectroscopic study of malonaldehyde (3- hydroxy-2-propenal). 2. Structure, dipole moment, and tunneling. J. Am. Chem. Soc. 1981, 103, 6296-6303.
(41)		Coe, J. D.; Martínez, T. J. Competitive Decay at Two- and Three-State Conical Intersections in Excited-State Intramolecular Proton Transfer. J. Am. Chem. Soc. 2005, 127, 4560-4561.
(42)		Coe, J. D.; Martínez, T. J. Ab Initio Molecular Dynamics of Excited-State Intramolecular Proton Transfer around a Three-State Conical Intersection in Malonaldehyde. J. Phys. Chem. A 2006, 110, 618-630.
(43)		Coe, J. D.; Ong, M. T.; Levine, B. G.; Martínez, T. J. On the Extent and Connectivity of Conical Intersection Seams and the Effects of Three-State Intersections. J. Phys. Chem. A 2008, 112, 12559-12567.
(44)		Xie, B.; Cui, G.; Fang, W.-H. Multiple-State Nonadiabatic Dynamics Simulation of Photoisomerization of Acetylacetone with the Direct ab Initio QTMF Approach. J. Chem. Theory Comput. 2017, 13, 2717-2729.
(45)		Sapunar, M.; Ayari, T.; Dosľ ic,́ N. Comparative study of the photodynamics of malonaldehyde and acetylacetone. Chem. Phys. 2018, 515, 622-627. Ultrafast Photoinduced Processes in Polyatomic Molecules:Electronic Structure, Dynamics and Spectroscopy (Dedicated to Wolfgang Domcke on the occasion of his 70th birthday).
(46)		Squibb, R. J.; Sapunar, M.; Ponzi, A.; Richter, R.; Kivimäki, A.; Plekan, O.; Finetti, P.; Sisourat, N.; Zhaunerchyk, V.; Marchenko, T.; Journel, L.; Guillemin, R.; Cucini, R.; Coreno, M.; Grazioli, C.; Di Fraia, M.; Callegari, C.; Prince, K. C.; Decleva, P.; Simon, M.; Eland, J. H. D.; Dosľ ic,́ N.; Feifel, R.; Piancastelli, M. N. Acetylacetone photodynamics at a seeded free-electron laser. Nat. Commun. 2018, 9, No. 63.
(47)		Marsili, E.; Prlj, A.; Curchod, B. F. E. Caveat when using ADC(2) for studying the photochemistry of carbonyl-containing molecules. Phys. Chem. Chem. Phys. 2021, 23, 12945-12949.
(48)		Severino, S.; Aleotti, F.; Mai, L.; Crego, A.; Medeghini, F.; Frassetto, F.; Poletto, L.; Lucchini, M.; Segatta, F.; Reduzzi, M.; Nisoli, M.; Nenov, A.; Borrego-Varillas, R. Mapping Excited-State Decay Mechanisms in Acetylacetone by Sub-20 fs Time-Resolved Photoelectron Spectroscopy. J. Am. Chem. Soc. 2025, 147, 3078530793.
(49)		Xu, S.; Park, S. T.; Feenstra, J. S.; Srinivasan, R.; Zewail, A. H. Ultrafast Electron Diffraction: Structural Dynamics of the Elimination Reaction of Acetylacetone. J. Phys. Chem. A 2004, 108, 6650-6655.
(50)		Scutelnic, V.; Tikhonov, D. S.; Marggi Poullain, S.; Haugen, E. A.; Attar, A.; Barreau, L.; Chang, K.; Fidler, A. P.; Gaynor, J. D.; Lin, Y.-C.; Yang, J.; Champenois, E. G.; Wang, X.; Shen, X.; Luo, D.; Ji, F.; Weathersby, S.; Vishnevskiy, Y. V.; Schnell, M.; Wolf, T. J. A.; Lin, M.-F.; Leone, S. R. Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction. The. J. Phys. Chem. Lett. 2025, 16, 5068-5075.
(51)		Poisson, L.; Roubin, P.; Coussan, S.; Soep, B.; Mestdagh, J.-M. Ultrafast Dynamics of Acetylacetone (2,4-Pentanedione) in the S2 State. J. Am. Chem. Soc. 2008, 130, 2974-2983.
(52)		Kotsina, N.; Candelaresi, M.; Saalbach, L.; Zawadzki, M. M.; Crane, S. W.; Sparling, C.; Townsend, D. Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone. Phys. Chem. Chem. Phys. 2020, 22, 4647-4658.
(53)		Bhattacherjee, A.; Pemmaraju, C. D.; Schnorr, K.; Attar, A. R.; Leone, S. R. Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge. J. Am. Chem. Soc. 2017, 139, 16576-16583.
(54)		Antonov, I.; Voronova, K.; Chen, M.-W.; Sztáray, B.; Hemberger, P.; Bodi, A.; Osborn, D. L.; Sheps, L. To Boldly Look Where No One Has Looked Before: Identifying the Primary Photoproducts of Acetylacetone. J. Phys. Chem. A 2019, 123, 54725490.
(55)		Rinaman, J. E.; Murray, C. Acetylacetone Photolysis at 280 nm Studied by Velocity-Map Ion Imaging. J. Phys. Chem. A 2023, 127, 6687-6696.
(56)		Prlj, A.; Hollas, D.; Curchod, B. F. E. Deciphering the Influence of Ground-State Distributions on the Calculation of Photolysis Observables. J. Phys. Chem. A 2023, 127, 7400-7409.
(57)		Finley, J.; Malmqvist, P.-Å.; Roos, B. O.; Serrano-Andrés, L. The multi-state CASPT2 method. Chem. Phys. Lett. 1998, 288, 299306.
(58)		Granovsky, A. A. Extended multi-configuration quasidegenerate perturbation theory: The new approach to multi-state multi-reference perturbation theory. J. Chem. Phys. 2011, 134, No. 214113.
(59)		Shiozaki, T.; Györffy, W.; Celani, P.; Werner, H.-J. Communication: Extended multi-state complete active space second-order perturbation theory: Energy and nuclear gradients. J. Chem. Phys. 2011, 135, No. 081106.
(60)		Shiozaki, T. BAGEL: Brilliantly Advanced General Electronicstructure Library. WIREs Comput. Mol. Sci. 2018, 8, No. e1331.
(61)		BAGEL, Brilliantly Advanced General Electronic-structure Library, Version 1.2.2. 2018 http://www.nubakery.org.
(62)		Park, J. W. Single-state single-reference and multistate multireference zeroth-order Hamiltonians in MS-CASPT2 and conical intersections. J. Chem. Theory Comput. 2019, 15, 3960-3973.
(63)		List, N. H.; Jones, C. M.; Martínez, T. J. Internal conversion of the anionic GFP chromophore: in and out of the I-twisted S1/S0 conical intersection seam. Chem. Sci. 2022, 13, 373-385.
(64)		Ceriotti, M.; Bussi, G.; Parrinello, M. Langevin Equation with Colored Noise for Constant-Temperature Molecular Dynamics Simulations. Phys. Rev. Lett. 2009, 102, No. 020601.
(65)		Ceriotti, M.; Bussi, G.; Parrinello, M. Colored-Noise Thermostats à la Carte. J. Chem. Theory Comput. 2010, 6, 1170-1180.
(66)		Prlj, A.; Marsili, E.; Hutton, L.; Hollas, D.; Shchepanovska, D.; Glowacki, D. R.; Slavíček, P.; Curchod, B. F. E. Calculating Photoabsorption Cross-Sections for Atmospheric Volatile Organic Compounds. ACS Earth Space Chem. 2022, 6, 207-217.
(67)		Ziegler, T. Approximate density functional theory as a practical tool in molecular energetics and dynamics. Chem. Rev. 1991, 91, 651667.
(68)		Kohn, W.; Becke, A. D.; Parr, R. G. Density functional theory of electronic structure. J. Phys. Chem. A 1996, 100, 12974-12980.
(69)		Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38, No. 3098.
(70)		Lee, C.; Yang, W.; Parr, R. G. Development of the Colle- Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, No. 785.
(71)		Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis. Can. J. Phys. 1980, 58, 1200-1211.
(72)		Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648-5652.
(73)		Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, No. 154104, DOI: 10.1063/1.3382344.
(74)		Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456-1465.
(75)		Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724-728.
(76)		Hehre, W. J.; Ditchfield, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XII. Further Extensions of Gaussian Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257-2261.
(77)		Hariharan, P. C.; Pople, J. A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chim. Acta 1973, 28, 213-222.
(78)		gle4md: Generalized Langevin Equations for Molecular Dynamics 2023 http://gle4md.org/. (accessed June).
(79)		Hollas, D.; Suchan, J.; Oncǒák, M.; Svoboda, O.; Slavíček, P. ABIN, Version 1.1. 2021 https://github.com/PHOTOX/ABIN.
(80)		Ufimtsev, I. S.; Martinez, T. J. Quantum chemistry on graphical processing units. 1. Strategies for two-electron integral evaluation. J. Chem. Theory Comput. 2008, 4, 222-231.
(81)		Ufimtsev, I. S.; Martinez, T. J. Quantum chemistry on graphical processing units. 2. Direct self-consistent-field implementation. J. Chem. Theory Comput. 2009, 5, 1004-1015.
(82)		Ufimtsev, I. S.; Martinez, T. J. Quantum chemistry on graphical processing units. 3. Analytical energy gradients, geometry optimization, and first principles molecular dynamics. J. Chem. Theory Comput. 2009, 5, 2619-2628.
(83)		Seritan, S.; Bannwarth, C.; Fales, B. S.; Hohenstein, E. G.; Kokkila-Schumacher, S. I.; Luehr, N.; Snyder, J. W., Jr; Song, C.; Titov, A. V.; Ufimtsev, I. S.; Martínez, T. J. TeraChem: Accelerating electronic structure and ab initio molecular dynamics with graphical processing units. J. Chem. Phys. 2020, 152, No. 224110.
(84)		Mai, S.; Marquetand, P.; González, L. Nonadiabatic dynamics: The SHARC approach. WIREs Comput.Mol. Sci. 2018, 8, No. e1370.
(85)		Tully, J. C. Molecular dynamics with electronic transitions. J. Chem. Phys. 1990, 93, 1061-1071.
(86)		Granucci, G.; Persico, M. Critical appraisal of the fewest switches algorithm for surface hopping. J. Chem. Phys. 2007, 126, No. 134114.
(87)		Malmqvist, P. k.; Pierloot, K.; Shahi, A. R. M.; Cramer, C. J.; Gagliardi, L. The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems. J. Chem. Phys. 2008, 128, No. 204109, DOI: 10.1063/1.2920188.
(88)		Delcey, M. G.; Sørensen, L. K.; Vacher, M.; Couto, R. C.; Lundberg, M. Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions. J. Comput. Chem. 2019, 40, 1789-1799.
(89)		Aquilante, F.; Pedersen, T. B.; Lindh, R. Low-cost evaluation of the exchange Fock matrix from Cholesky and density fitting representations of the electron repulsion integrals. J. Chem. Phys. 2007, 126, No. 194106, DOI: 10.1063/1.2736701.
(90)		Vidal, M. L.; Krylov, A. I.; Coriani, S. Dyson orbitals within the fc-CVS-EOM-CCSD framework: theory and application to X-ray photoelectron spectroscopy of ground and excited states. Phys. Chem. Chem. Phys. 2020, 22, 2693-2703.
(91)		Söderström, J.; Mårtensson, N.; Travnikova, O.; Patanen, M.; Miron, C.; Sæthre, L. J.; Børve, K. J.; Rehr, J. J.; Kas, J. J.; Vila, F. D.; Thomas, T. D.; Svensson, S. Nonstoichiometric Intensities in Core Photoelectron Spectroscopy. Phys. Rev. Lett. 2012, 108, No. 193005.
(92)		Feyer, V.; Prince, K. C.; Coreno, M.; Melandri, S.; Maris, A.; Evangelisti, L.; Caminati, W.; Giuliano, B. M.; Kjaergaard, H. G.; Carravetta, V. Quantum Effects for a Proton in a Low-Barrier, Double-Well Potential: Core Level Photoemission Spectroscopy of Acetylacetone. J. Phys. Chem. Lett. 2018, 9, 521-526.
(93)		Capron, N.; Casier, B.; Sisourat, N.; Piancastelli, M. N.; Simon, M.; Carniato, S. Probing keto-enol tautomerism using photoelectron spectroscopy. Phys. Chem. Chem. Phys. 2015, 17, 19991-19996.
(94)		Malmqvist, P.; Roos, B. O. The CASSCF state interaction method. Chem. Phys. Lett. 1989, 155, 189-194.
(95)		Fdez Galván, I.; Vacher, M.; Alavi, A.; Angeli, C.; Aquilante, F.; Autschbach, J.; Bao, J. J.; Bokarev, S. I.; Bogdanov, N. A.; Carlson, R. K.; Chibotaru, L. F.; Creutzberg, J.; Dattani, N.; Delcey, M. G.; Dong, S. S.; Dreuw, A.; Freitag, L.; Frutos, L. M.; Gagliardi, L.; Gendron, F.; Giussani, A.; González, L.; Grell, G.; Guo, M.; Hoyer, C. E.; Johansson, M.; Keller, S.; Knecht, S.; Kovacě vic,́ G.; Källman, E.; Li Manni, G.; Lundberg, M.; Ma, Y.; Mai, S.; Malhado, J. P.; Malmqvist, PÅ.; Marquetand, P.; Mewes, S. A.; Norell, J.; Olivucci, M.; Oppel, M.; Phung, Q. M.; Pierloot, K.; Plasser, F.; Reiher, M.; Sand, A. M.; Schapiro, I.; Sharma, P.; Stein, C. J.; Sørensen, L. K.; Truhlar, D. G.; Ugandi, M.; Ungur, L.; Valentini, A.; Vancoillie, S.; Veryazov, V.; Weser, O.; Wesołowski, T. A.; Widmark, P.-O.; Wouters, S.; Zech, A.; Zobel, J. P.; Lindh, R. OpenMolcas: From Source Code to Insight. J. Chem. Theory Comput. 2019, 15, 5925-5964.
(96)		Aquilante, F.; Autschbach, J.; Baiardi, A.; Battaglia, S.; Borin, V. A.; Chibotaru, L. F.; Conti, I.; De Vico, L.; Delcey, M.; Fdez Galván, I.; Ferré, N.; Freitag, L.; Garavelli, M.; Gong, X.; Knecht, S.; Larsson, E. D.; Lindh, R.; Lundberg, M.; Malmqvist, PÅ.; Nenov, A.; Norell, J.; Odelius, M.; Olivucci, M.; Pedersen, T. B.; Pedraza-González, L.; Phung, Q. M.; Pierloot, K.; Reiher, M.; Schapiro, I.; Segarra-Martí, J.; Segatta, F.; Seijo, L.; Sen, S.; Sergentu, D.-C.; Stein, C. J.; Ungur, L.; Vacher, M.; Valentini, A.; Veryazov, V. Modern quantum chemistry with [Open]Molcas. J. Chem. Phys. 2020, 152, No. 214117.
(97)		Li Manni, G.; Fdez Galván, I.; Alavi, A.; Aleotti, F.; Aquilante, F.; Autschbach, J.; Avagliano, D.; Baiardi, A.; Bao, J. J.; Battaglia, S.; Birnoschi, L.; Blanco-González, A.; Bokarev, S. I.; Broer, R.; Cacciari, R.; Calio, P. B.; Carlson, R. K.; Couto, R. C.; Cerdán, L.; Chibotaru, L. F.; Chilton, N. F.; Church, J. R.; Conti, I.; Coriani, S.; Cuéllar- Zuquin, J.; Daoud, R. E.; Dattani, N.; Decleva, P.; de Graaf, C.; Delcey, M. G.; De Vico, L.; Dobrautz, W.; Dong, S. S.; Feng, R.; Ferré, N.; Filatov(Gulak), M.; Gagliardi, L.; Garavelli, M.; González, L.; Guan, Y.; Guo, M.; Hennefarth, M. R.; Hermes, M. R.; Hoyer, C. E.; Huix-Rotllant, M.; Jaiswal, V. K.; Kaiser, A.; Kaliakin, D. S.; Khamesian, M.; King, D. S.; Kochetov, V.; Krosń icki, M.; Kumaar, A. A.; Larsson, E. D.; Lehtola, S.; Lepetit, M.-B.; Lischka, H.; López Ríos, P.; Lundberg, M.; Ma, D.; Mai, S.; Marquetand, P.; Merritt, I. C. D.; Montorsi, F.; Mörchen, M.; Nenov, A.; Nguyen, V. H. A.; Nishimoto, Y.; Oakley, M. S.; Olivucci, M.; Oppel, M.; Padula, D.; Pandharkar, R.; Phung, Q. M.; Plasser, F.; Raggi, G.; Rebolini, E.; Reiher, M.; Rivalta, I.; Roca-Sanjuán, D.; Romig, T.; Safari, A. A.; Sánchez- Mansilla, A.; Sand, A. M.; Schapiro, I.; Scott, T. R.; Segarra-Martí, J.; Segatta, F.; Sergentu, D.-C.; Sharma, P.; Shepard, R.; Shu, Y.; Staab, J. K.; Straatsma, T. P.; Sørensen, L. K.; Tenorio, B. N. C.; Truhlar, D.
G.; Ungur, L.; Vacher, M.; Veryazov, V.; Voß, T. A.; Weser, O.; Wu, D.; Yang, X.; Yarkony, D.; Zhou, C.; Zobel, J. P.; Lindh, R. The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry. J. Chem. Theory Comput. 2023, 19, 69336991.
(98)		Zhu, X.; Thompson, K. C.; Martínez, T. J. Geodesic interpolation for reaction pathways. J. Chem. Phys. 2019, 150, No. 164103.
(99)		Lowrey, A. H.; George, C.; d’Antonio, P.; Karle, J. Structure of acetylacetone by electron diffraction. J. Am. Chem. Soc. 1971, 93, 6399-6403.
(100)		Andreassen, A.; Bauer, S. The structures of acetylacetone, trifluoroacetyl-acetone and trifluoroacetone. J. Mol. Struct. 1972, 12, 381-403.
(101)		Srinivasan, R.; Feenstra, J. S.; Park, S. T.; Xu, S.; Zewail, A. H. Direct determination of hydrogen-bonded structures in resonant and tautomeric reactions using ultrafast electron diffraction. J. Am. Chem. Soc. 2004, 126, 2266-2267.
(102)		Iijima, K.; Ohnogi, A.; Shibata, S. The molecular structure of acetylacetone as studied by gas-phase electron diffraction. J. Mol. Struct. 1987, 156, 111-118.
(103)		Caminati, W.; Grabow, J.-U. The C2v Structure of Enolic Acetylacetone. J. Am. Chem. Soc. 2006, 128, 854-857.
(104)		Qu, C.; Conte, R.; Houston, P. L.; Bowman, J. M. Fulldimensional potential energy surface for acetylacetone and tunneling splittings. Phys. Chem. Chem. Phys. 2021, 23, 7758-7767.
(105)		Qu, C.; Houston, P. L.; Conte, R.; Nandi, A.; Bowman, J. M. Breaking the coupled cluster barrier for machine-learned potentials of large molecules: The case of 15-atom acetylacetone. J. Phys. Chem. Lett. 2021, 12, 4902-4909.
(106)		Käser, S.; Unke, O. T.; Meuwly, M. Reactive dynamics and spectroscopy of hydrogen transfer from neural network-based reactive potential energy surfaces. New J. Phys. 2020, 22, No. 055002.
(107)		El-Sayed, M. A. Triplet state. Its radiative and nonradiative properties. Acc. Chem. Res. 1968, 1, 8-16.
(108)		List, N. H.; Dempwolff, A. L.; Dreuw, A.; Norman, P.; Martínez, T. J. Probing competing relaxation pathways in malonaldehyde with transient X-ray absorption spectroscopy. Chem. Sci. 2020, 11, 4180-4193.
(109)		Weller, A. Uber die Fluoreszenz der Salizylsäure und verwandter Verbindungen. Naturwissenschaften 1955, 42, 175-176.
(110)		Goodman, J.; Brus, L. E. Proton transfer and tautomerism in an excited state of methyl salicylate. J. Am. Chem. Soc. 1978, 100, 7472-7474.
(111)		Rao, C. N. R.; Goldman, G. K.; Balasubramanian, A. Structural and solvent effects on the n→ π*(′U← ′A) transitions in aliphatic carbonyl derivatives: evidence for hyperconjugation in the electronically excited states of molecules. Can. J. Chem. 1960, 38, 2508-2513. (112)	Walzl, K. N.; Xavier, J.; I, M.; Kuppermann, A. Electronimpact spectroscopy of various diketone compounds. J. Chem. Phys. 1987, 86, 6701-6706.
(113)		Wolf, T. J. A.; Myhre, R. H.; Cryan, J. P.; Coriani, S.; Squibb, R. J.; Battistoni, A.; Berrah, N.; Bostedt, C.; Bucksbaum, P.; Coslovich, G.; Feifel, R.; Gaffney, K. J.; Grilj, J.; Martinez, T. J.; Miyabe, S.; Moeller, S. P.; Mucke, M.; Natan, A.; Obaid, R.; Osipov, T.; Plekan, O.; Wang, S.; Koch, H.; Gühr, M. Probing ultrafast ππ*/ nπ*internal conversion in organic chromophores via K-edge resonant absorption. Nat. Commun. 2017, 8, No. 29.
(114)		Lessard, R.; Cuny, J.; Cooper, G.; Hitchcock, A. P. Inner-shell excitation of gas phase carbonates and α,γ-dicarbonyl compounds. Chem. Phys. 2007, 331, 289-303.
(115)		Rohatgi, A. WebPlotDigitizer, Version 5.2. 2024 https:// automeris.io/WebPlotDigitizer/.