# Electronic and Inertial Effects of Methylation on Excited-State Hydrogen Transfer

## TL;DR

This paper studies how methylation affects ultrafast hydrogen transfer in molecules using advanced computational methods.

## Contribution

The study reveals how methylation alters electronic and inertial dynamics in excited-state hydrogen transfer.

## Key findings

- Methylation destabilizes the S1 state and reduces the S2/S1 energy gap.
- Methylation introduces inertial mismatch leading to distinct S1 behaviors.
- Results align with time-resolved photoelectron spectroscopy data.

## 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.