Chemical substituents
can influence photodynamics by
altering the
location of critical points and the topography of the potential energy
surfaces (electronic effect) and by selectively modifying the inertia
of specific nuclear modes (inertial effects). Using nonadiabatic dynamics
simulations, we investigate the impact of methylation on S2(ππ*) internal conversion in acrolein,
the simplest linear α,β-unsaturated carbonyl. Consistent
with time constants reported in a previous time-resolved photoelectron
spectroscopy study, S2→ S1deactivation
occurs on an ultrafast time scale (∼50 fs). However, our simulations
do not corroborate the sequential decay model used to fit the experiment.
Instead, upon reaching the S1state, the wavepacket bifurcates:
a portion undergoes ballistic S1→ S0deactivation (∼90 fs) mediated by fast bond-length alternation
motion, while the remaining decays on the picosecond time scale. Our
analysis reveals that methyl substitution, generally assumed to mainly
exert inertial influence, is also manifested in important electronic
effects due to its weak electron-donating ability. While methylation
at the β C atom gives rise to effects principally of an inertial
nature, such as retarding the twisting motion of the terminal −CHCH3group and increasing its coupling with pyramidalization,
methylation at the α or carbonyl C atom modifies the potential
energy surfaces in a way that also contributes to altering the late
S1-decay behavior. Specifically, our results suggest that
the observed slowing of the picosecond component upon α-methylation
is a consequence of a tighter surface and reduced amplitude along
the central pyramidalization, effectively restricting the access to
the S1/S0-intersection seam. Our work offers
new insight into the S2(ππ*) internal conversion mechanisms in acrolein and its methylated
derivatives and highlights site-selective methylation as a tuning
knob to manipulate photochemical reactions.