Photo-induced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic-structure and molecular-dynamics calculations of the excitedand ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state is shown to be driven by ballistic S-C bond extension and to be complete within 350 femtoseconds. Theory and experiment also enable visualization of the rich ground-state dynamics -involving formation of, and interconversion between, ring-opened isomers and the cyclic structure, and fragmentation over much longer timescales.
Volatile
organic compounds (VOCs) are ubiquitous atmospheric molecules
that generate a complex network of chemical reactions in the troposphere,
often triggered by the absorption of sunlight. Understanding the VOC
composition of the atmosphere relies on our ability to characterize all of their
possible reaction pathways. When considering reactions of (transient)
VOCs with sunlight, the availability of photolysis rate constants,
utilized in general atmospheric models, is often out of experimental
reach due to the unstable nature of these molecules. Here, we show
how recent advances in computational photochemistry allow us to calculate
in silico
the different ingredients of a photolysis rate
constant, namely, the photoabsorption cross-section and wavelength-dependent
quantum yields. The rich photochemistry of
tert
-butyl
hydroperoxide, for which experimental data are available, is employed
to test our protocol and highlight the strengths and weaknesses of
different levels of electronic structure and nonadiabatic molecular
dynamics to study the photochemistry of (transient) VOCs.
We present a detailed study of the decoherence correction to surface hopping that was recently derived from the exact factorization approach. Ab initio multiple spawning calculations that use the same initial conditions and the same electronic structure method are used as a reference for three molecules: ethylene, the methaniminium cation, and fulvene, for which nonadiabatic dynamics follows a photoexcitation. A comparison with the Granucci−Persico energy-based decoherence correction and the augmented fewest-switches surface-hopping scheme shows that the three decoherence-corrected methods operate on individual trajectories in a qualitatively different way, but the results averaged over trajectories are similar for these systems.
The
ultrafast time evolution of a single-stranded adenine DNA is
studied using a hybrid multiscale quantum mechanics/molecular mechanics
(QM/MM) scheme coupled to nonadiabatic surface hopping dynamics. As
a model, we use (dA)
20
where a stacked adenine tetramer
is treated quantum chemically. The dynamical simulations combined
with on-the-fly quantitative wave function analysis evidence the nature
of the long-lived electronically excited states formed upon absorption
of UV light. After a rapid decrease of the initially excited excitons,
relaxation to monomer-like states and excimers occurs within 100 fs.
The former monomeric states then relax into additional excimer states
en route to forming stabilized charge-transfer states on a longer
timescale of hundreds of femtoseconds. The different electronic-state
characters is reflected on the spatial separation between the adenines:
excimers and charge-transfer states show a much smaller spatial separation
than the monomer-like states and the initially formed excitons.
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