Non-adiabatic dynamic of atmospheric unimolecular photochemical
reactions of 4,4-difluoro-crotonaldehyde using TD-DFT and TSH approaches
Abstract
Photochemical reactions of small molecules occur upon irradiation by
ultraviolet or visible light, and they are a very important and
controversial chemical process in the Earth’s atmosphere because they
impact our quality of life and health. Small-unsaturated carbonyl
compounds play an important role in the chemistry of the polluted
troposphere. The fluorinated aldehydes are very reactive under the
sunlight driving to species that trigger more atmospheric reactions.
This paper is focused on a theoretical study of the photochemistry of
difluoro-crotonaldehyde using static and dynamic calculations by
combination of Global Reaction Route mapping (GRRM) and Trajectory
Surface Hopping (TSH) approach. The static analysis of the electronic
and geometrical structures at the critical points allowed to rationalize
the possible pathways that interconnect the stationary and crossing
points in order to get a map of the unimolecular photochemical reactions
which take place. The time evolution of the electronic states and the
degrees of freedom enabled the identification of the requirements to
follow the most probable deactivation pathways. This article reports the
unimolecular deactivation pathways after the electronic excitation of
the trans and cis isomers. In both cases, the excitation energies were
calculated and compared with the analogous in the crotonaldehyde in
order to elucidate the effect of fluorine atoms on the electronic
structure and stabilities. After the initial excitations to the ππ*
excited states, the main deactivation channels follow non-adiabatic
pathways via S1/S0 conical
intersections. Ultrafast processes leading to the early activation of
the S1 govern the decay of the difluoro-crotonaldehyde.
Depending on the nature of the S1 state before the
crossing with the S0, the system can follow several
reaction pathways. The main photochemical processes observed were the
cis-trans isomerization, the Norrish type I reaction (α-cleavage),
Norrish type II reaction (γ-hydrogen abstraction) and fluorine
photodissociation. The time scale, the molecular deformations and the
electronic states implied for the different photochemical processes, as
well as how these compete with the photophysical deactivation are
discussed.