Photoisomerization of Methyl Vinyl Ketone and Methacrolein in the Troposphere: A Theoretical Investigation of Ground-State Reaction Pathways

The ground-state rearrangement and
decomposition of methyl vinyl
ketone (MVK) and methacrolein (MACR) has been investigated using quantum
chemical calculations and RRKM theory/master equation simulations.
MVK and MACR absorb actinic radiation at around 380–280 nm,
and we have identified a number of isomerization pathways with barriers
that are accessible from the longer wavelength end of this range (visible/near-UV).
Assuming that radiationless transitions dominate, master equation
simulations of the reactions on the vibrationally excited ground-state
potential-energy surface predict that isomerization to 2-hydroxybutadiene
and 1-hydroxymethylallene from MVK, and isomerization to dimethylketene
from MACR, are the major tropospheric reaction channels. Despite these
processes having low quantum yields, they are prevalent because of
the coincidence of high absorption cross sections with significant
solar photon fluxes at around 320–330 nm, where photodissociation
does not occur. This work suggests that photoisomerization may be
an important process in the photolysis of these compounds in the troposphere,
particularly for MVK, which, in comparison with MACR, has both a shorter
lifetime with respect to photolysis and a longer lifetime with respect
to reaction with the •OH radical.