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.