Photoinduced Double Proton Transfer in the Glyoxal–Methanol Complex Revisited: The Role of the Excited States

Under irradiation in the visible
range, the glyoxal–methanol
complex in a cryogenic argon matrix undergoes a double proton transfer
(DPT) reaction through which the glyoxal molecule isomerizes into
hydroxyketene. In this work, we employ electronic structure simulations
in order to shed more light on the underlying mechanism. Rewardingly,
we find that the lowest singlet excited state (S1) of the
complex acts as a gateway to two previously unknown isomerization
pathways, of which one takes place entirely in the singlet manifold
and the other also involves the lowest triplet state (T1). Both of these pathways are fully compatible with the available
experimental data, implying that either or both are operative under
experimental conditions. In either pathway, the methanol molecule
acts as a proton shuttle between the proton-donating and proton-accepting
sites of glyoxal, resulting in a dramatic lowering of the potential
energy barrier to isomerization with respect to the case of isolated
glyoxal. The occurrence of DPT in the singlet manifold is demonstrated
directly with the use of nonadiabatic molecular dynamics simulations
at the spin-flip time-dependent density functional theory level.