Probing mechanistic photochemistry of glyoxal in the gas phase by ab initio calculations of potential-energy surfaces and adiabatic and nonadiabatic rates

In the present work, the wavelength-dependent mechanistic photochemistry of glyoxal in the gas phase has been explored by ab initio calculations of potential-energy surfaces, surface crossing points, and adiabatic and nonadiabatic rates. The CHOCHO molecules in $S1$ by photoexcitation at 393-440 nm mainly decay to the ground state via internal conversion, which is followed by molecular eliminations to form CO, $H2CO,H2$⁠, and HCOH. Upon photodissociation of CHOCHO at 350-390 nm, intersystem crossing to $T1$ followed by the C–C bond cleavage is the dominant process in this wavelength range, which is responsible for the formation of the CHO radicals. The C–C and C–H bond cleavages along the $S1$ pathway are energetically accessible upon photodissociation of CHOCHO at 290-310 nm, which can compete with the $S1→T1$ intersystem crossing process. The present study predicts that the C–H bond cleavage on the $S1$ surface is probably a new photolysis pathway at high excitation energy, which has not been observed experimentally. In addition, the trans-cis isomerization is predicted to occur more easily in the ground state than in the excited states.