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.