Mechanistic photodissociation of glycolaldehyde: insights from ab initio and RRKM calculations

Herein we report a theoretical study on mechanistic photodissociation of glycolaldehyde, HOCH(2) CHO. Equilibrium structures, transition states, and intersection structures for the α-C-C and -C-H bond fissions and the β-C-O bond fission in the excited states are determined by the complete active space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations are refined by performing single-point calculations using the multi-state multi-reference CASSCF second order perturbation (MS-MR-CASPT2) method. With a low excitation energy of 280-340 nm, the T(1) α-C-C and β-C-O bond fissions following intersystem crossing from the S(1) state are the predominant and comparable channels, whereas the α-C-H bond fissions both in the S(1) and in the T(1) states are nearly prohibited due to the relevant high barriers. The rate constants for the T(1) α-C-C and β-C-O bond fissions are also calculated by RRKM theory. Furthermore, the S(0) reactions can occur as a consequence of intersystem crossing via T(1)/S(0) intersection points resulting from the T(1) C-C and C-O bond cleavages. This photodissociation mechanism is consistent with recent experimental studies.