Photohrittated H-Atom Reactions in C0,-HBr Complexes

Photoinitiated reactions of H atoms with C 0 2 in C02-HBr complexes have been examined by using ab initio generalized valence bond-configuration interaction calculations. Recent experiments have shown that C02-HBr complexes yield OH -40 times more efficiently than C02-HCI complexes for similar values of hu-&(HX). We find that the calculated equilibrium geometry for the T-shaped C02-HBr complex is in excellent agreement with the experimental geometry, which is inertially asymmetric with the Br< line almost perpendicular to the C 0 2 axis and the H-Br bond nearly parallel to the C02 molecular axis. We find a linear isomer of C02-HBr to also be stable; this species has not been observed experimentally. We find that photoexcitation of the T-shaped HBr moiety leads to two directly dissociative excited states of A’ and A” symmetries. For both states, the HBr bond expands rapidly following photoexcitation with most H atoms leaving unreactively from the complex. However, for those H atoms that approach C 0 2 and form HOCO, the two excited states show dissimilar interactions due to interaction of the Br atom with HOCO. The A’ state has a weak in-plane bonding interaction between the Br p orbital and the C radical orbital of HOCO, whereas the A” state leads to repulsion. These results are consistent with the experimental observations and underscore the greater variety possible in such relations taking place in complexes vs under single-collision, gas-phase conditions.