PHOTOINITIATED H-ATOM REACTIONS IN CO2-HBR COMPLEXES

Abstract

Photoinitiated reactions of H atoms with CO2 in CO2-HBr complexes have been examined by using ab initio generalized valence bond-configuration interaction calculations. Recent experiments have shown that CO2-HBr complexes yield OH approximately 40 times more efficiently than CO2-HCl complexes for similar values of h-nu-D0(HX). We find that the calculated equilibrium geometry for the T-shaped CO2-HBr complex is in excellent agreement with the experimental geometry, which is inertially asymmetric with the Br-C line almost perpendicular to the CO2 axis and the H-Br bond nearly parallel to the CO2 molecular axis. We find a linear isomer of CO2-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 CO2 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.