THEORETICAL-STUDY OF THE LOWEST POTENTIAL-ENERGY SURFACES FOR THE REACTION O(P-3)+HBR(X(1)SIGMA(+))-]OH(X(2)PI)+BR(P-2)

Potential energy surfaces for the reaction of O(P-3) with HBr(X (1) Sigma(+)) to OH(X (2) Pi)+Br(P-2) have been calculated by means of non-relativistic quantum chemical ab initio methods. Dynamic correlation effects have a rather large influence on the height and the position of the barrier as well as on the exoergicity of the reaction, therefore all calculations have been performed at the SCF or CASSCF levels and with inclusion of electronic correlation using the single-reference or multi-reference CEPA method. For collinear geometries, the (spatially) triply degenerate P-3 ground state of the O atom is split into a (3) Pi and a (3) Sigma(-) component. The reaction to OH+Br is possible on both potential surfaces; the one of the (3) Pi state possesses a slightly lower barrier (0.42 eV at R(OH) = 2.45a(0) and R(HBr)=2.90a(0)) than that of the 3 Sigma(-) state (0.58 eV at R(OH)=2.50a(0) and R(HBr)= 3.05a(0)). Upon bending, the (3) Sigma(-) state ((3)A'') is lowered below the two components of (3) Pi((3)A', (3)A''); this gives rise to a true and an avoided crossing. The overall barrier is shifted to a bent conformation with an OHBr angle of similar to 150 degrees on the 1 (3)A'' surface, but the height of the barrier is lowered only by 0.05 eV.