Generalized valence-bond investigation of the reaction H+Br2 →HBr+Br

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A b initio quantum mechanical calculations have been carried out to predict the H+Br2→HBr+Br potential energy surface. We used ab initio effective core potentials and an extended valence basis set including polarization functions on each center and carried out open-shell self-consistent-field calculations in the generalized valence bond–perfect-pairing (GVB–PP) approximation. The orbitals of this calculation were used as a starting point for eight-configuration–configuration-interaction (GVB–CI) calculations. The CI calculations not only bring in electron correlation effects but also make up to a large extent the inability of the GVB–PP calculation to adequately treat the recoupling of orbitals which occurs near the transition state. The classical barrier height is predicted to be about 12 kcal mole−1 by the GVB–PP calculations and about 3.0 kcal mole−1 by the GVB–CI calculations. The latter value is in reasonable agreement with the experimental Arrhenius activation energy. The saddle point is predicted from the GVB–CI calculations to occur for a linear geometry with an H–Br separation 46% greater than in HBr but a Br–Br separation only 6% greater than in Br2. The CI calculations lead to only 30% attractive energy release along a rectilinear path and predict that the energy increases only about 2.4 kcal mole−1 for a 25° bend near the saddle point. We present results for a wide range of geometries which illustrate the phenomenon of dual surfaces for a GVB–PP self-consistent-field calculation on a reactive system.