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

Paul Baybutt, Frank W. Bobrowicz, Luis R. Kahn, Donald G. Truhlar

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23 Scopus citations


Ab 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.

Original languageEnglish (US)
Pages (from-to)4809-4822
Number of pages14
JournalThe Journal of chemical physics
Issue number11
StatePublished - 1978


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