Unified statistical model for ’’complex’’ and ’’direct’’ reaction mechanisms: A test on the collinear H+H2 exchange reaction

Abstract

Miller’s unified statistical theory for bimolecular chemical reactions is tested on the collinear H+H2 exchange reaction, treated classically. The reaction probability calculated from unified statistical theory is more accurate than that calculated from ordinary transition state theory or from variational transition state theory; in particular, unified statistical theory predicts the highenergy falloff of the reaction probability, which transition state theory does not. A derivation of unified statistical theory is presented that emphasizes the dynamical and statistical assumptions that are the foundation of the theory. We show how these assumptions unambiguously define the ’’collision complex’’ in unified statistical theory, and we test these assumptions in detail on the H+H2 reaction. Finally, a lower bound on the reaction probability is derived; this bound complements the upper bound provided by transition state theory and is significantly more accurate, for the H+H2 reaction, than either transition state theory or unified statistical theory.