Theoretical studies of the energetics and mechanisms of chemical reactions: abstraction reactions

Abstract

First methods developed to locate and characterize critical points (minimia and saddle points) on molecular potential energy surfaces are reviewed. Then briefly the computational advances which are currently driving the development of algorithms for calculating potential energy surfaces are discussed. Finally, a sampling of the studies of simple abstraction reactions carried out in the Theoretical Chemistry Group at ANL over the past five years is presented. In this latter section focus is on a series of prototypical chemical reactions (O + H/sub 2/, H + HX, CH + H/sub 2/, H + HCO, OH + H/sub 2/, H + HCN, and H/sub 2/ + C/sub 2/H). Most of the above reactions involve direct processes; however, for the CH + H/sub 2/ and H + HCO reactions indirect, addition-elimination pathways also play a role. For the homologous series of abstractions, H + HX (X = F-I), simple exponential relationships are found both between the barrier heights and the reaction exoergicities and between the two bond extensions at the saddle point. Finally, the last two reactions, H/sub 2/ + CN and H/sub 2/ + C/sub 2/H, are both highly exoergic and are predicted to have very early transition states. 94 references.