INTRINSIC ACTIVATION BARRIERS AS A GUIDE TO MECHANISMS OF REACTIONS IN THE GAS-PHASE AND ON SOLID-SURFACES

This paper shows that a simple heuristic, that the intrinsic barriers for C-C and C-O bond scission is 20-40 kcal/mole higher than the intri nsic barriers for C-H bond scission, provides useful insights into rea ctions on surfaces. Quantum mechanical calculations at various levels up to QCISD(T)/6-311G(d,p) are used to show that the gas-phase intrins ic barrier for dehydrogenation reactions of the form . H + CH(3)CH(2)R --> H-2 + . CH(2)CH(2)R are 33 +/- 2 kcal/mole less than the intrinsi c barriers for hydrogenolysis reactions of the form . H + CH(3)CH(2)R --> CH4 + . CH(2)R with R groups comprising a wide range of Taft param eters. We then assume that the difference in intrinsic barriers is the same, in the gas phase and on a surface, and use that assumption to p redict mechanisms of reactions on metal surfaces. We find quite good a greement with the experiment for a wide variety of systems. In particu lar we explain why most hydrocarbon decomposition processes go via seq uential decomposition mechanisms. We also explain why hydrogenolysis r eactions have much higher activation barriers than most hydrogenation/ dehydrogenation reactions on transition metal surfaces. These results imply that an examination of intrinsic barriers may provide a wide fra mework to characterize many types of surface reactions. (C) 1997 Acade mic Press, Inc.