INTERMEDIATE COMPLEXES AND TRANSITION STRUCTURES FOR THE REACTIONS CH3+HX-]CH4+X (X = CL, BR) - APPLICATION OF G1 THEORY

Parts of the potential energy surface for the reactions CH3 + HX --> CH4 + X (X = Cl, Br) were evaluated at the GAUSSIAN-1 (G1) level of theory. Both reactions proceed via a loosely bonded complex (1a, 1b) which is formed without activation energy. For each reaction, the complex is hydrogen bridged, has C3-upsilon symmetry, and is lower in energy at G1 level than reactants by 0.67 and 0.28 kcal/mol for CH3 + HCl and CH3 + HBr systems, respectively. The transition state structures (2a, 2b) for decomposition of the complexes to products were located. The transition state (2a) for the CH3 + HCl reaction has energy 2.53 kcal/mol relative to the reactants, while for the CH3 + HBr system the transition state (2b) is calculated to lie only 0.67 kcal/mol above reactants at G1 level. Heats of reaction for the two reactions compared to experimental values at the same temperature reaffirm error limits for G1 theory, 1.3 kcal/mol (X = Cl) and 2.2 kcal/mol (X = Br).