The least-motion insertion reaction methylene(1A1) + molecular hydrogen .fwdarw. methane. Theoretical study of a process forbidden by orbital symmetry

Abstract

Ab initio electronic structure theory has been applied to the insertion reaction of singlet methylene with molecular hydrogen. Since the molecular orbital descriptions of CH/sub 2/(/sup 1/A/sub 1/) + H/sub 2/ and CH/sub 4/ differ by two electrons, the least-motion approach considered here is forbidden in the sense of Woodward and Hoffmann. Electron correlation was explicitly taken into account via configuration interaction (Cl). The Cl included all singly and doubly excited configureations (a total of 1192) with respect to three reference configurations. A primary goal was the location of the saddle point or transition state (within the constraints of the least motion approach adopted) geometry with R = 2.20 A, r = 0.76 A, and theta = 172/sup 0/. This stationary point on the potential energy surface lies 26.7 kcal/mol above separated CH/sub 2/(/sup 1/A/sub 1/) + H/sub 2/. The portion of the minimum energy path near the saddle point has been obtained by following the gradient of the potential energy in the direction of most negative curvature. The electronic structure at the transition state is compared with that of the reactants and product in terms of the natural orbitals resulting from the wave functions.