THEORETICAL-STUDIES OF THE REACTIONS H+CH-]C+H2 AND C+H2-]CH2 USING AN ABINITIO GLOBAL GROUND-STATE POTENTIAL SURFACE FOR CH2

Ab initio, multireference, configuration interaction (CI) calculations have been used to characterize the groundstate potential surface of methylene. The calculations employ a full-valence, complete-active-space reference wave function and a (4s,3p,2d,1f/3s,2p,1d) basis set. The calculations were carried out at approximately 6000 points, and the resulting energies were fit to a many-body expansion including conical intersections between the B-3(1) and 3A2 states for C2v geometries and between the 3PI and 3SIGMA- states for linear geometries. The calculations show that the C2v conical intersection provides a zero barrier route for the C(3P) + H-2 insertion reaction. Quasiclassical trajectory studies of the title reactions using the global surface indicate that both reactions proceed without activation energy and that the thermal rate constants are nearly independent of temperature. The rate constants for H + CH agree with high-temperature (2000 K) experimental studies but are substantially higher (factor of 10) than a measurement at 297 K. For C + H-2, the CH2 lifetimes at thermal energies are approximately 20 ps which is in agreement with an experimentally derived estimate.