Activation of C2H6 and C3H8 by Gas-Phase Mo+: Potential Energy Surfaces and Reaction Mechanisms

Abstract

A theoretical investigation of the reaction mechanisms for C−H and C−C bond activation processes in the reaction of Mo+ (6S) with ethane and propane is carried out. Results obtained at the B3LYP/HW/6-311++G(3df,3p) level of theory are compared with guided ion beam mass spectrometry studies provided in the preceding paper. A complete exploration of the potential energy surfaces is conducted for ethane, whereas some limitations are imposed on calculations for the more complicated propane system. In all cases, intermediates and transition states along the reaction paths of interest are characterized. It is found that both the C−H and C−C bond activation processes are limited by the initial activation step, with a transition state having an energy in reasonable agreement with experimental observations. The rate-limiting TS is located on the quartet surface for C−H activation and on the sextet surface for C−C activation. This difference can be traced to the directionality of the sp3-hybridized orbital on methyl compared to the spherical orbital on the H atom, which raises the relative energy on the quartet surface, where the metal ion binds covalently to both fragments, but less so on the sextet surface, where one of the fragments is not covalently bound to Mo+. In the propane system, the calculations show that methane elimination can plausibly occur either by initial C−C or C−H bond activation, although the former pathway seems more likely.