Abstract
The Zn + CH4 reaction has been studied through self-consistent field followed by extensive variational and perturbational second-order Möller-Plesset multi-reference configuration interaction (CIPSI) calculations using extended Gaussian basis sets. Results indicate that for the formation of the HZnCH3 intermediate a previous excitation of Zn(1S) to its (1P) excited state is necessary. The following mechanism is proposed: the Zn(1P) + CH4 reactants follow the 2A′ attractive surface until they reach the originally repulsive 1A′ surface originating from the Zn(1S) + CH4 reactants. An avoided crossing is generated implying a non-adiabatic transition. When the 1A′ surface reaches the avoided crossing it becomes attractive leading to the C3v HZnCH3 intermediate which is stable and needs 63·9 kcal mol-1 to yield, without an activation barrier, the HZn + CH3 products or 69·3 kcal mol-1 to yield the H + ZnCH3 products. The Zn(3P) + CH4 reaction pathway presents a transition state 18 kcal mol-1 above the initial reactants and the formation of a stable 3A complex is achieved at a H-Zn-C angle of 68°. However, reactivity on this excited surface is not observed because it is not reachable from the ground state reactants by a photoexcitation process.