Theoretical study of the activation of methane by photoexcited zinc atoms

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 C_3v 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.