THEORETICAL-STUDY OF THE ACTIVATION OF METHANE BY PHOTOEXCITED ZINC ATOMS

The Zn+ CH4 reaction has been studied through self-consistent field fo llowed by extensive variational and perturbational second-order Moller Plesset multi-reference configuration interaction (CIPSI) calculation s using extended Gaussian basis sets. Results indicate that for the fo rmation of the HZnCH3 intermediate a previous excitation of Zn(1S) to its (1P) excited state is necessary. The following mechanism is propos ed: 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 n on-adiabatic transition. When the 1A' surface reaches the avoided cros sing it becomes attractive leading to the C3v HZnCH3 intermediate whic h 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 stabl e 3A complex is achieved at a H-Zn-C angle of 68-degrees. However, rea ctivity on this excited surface is not observed because it is not reac hable from the ground state reactants by a photoexcitation process.