THEORETICAL-STUDY OF THE QUENCHING OF THE LOW-LYING EXCITED-STATES OF LI AND MG BY CH4

The reactivity of the ground state (GS) and the low excited states of Li and Mg atoms with CH4 has been studied on the basis of SCF-CI ab initio calculations. No attractive surface is found starting from 2S (GS) and 2P states of Li and 1S, 3P, and 1P states of Mg. No energy barrier above endothermicity is found for the CH4 + M → CH3 + MH reactions in the case of Li, for both insertion and abstraction model mechanisms. In the case of Mg, the relative height of energy barriers indicates that the insertion mechanism is preferred when starting from the 1P state. The existence of a CH3·MgH bent exciplex with a small singlet-triplet gap accounts for a possible intersystem crossing. Some consequences and a comparison with H2 are discussed concerning the physical and chemical quenching of the excited metal atoms. © 1990 American Chemical Society.