Potential Energy Surfaces for LiH2 and Photochemical Reactions Li*+ H2 ↔ LiH + H

Abstract

Potential energy surfaces for the Li*+ H2 → LiH + H and the reverse reactions are calculated using ab initio methods. Extensive configuration interactions have been done for a large number of collinear, C2v, and Cs geometrical forms using a large basis set to describe the 2s−3d atomic states of the lithium atom and the neutral and anionic hydrogen molecule. The Li(3s) channel has a small activation barrier and leads to a stable intermediate, 4 2A‘ (LiH2)*, but a diabatic coupling between the 3 and 4 2A‘ surfaces would preferentially lead to a nonreactive inelastic collision producing Li(2p). The Li(2p) channel leads to one entirely repulsive potential surface (3 2A‘) and two attractive potential surfaces (2 2A‘ and 1 2A‘ ‘) resulting in two stable intermediates. The Li(2p) atoms with enough collision energy to overcome the endothermicity of 1624 cm-1 can lead to the reaction producing LiH. Those three excited intermediate complexes are bent (planar) and not linear. The charge transfer from the metal atom to the hydrogen molecule does not occur in the course of the collision but much later, when one hydrogen atom breaks away, implying that the harpooning model does not apply to this case. The reverse reaction LiH + H → Li + H2 can produce (i) the high-energy Li(2s) directly, (ii) the thermal Li(2p), and (iii) the thermal Li(2s) accompanied with a chemiluminescence. The 2 2A‘ intermediate may play an important role in the reverse reaction, too.