The reaction of lithium with hydrogen. a model ab initio study

Abstract

The overall reaction of H 2 with Li 2 to give (LiH) 2 dimer (D 2h) is exothermic by 18.5 kcal mol (MP4SDTQ/6-311G ★★ + ZPE) but requires an activation energy of 30.6 kcal mol . The transition structure (MP2/6-31G ★★) has a nearly triangular geometry with two short (≈ 1.68 A) and one long (1.98 A) LiH distances and considerable H⋯H separation (1.36 A). The symmetry-forbidden character usually associated with 2σ + 2σ cycloadditions is precluded by the highly ionic character of this transition structure, which can be described as a triangular (Li 2) +(H 2) − singlet complex in which half of the eventual electron transfer from the lithiums to the hydrogens has already taken place.