Method of Diatomics in Molecules. V. Theoretical Prediction of Stable Li2H+ and Almost Stable LiH2+

Abstract

Diatomics‐in‐molecules theory is applied to the molecule—ions LiH₂⁺ and Li₂H⁺ using as a basis three canonical valence‐bond wavefunctions in each case. The input for this theory consists of the valence‐bond wavefunctions and potential‐energy curves for the ground electronic states of H₂, Li₂, and LiH and for the ground and first excited states of H₂⁺, Li₂⁺, and LiH⁺. Experimental diatomic potential‐energy curves are used if available; otherwise, best possible theoretical or semiempirical curves are used. Diatomics‐in‐molecules theory in this approximation leads to predictions that ground‐state LiH₂⁺ may be stable by about 2.5 kcal with respect to Li⁺+H₂ and that ground‐state Li₂H⁺ should be stable by about 10 kcal with respect to Li⁺+LiH. The equilibrium shape for LiH₂⁺ turns out to be a symmetrical isosceles triangle with R_LiH=5.0 bohrs and R_HH=1.4 bohrs; Li₂H⁺ is symmetrical and linear with R_LiH=3.4 bohrs. A stable linear excited state (¹Σ_u⁺) is predicted at about 35 kcal above the ground state (¹Σ_g⁺) of Li₂H⁺.