Ab initio investigations of Li^-+nH₂→LiH₂^-(H ₂)_n-1,n=1-3

Ab initio investigations at the coupled-cluster single double (triple) [CCSD(T)] and MRCISD level with augmented triple and quadruple zeta basis sets have identified various stationary points on the Li^-/(H₂)_n,=1-3, hypersurfaces. The electrostatic complexes, Li^-(H₂)_n, are very weakly bound (D_e<0.25 kcal/mol with respect to H₂ loss) and H₂/H₂ interactions play a contributing role in determining the equilibrium structures within the electrostatic constraint of a linear or near-linear Li^--H-H orientation. The covalent molecular ion, LiH₂^-, is found to have a linear centrosymmetric structure and to be bound with respect to Li^-+H₂ in agreement with previous calculations. The interaction of LiH₂^- with additional H₂ is purely electrostatic but with a De larger than those of the Li^-(H₂)_n complexes. LiH₂^-(H₂) is found to have a linear equilibrium structure and LiH₂^-(H₂)₂ is found to have two almost isoenergetic structures: linear with an H₂ on either end of the LiH₂^-, and C_2v with both H₂ on the same end of the LiH₂^-. Of particular interest is the dramatic change in the nature of the transition state for LiH₂^- production depending on the number of H₂ molecules present. For N=1, the reaction proceeds through a conical intersection between the lowest energy ¹B₂ and ¹A₁ electronic surfaces in C_2v symmetry. For n=2, the reaction occurs on a single surface in a pericyclic mechanism through a transition state consisting of a planar five-member ring where simultaneously two H₂ bonds are broken while two LiH bonds and one new H₂ bond are formed. For n=3, the reaction proceeds by direct insertion of Li^- into one of the H₂ molecules with the two additional H₂ molecules providing substantial stabilization of the transition state by taking on part of the negative charge in a weakly covalent interaction. The results are discussed in comparison to the isoelectronic B⁺/(H₂)_n systems where significant sigma bond activation through a cooperative interaction mechanism has been identified recently.