Ab initio investigations of Li−+nH2→LiH2−(H2)n−1,n=1–3

Abstract

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,n=1–3, hypersurfaces. The electrostatic complexes, Li−(H2)n, are very weakly bound (De<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 C2v 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 B21 and A11 electronic surfaces in C2v 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.