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 (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 1B2 and 1A1 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.
|Number of pages||10|
|Journal||Journal of Chemical Physics|
|State||Published - Oct 15 2000|