## 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^{-}/(H_{2})_{n},=1-3, hypersurfaces. The electrostatic complexes, Li^{-}(H_{2})_{n}, are very weakly bound (D_{e}<0.25 kcal/mol with respect to H_{2} loss) and H_{2}/H_{2} 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, LiH_{2}^{-}, is found to have a linear centrosymmetric structure and to be bound with respect to Li^{-}+H_{2} in agreement with previous calculations. The interaction of LiH_{2}^{-} with additional H_{2} is purely electrostatic but with a De larger than those of the Li^{-}(H_{2})_{n} complexes. LiH_{2}^{-}(H_{2}) is found to have a linear equilibrium structure and LiH_{2}^{-}(H_{2})_{2} is found to have two almost isoenergetic structures: linear with an H_{2} on either end of the LiH_{2}^{-}, and C_{2v} with both H_{2} on the same end of the LiH_{2}^{-}. Of particular interest is the dramatic change in the nature of the transition state for LiH_{2}^{-} production depending on the number of H_{2} molecules present. For N=1, the reaction proceeds through a conical intersection between the lowest energy ^{1}B_{2} and ^{1}A_{1} 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 H_{2} bonds are broken while two LiH bonds and one new H_{2} bond are formed. For n=3, the reaction proceeds by direct insertion of Li^{-} into one of the H_{2} molecules with the two additional H_{2} 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^{+}/(H_{2})_{n} systems where significant sigma bond activation through a cooperative interaction mechanism has been identified recently.

Original language | English |
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Pages (from-to) | 6122-6131 |

Number of pages | 10 |

Journal | Journal of Chemical Physics |

Volume | 113 |

Issue number | 15 |

State | Published - Oct 15 2000 |