Infrared Spectra and Theoretical Calculations of Lithium Hydride Clusters in Solid Hydrogen, Neon, and Argon

2007-07-12T00:00:00Z (GMT) by Xuefeng Wang Lester Andrews
A matrix isolation IR study of laser-ablated lithium atom reactions with H<sub>2</sub> has been performed in solid <i>para</i>-hydrogen, normal hydrogen, neon, and argon. The LiH molecule and (LiH)<sub>2,3,4</sub> clusters were identified by IR spectra with isotopic substitution (HD, D<sub>2</sub>, and H<sub>2</sub> + D<sub>2</sub>) and comparison to frequencies calculated by density functional theory and the MP2 method. The LiH diatomic molecule is highly polarized and associates additional H<sub>2</sub> to form primary (H<sub>2</sub>)<sub>2</sub>LiH chemical complexes surrounded by a physical cage of solid hydrogen where the ortho and para spin states form three different primary complexes and play a role in the identification of the bis-dihydrogen complex and in characterization of the matrix cage. The highly ionic rhombic (LiH)<sub>2</sub> dimer, which is trapped in solid matrices, is calculated to be 22 kcal/mol more stable than the inverse hydrogen bonded linear LiH−LiH dimer, which is not observed here. The cyclic lithium hydride trimer and tetramer clusters were also observed. Although the spontaneous reaction of 2 Li and H<sub>2</sub> to form (LiH)<sub>2</sub> occurs on annealing in solid H<sub>2</sub>, the formation of higher clusters requires visible irradiation. We observed the simplest possible chemical reduction of dihydrogen using two lithium valence electrons to form the rhombic (LiH)<sub>2</sub> dimer.