Infrared spectra of the Li +_(H 2)n(n=1-3) cation complexes

Authors

C Emmeluth, University Of Melbourne
B L. J Poad, University of WollongongFollow
C D. Thompson, University Of Melbourne
G H. Weddle, University Of Melbourne
E J. Bieske, University Of Melbourne

RIS ID

72193

Publication Details

Emmeluth, C., Poad, B. L. J., Thompson, C. D., Weddle, G. H. & Bieske, E. J. (2007). Infrared spectra of the Li +_(H 2)n(n=1-3) cation complexes. Journal of Chemical Physics, 126 (20), 204309-1-204309-9.

Abstract

The Li⁺–(H₂)_n n = 1–3 complexes are investigated through infrared spectra recorded in the H–H stretch region (3980–4120 cm⁻¹) and through ab initio calculations at the MP2∕aug-cc-pVQZ level. The rotationally resolved H–H stretch band of Li⁺–H₂ is centered at 4053.4 cm⁻¹ [a −108 cm⁻¹ shift from the Q₁(0) transition of H₂]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li⁺ ion attached to a slightly perturbed H₂ molecule. Around 100 rovibrational transitions belonging to parallel K_a = 0‐0, 1-1, 2-2, and 3-3 subbands are observed. The K_a = 0‐0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 Å increasing by 0.004 Å when the H₂ subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li⁺–H₂ potential energy surfaces [ Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Špirko, Chem. Phys. 330, 190 (2006) ]. The H–H stretch band of Li⁺–(H₂)₂, which is centered at 4055.5 cm⁻¹ also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H₂–Li⁺–H₂ geometry, in which the two H₂ molecules are disposed on opposite sides of the central Li⁺ ion. The two equivalent Li⁺⋯H₂ bonds have approximately the same length as the intermolecular bond in Li⁺–H₂. The Li⁺–(H₂)₃ cluster is predicted to possess a trigonal structure in which a central Li⁺ ion is surrounded by three equivalent H₂ molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm⁻¹.