Infrared Spectra of the Lithium Halide Monomers and Dimers in Inert Matrices at Low Temperature

The infrared absorption spectra of lithium fluoride, chloride, and bromide vaporization products have been investigated between 2–37 μ in solid argon, krypton, xenon, and nitrogen matrices at liquid-hydrogen and liquid-helium temperatures. Both the natural lithium salts containing 93% 7Li and 6Li salts as well as equimolar isotopic mixtures were studied. The solvent shifts of the monomers were found to confirm the isotopic invariance of the ratio Δω/ω as predicted by Buckingham. Serious deviations from the Kirkwood—Bauer—Magat relation were, however, observed. These shifts have also been discussed in terms of specific solute—solvent interactions. Two dimer vibrational frequencies were definitely characterized by means of isotopic substitutions and these frequencies were assigned. They represent in-plane stretching modes of the planar rhombus, the higher energy vibration being of symmetry b3u (X—X axis polarized) and the lower energy vibration b2u, (Li–Li axis polarized).