Infrared spectra of matrix isolated alkali metal perchlorate ion pairs

Abstract

The infrared spectra of the vapors over alkali metal perchlorate salts at temperatures in the 400−450° range have been measured by isolation of molecular beams of the vapor species in various matrices at 12°K. The data indicate that at these temperatures the dominant vapor species are the monomeric ion pairs, M+ClO−4, though the LiClO4 vapor, in particular, may contain a large percentage of dimers. Observation of the monomer ion−pair spectra for argon matrices shows a strong splitting of the ν3(f) and ν4(f) threefold degeneracies with the magnitude of the ν3(f) splittings in the same range (200 cm−1) as previously reported for M+NO−3 monomers and, therefore, suggestive of a strong distortion of the tetrahedral perchlorate structure. The appearance of three distinct widely spaced ν3 components indicates a bidentate or bidentate−like cation−perchlorate pairing and may require an examination of the bases for the conclusion that the pairing is unidentate in M+NO−3 and M+ClO−3 ion pairs. The perchlorate ion distortion is reduced by as much as a factor of six in H2O and NH3 matrices, much as observed for the nitrates, apparently as a result of cancellation of the cation charge via electron transfer from the matrix molecules.