Infrared spectra of the M+HBr2− and the M+HClBr− ion pairs and their deuterium analogs isolated in argon matrices at 15 K

Abstract

The matrix reactions of alkali bromide salts and HBr and DBr have been investigated in argon matrices. In each case, an intense band was observed from the reaction product, which was assigned to the antisymmetric stretch, ν₃, of the HBr₂⁻ and DBr₂⁻ anions in the M⁺HBr₂⁻ and M⁺DBr₂⁻ ion pairs. Comparisons to literature spectra, in conjunction with quartic deuterium shifts, indicate that the anion has a linear, centrosymmetric geometry in the isolated ion pair. A less intense band was also observed which has been assigned to the combination band ν₁+ν₃ for the HBr₂⁻ anion. Reaction of alkali bromide salts with HCl and alkali chloride salts with HBr have been shown to yield identical absorptions for the M⁺HClBr⁻ ion pairs. The spectra suggest that both type I and type II HClBr⁻ anions exist, and the resulting form may be determined by cation position.