Infra-red spectra of the hydrates of hydrogen chloride and hydrogen bromide. Absorption bands of the H5O +2 species

Abstract

Infra-red spectra have been obtained from a number of crystalline phases formed by reactions between hydrogen halides (HCl and HBr) and water at low temperatures. Evidence is presented for the assignment of these spectra to the hydrates HX·xH₂O (X = Cl, Br; x= 1–4), several of which have been examined previously by other workers using X-ray diffraction techniques. Comparison of spectra from samples of the chloride dihydrate containing various amounts of deuterium show the presence of a very strong central hydrogen bond in H₅O⁺₂, and other spectra indicate the presence of H₂O, H₃O⁺ or H₅O⁺₂ units consistent with the known crystal structures. The spectra of the H₅O⁺₂ ion in these compounds is discussed in relation to other very short OHO bonds. This work appears to provide the first report of a crystalline phase corresponding in composition to the tetrahydrate of hydrogen chloride.