Infrared spectra of the ammonium ion in crystals: Part V. Tutton's salts: a correlation of vibrational frequency and hydrogen-bonded N⋯O distance for the ammonium ion

Abstract

The infrared spectra of the isotopically isolated NH 3D + and HDO species have been examined in seven ammonium Tutton salts. The observed spectra are in good agreement with predictions based on the known crystallographic features of these salts. Linear regression of the ND stretching frequencies v 1(NH 3D +) of the isotopically isolated NH 3D + ion on hydrogen-bonded distance d(N ⋯ O) indicated the existence of a correlation ; subsequent fitting of the data to a more plausible empirical function v 1(NH 3D +) = v 1,∞,- k 1 exp(- k 2, d) resulted in a coefficient of determination of 0.94 and a standard deviation of 10 cm −1 for the goodness of fit. The structural differences caused by the distortion of the metal coordination octahedron in the copper(II) Tutton salts are discussed. For this purpose the spectra of isotopically dilute HDO in the salts M 2 i[Cu(H 2O) 6](SO 4) 2 (M i = K, Rb, Cs) have also been measured. No evidence of phase transformations between room and liquid-nitrogen temperatures was detected in the spectra of any of the saltri studied in this work.