The influence of pressure on the infrared spectra of hydrogen-bonded solids. VI. Ammonium salts

Abstract

The infrared spectra of 33 polycrystalline ammonium salts have been measured at 25°C, at pressures up to 45 kbar. The N-H stretching and bending bands of the hydrogen-bonded NH₄⁺ ions of most of the salts shift anomalously to higher and to lower frequencies, respectively, as the pressure is raised. In this sense, the salts behave as if they had very strong hydrogen bonds, instead of quite weak ones.    

A fairly good correlation exists between the N-H stretching frequencies of salts with N⁺_-H...O bonds and their hydrogen bond strengths as measured by the minimum N⁺...O distances in their crystals.