The effect of pressure on the infrared spectrum of ammonium formate: HCO₂NH₄, DCO₂NH₄, HCO₂ND₄, DCO₂ND₄. An unusual intermolecular coupling of non-identical vibrations in phase II

Abstract

The infrared spectra of the title compounds have been measured over the pressure range 0-42 kbar at 25°C. The phase change at c. 12 kbar is accompanied by an increase in the NH stretching frequencies and other well-defined spectral changes. Vibrational coupling in phase II of HCO₂NH₄ between CH in-plane bending in HCO₂^- and a bending motion in NH₄⁺, and its counterpart for DCO₂ND₄, account for some unusual frequency changes arising from isotopic substitution in the counter ion. The spectral changes suggest that during the phase change I → II the NH...O hydrogen bonds are weakened and the cations move to positions in close proximity beside the CH bonds.