Pressure-tuned vibrational resonance coupling of intramolecular fundamentals in ammonium azide (NH4N3)

The peculiar behavior of the symmetric stretch of the N3-anion and the bending mode of the NH4-cation in the high-pressure phase II (above 3 GPa) of the molecular ionic crystal NH4N3 has been studied by pressure-dependent Raman spectroscopy up to a pressure of 20 GPa. The spectra in the pressure range above 10 GPa reveal a remarkable intensity transfer between bands and a level repulsion (characteristic of vibrational resonance coupling) that is mediated by hydrogen bonding between the N3- and NH4-ions. These observations may be useful for theoretical studies of the potential energy surface and for studying the nature of hydrogen bonding in ammonium salts. With respect to the biological activity of the ammonium ion, the observed resonance coupling may be relevant to vibrational energy transfer in organic systems.