Raman spectra of NH4Cl and NH4Br: Dependence of the librational and the internal modes of the NH4+ ion on volume and on nitrogen-halogen distance

Abstract

The wavenumbers for the librational and the high frequency modes in the ``disordered'' NH<sub>4</sub>Cl and NH<sub>4</sub>Br were obtained from polarized Raman spectra with values for unresolved peaks determined by fitting the Raman intensity to damped uncoupled oscillators. The high frequency and the librational modes were studied isothermally (296 K) as a function of volume or of nitrogen‐halogen distance over a decrease in distance of 4.4% in NH<sub>4</sub>Cl and 5.0% in NH<sub>4</sub>Br. The librational frequency increases with decrease in volume in both halides with anharmonicity being given by the Grüneisen constants (γ<sub>6</sub>): 1.30 (``disordered'' NH₄Cl V); 0.29 (ordered NH₄Cl IV); 0.72 (``disordered'' NH₄Br II) and 0.46 (NH₄Br V). The volume dependence of the librational motion does not show the behavior expected for a motion in a simple potential. The internal modes of the ${\mathrm{NH}}_4^{+}$ ion are insensitive to phase transition in both halides but depend to a small degree on internuclear distance; for example, the Grüneisen constants for NH₄Br are: γ₁ =(negative); γ₂=+0.022; γ₃=−0.036; γ₄(TO)=−0.044, and γ₄(LO)=−0.065. These interesting results on the internal modes provide direct evidence that anharmonic effects which can be attributed to strong proton‐halogen coupling or hydrogen bonding effects are present in NH₄Cl and NH₄Br.