Infrared Spectra of the Methylammonium Halides: Effects of Orientational Disorder about One Axis

Abstract

The spectroscopic effects of orientational disorder of molecules about one axis only have been identified in the spectra of the α‐solid phases of the methylammonium halides published by others. The $C_{\text{3υ}}$ methylammonium ion occupies a crystallographic tetragonal axis, and so must be either disordered or freely rotating. The entropy of transition from the ordered phase is nearly $R$ ln4, and there is no drop of heat capacity that can be associated with the onset of free rotation. It seems likely that the anion is disordered among four equivalent positions. The $A_1$ vibrations of the methylammonium ion are little affected by the disorder, and most of the intensity resides in the zero‐wave vector vibrations The coupling of the $E$ vibrations, on the other hand, is greatly affected by the disorder, and all coupled vibrations are active. The $A_1$ vibrations therefore cause sharp bands whereas the $E$ vibrations cause broad bands. The expected effects occur clearly in the NH₃ and CH₃ bending vibrations.