Infrared spectra are presented for three crystalline phases of methyl ammonium chloride which occur at different temperatures. The spectrum of the high temperature structure reveals apparently degenerate vibrational frequencies for the perpendicular bands indicating little or no distortion from a threefold symmetry axis for the methyl ammonium ion. The intermediate and low temperature structures show increasing distortion of the cation as evidenced by increasing splitting of the degenerate absorption bands amounting to about 10ndash;40 cmminus;1in the low temperature form, where the torsional vibration about the Csngbnd;N axis becomes infrared active.An assignment of frequencies is made in satisfactory agreement with a valencehyphen;force normal coordinate treatment and the isotope product rule. A simplified calculation of the height of the potential barrier to internal rotation was made yielding the approximate value,V0=1670 cmminus;1.
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