Infrared Absorption of the v3 Fundamental of Liquid and Solid CH4 and CD4

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Abstract
The v3 fundamental of CH4, CD4, and solutions of CD4 in CH4 has been studied in the liquid and solid state from 105° to 28°K. The broad liquid‐state absorption approximates the gas‐phase vibration—rotation envelope in the wings, but has significantly more absorption at the band center. This deviation is consistent with the assumption that molecules in the liquid can undergo hindered rotation. One consequence of this model is that rotatory motion of molecules is in part librational in character and selection rules predict an intense Q branch and weaker P, R, S, and O branches. Another consequence is that the liquid environment may induce molecular dipoles causing Raman selection rules to be operative which contribute further to an intensification of the Q branch. The bandwidth decreases gradually with temperature but undergoes no discernable discontinuous change on passing through the freezing point, which suggests that rotatory motion persists in this solid phase. The bandwidth of the fundamental of liquid and solid CD4 is narrower than CH4 but behaves in a similar fashion with temperature. The band shape and position of v3 of CD4 in solution with CH4 is indistinguishable from that of pure liquid CD4, and suggests that resonance broadening or coupling of rotatory motion is not important for this phase. In the solid state, however, the v3 bandwidth of CD4 in the CD4—CH4 solution is slightly larger than the v3 bandwidth of pure CD4.