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

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.