Rotational diffusion in methyl iodide: Bandshape analysis of degenerate vibrational modes

The Raman spectral bandshapes of the ν 1 ( A 1 ), ν 3 ( A 1 ) and ν 4 ( E ) vibrations of CH 3 I were studied as a function of temperature in the liquid phase. Perpendicular diffusion coefficients calculated from ν 3 agreed well with earlier reported results. The degenerate CH 3 stretching vibration, ν 4 , was analyzed via fitting by a model incorporating two Lorentzian lineshapes. Values of the parallel diffusion constants, D ∥ , determined using only the narrower component were in excellent semi‐quantitative agreement with those predicted by the free rotor model of molecular reorientation. Analysis of the room‐temperatur Raman spectrum of ν 5 ( E ) yielded the same value of D ∥ as that calculated from ν 4 . The room‐temperature infrared spectral intensities of ν 5 were fitted by a model with a narrow Lorentzian containing contributions from rotational and vibrational relaxation superposed on a second component introduced to account for the broad background absorption. D ∥ determined from the IR bandwidth was in good agreement with values obtained from the Raman spectra of ν 4 and ν 5 . These results demonstrate the utility of analyzing degenerate vibrational modes to determine the parallel diffusion coefficients in C 3v molecules.