Absorption Band Shapes and Molecular Reorientation of Liquid Methyl Cyanide

The shapes and intensities of the fundamental bands of liquid methyl cyanide were observed by conventional transmittance method. Using the refractive indices observed by the ATR method, the observed absorption indices were corrected for the dielectric-field effect. The band shapes were analyzed from the point of view of moment analysis or correlation-function analysis. The results indicate that the shapes of a1-type vibrations are good Lorentzian shapes, while those of the e-type can be explained as superpositions of Lorentzian and Gaussian functions. It is also found that the band-widths of the e-type vibrations were quite different from each other. Taking into account the effect of vibration-rotation interaction on the band-widths, the disagreement could be explained well. From the analyses of the band shapes of ν7 and nu8, the life-time function, ψ(t), was obtained; it represents the decay of the fraction of freely-rotating molecules.