INFRARED E-TYPE BAND SHAPES OF THE LIQUID CD3I AND CD3CN - ORIENTATIONAL DIFFUSION AND FREE ROTATION

Infrared absorption spectra of liquid methyl iodide-d(3) (CD3I) and ac etonitrile-d(3) (CD3CN) have been studied in wide temperature ranges ( 212-317 K and 234-346 K, respectively). IR spectra in the regions of d egenerate (E-type) bands belonging to CD3-stretching and deformational vibrations (nu(4), nu(5), nu(6) of CD3I and nu(6), nu(7) of CD3CN) we re fitted by the sum of Cauchy-Gauss components. Each E-type band was reproduced by the sum of two components: the narrower (n) and the broa der (b) ones. The different temperature behaviour of the components ha s been found: the integrated intensities of the narrower components (I -n) decrease with the temperature, while the intensities of the broade r ones (I-b) increase. The narrower components of the bands belonging to deformational CD3-vibrations were interpreted within the framework of the orientational diffusion mechanism. The broader components of th ese bands were attributed to the unresolved gas-like vibration-rotatio nal absorption of the molecules. The enthalpy differences between the molecules absorbing via two different mechanisms (Delta H) were determ ined from the dependencies of In(I-n/I-b) upon T-1: 0.59 +/- 0.15 (CD3 I) and 1.10 +/- 0.20 koal mol(-1) (CD3CN). These values are close to t hose determined previously for nondeuterated methyl iodide and acetoni trile, respectively. The shape of the CD3-stretching E-type band of CD 3I is assumed to be mainly due to interactions of the C-D stretching v ibrations with single particle and collective motions of molecular dip oles. An attempt is made to separate the widths of the narrower compon ents into the contributions of orientational and vibrational relaxatio n. Various experimental and theoretical approaches to molecular relaxa tion an considered in view of the obtained data. (C) 1998 Elsevier Sci ence B.V. All rights reserved.