Infrared E-type band shapes of the liquid CD3I and CD3CN: Orientational diffusion and free rotation

Abstract

Infrared absorption spectra of liquid methyl iodide-d3 (CD3I) and acetonitrile-d3 (CD3CN) have been studied in wide temperature ranges (212-317 K and 234-346 K, respectively). IR spectra in the regions of degenerate (E-type) bands belonging to CD3-stretching and deformational vibrations (v4, v5, v6 of CD3I and v6, v7 of CD3CN) were 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 broader (b) ones. The different temperature behaviour of the components has been found: the integrated intensities of the narrower components (In) decrease with the temperature, while the intensities of the broader ones (Ib) 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 these bands were attributed to the unresolved gas-like vibration-rotational absorption of the molecules. The enthalpy differences between the molecules absorbing via two different mechanisms (ΔH) were determined from the dependencies of ln(In/Ib) upon T-1: 0.59 ± 0.15 (CD3I) and 1.10 ± 0.20 koal mol-1 (CD3CN). These values are close to those determined previously for nondeuterated methyl iodide and acetonitrile, respectively. The shape of the CD3-stretching E-type band of CD3I is assumed to be mainly due to interactions of the C-D stretching vibrations with single particle and collective motions of molecular dipoles. An attempt is made to separate the widths of the narrower components into the contributions of orientational and vibrational relaxation. Various experimental and theoretical approaches to molecular relaxation are considered in view of the obtained data. © 1998 Elsevier Science B.V. All rights reserved.