Infrared E-type band profiles of acetonitrile in condensed media: orientational diffusion and free rotation

Infrared spectra of liquid acetonitrile (CH3CN) and its solutions in CCl4, CS2, chloroform, dichloromethane, benzene-d(6), acetone-d(6) and dimethyl s ulfoxide-d(6) have been studied. E-type bands under investigation (v(5) = 3 009, v(6) = 1448 and v(7) = 1041 cm (-1)) were reproduced by the sum of two Cauchy-Gauss components, the narrower (n) and the broader (b) ones. The di fferent temperature behaviour of the components has been found:the integrat ed intensities of the narrower components, I-n, decrease with the temperatu re, while the intensities of the broader ones, I-b, increase. The narrower components of the bands were explained within the framework of the orientat ional diffusion mechanism. The broader components of v(6) and v(7) were att ributed to the unresolved gas-like vibration-rotational absorption of the m olecules. The enthalpy difference between the molecules absorbing via these two different mechanisms was determined from the dependence of In(I-n/I-b) upon T (-1):Delta H-0 = 1.26 +/- 0.15 kcal mol (-1). The broader component of v(5) is assumed to be mainly due to interactions of C-H stretching vibr ations with single particle and collective motions of molecular dipoles. Th e narrower components' widths were used for evaluating the spinning diffusi on constant of CH3CN. The absorption in C-H stretching region was found to be strongly affected by solvent. These effects were explained within the fr amework of hydrogen bond formation between the CH3-group of acetonitrile an d I-I-bond acceptor groups of the solvent molecules. (C) 2000 Elsevier Scie nce B.V. All rights reserved.