Temperature and solvent effects on the infrared e-type bands of methyl iodide: orientational diffusion and free rotation

Infrared absorption spectra of liquid methyl iodide (CH3I) and its solutions in CCl4, CS2, heptane, benzene, chloroform and deuterated acetone-d6 have been studied. Infrared spectra in the regions 3400-2700, 1700-1100 and 1000-750 cm-1 were fitted by the sum of components, with the form of multiplication of the Lorentzian and Gaussian functions. E-Type bands under investigation (v4 = 3047, v5 = 1428, and v6 = 885 cm-1) were reproduced by the sums of two components: the narrower (n) and the broader (b) ones. A different temperature behaviour of the components has been found: the integrated intensity of the narrower component (In) decreases with the temperature, while the intensity of the broader one (Ib) increases. The narrower components of v5 and v6 were attributed to CH3l molecules moving according to the orientational diffusion mechanism; the broader ones were attributed to molecules, freely rotating about the C3v axis. Some additional mechanism (probably the interactions between CH stretching vibrations with single particle and collective motions of molecular dipoles) was proposed to play a part in forming the v4 bandshape. The enthalpy difference between freely rotating molecules and those moving via an orientational diffusion mechanism (ΔH) have been determined by the slopes of the dependencies of ln(In/Ib) upon T-1: ΔH = 0.8 ± 0.1 kcal mol-1. The temperature behaviour of δn has been studied in the 210-340 K temperature range, and Rakov's approach has been used to determine the activation enthalpy (ΔH*) and entropy (ΔS*) of parallel orientational diffusion in the pure liquid: ΔS* = -4.5 ± 0.2 cal mol-1 K-1, ΔH* = 0.1 ± 0.1 kcal mol-1. The CH3 stretching range was found to be strongly affected by a solvent. Total integral absorption coefficients of v1 and v4 bands increase two-fold when going from CCl4 to acetone-d6 solution, while δn values decrease by 3-9 cm-1. The observed effects were explained in terms of the existence of complexes with weak ICH3 · acetone hydrogen bonding. The strength of the hydrogen bonding was characterized by enthalpies of specific interaction ΔHint.CH3I/S (sp.). These values were estimated by the "intensity rule": ΔHint. CH3I/S (sp.) = 0.13 kcal mol-1 for self-association in pure CH3I and 0.4 kcal mol-1 for solution in acetone-d6. © 1995