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 sol utions in CCl4, CS2, heptane, benzene, chloroform and deuterated aceto ne-d(6) have been studied. Infrared spectra in the regions 3400-2700, 1700-1100 and 1000-750 cm(-1) were fitted by the sum of components, wi th the form of multiplication of the Lorentzian and Gaussian functions . E-Type bands under investigation (nu(4) = 3047, nu(5) = 1428, and nu (6) = 885 cm(-1)) were reproduced by the sums of two components: the n arrower (n) and the broader (b) ones. A different temperature behaviou r of the components has been found: the integrated intensity of the na rrower component (I-n) decreases with the temperature, while the inten sity of the broader one (I-b) increases. The narrower components of nu (5) and nu(6) were attributed to CH3I molecules moving according to th e orientational diffusion mechanism; the broader ones were attributed to molecules, freely rotating about the C-3 nu axis. Some additional m echanism (probably the interactions between CH stretching vibrations w ith single particle and collective motions of molecular dipoles) was p roposed to play a part in forming the nu(4) bandshape. The enthalpy di fference between freely rotating molecules and those moving via an ori entational diffusion mechanism (Delta H) have been determined by the s lopes of. the dependencies of In(l(n)/l(b)) upon T-1: Delta H = 0.8+/- 0.1 kcal mol(-1). The temperature behaviour of delta(n) has been studi ed in the 210-340 K temperature range, and Rakov's approach has been u sed to determine the activation enthalpy (Delta H) and entropy (Delta S) of parallel orientational diffusion in the pure liquid: Delta S* = -4.5+/-0.2 cal mol(-1) K-1, Delta H = 0.1+/-0.1 kcal mol(-1). The C H3 stretching range was found to be strongly affected by a solvent. To tal integral absorption coefficients of nu(1) and nu(4) bands increase two-fold when going from CCl4 to acetone-d(6) solution, while delta(n ) values decrease by 3-9 cm(-1). The observed effects were explained i n terms of the existence of complexes with weak ICH3 ... acetone hydro gen bonding. The strength of the hydrogen bonding was characterized by enthalpies of specific interaction Delta H-int(CH3I/SIS) (sp.). These values were estimated by the ''intensity rule'': Delta H-int.(CH3I/S) (sp.) = 0.13 kcal mol(-1) for self-association in pure CH3I and 0.4 k cal mol(-1) for solution in acetone-d(6).