Density and temperature effects on the molecular reorientation and vibrational relaxation in liquid methyl iodide

Abstract

The reorientational correlation function for liquid methyl iodide has been measured by analysis of the Raman 526 cm⁻¹ ν₃ (a₁) band as a function of pressure up to 2.5 kbar within the temperature range 0 to 90°C. These reorientational correlation functions have been obtained from the anisotropic component of the vibrational Raman scattered light using a method of Fourier deconvolution. Density and viscosity of methyl iodide over the same range of temperatures and pressures have also been determined. In order to obtain information about the effects of density and temperature on the internal C₃ rotation of the methyl group the NMR deuteron spin‐lattice relaxation times in liquid methyl iodide‐d₃ have been measured under the same experimental conditions. In addition the vibrational relaxation rates have been determined from the isotropic Raman band shapes. The Raman experiments enable us to calculate the reorientational correlation function describing the reorientational motion of the CH₃I molecules about an axis perpendicular to the main symmetry axis for the following cases: (i) constant density ρ‐variable temperature; (ii) constant T‐variable ρ; and (iii) constant pressure‐variable T. Activation energies and activation volumes for the reorientation process are also discussed. All experimental results show unambiguously the large effects of density on molecular motions and the importance of constant density experiments. Assuming a rotational diffusion process the Raman data furnish the rotational diffusion constant D_⊥. By using this D_⊥ and the experimental NMR deuteron spin‐lattice relaxation times in CD₃I we calculate the rotational diffusion constant, D_∥, characterizing the internal rotation of the methyl group about the symmetry axis. In order to interpret the experimental data we propose that the deuteron field gradient deviates from the C–D bond axis. Both the rotational diffusion model and the Langevin model treatments of the internal rotation indicate very strong inertial effects in the methyl group rotation. As the density changes the rotation of the methyl group about the C₃ axis remains largely unaffected. In this study it has also been possible to obtain the vibrational relaxation rate 1/τ_vib from the isotropic component of the vibrational Raman band shape. The experimental results can be summarized as follows: (i) as ρ increases the relaxation rate increases; (ii) as T increases at constant ρ, 1/τ_vib remains approximately constant, and (iii) at constant pressure increasing T causes a significant decrease in the vibrational relaxation rate. The experimental data are discussed in terms of a binary collision model as developed by Litovitz and the time between collisions is estimated from the Enskog theory, the cell model, and the J‐diffusion model. Based on our earlier results we assume a temperature dependence of the effective hard sphere diameter in order to explain the experimental trends in the vibrational relaxation rate. On the basis of this study one arrives at a conclusion of general validity that any serious experimental attempt to characterize the molecular dynamics in a liquid must include the separation of the effects of density and temperature on the molecular motions.