Far-infrared absorption and dispersion studies on methyl iodide solutions

Abstract

A far-infrared dispersive liquid cell has been developed in collaboration with the National Physical Laboratory. This New Liquid Cell enables both the absorption coefficient and refractive index of a liquid to be accurately determined over the complete far-infrared frequency range. The New Liquid Cell has been used to make dispersive measurements of methyl iodide liquid and methyl iodide in a range of non-polar solvents. These were: carbon disulphide; carbon tetrachloride; n-heptane; n-decane; n-hexadecane and Santotrac 40 (an industrial traction fluid). This data has been combined with microwave measurements, enabling the total orientational correlation functions and the band moments to be determined. Results have also been obtained on the rates of reorientation and static angular correlations of methyl iodide molecules. This information on the liquid dynamics of methyl iodide has been used to elucidate the molecular environment of the solvents using methyl iodide as a probe molecule, which has given evidence for the presence of discrete solute 'pools' within the solvent environment for the longer chain length n-alkanes. Two theoretical models for molecular reorientation have been fitted to the experimental data. The first, a second order truncation of the Mori formalism though giving a good fit to the experimental far-infrared absorption coefficient and refractive index spectra, gave values for the molecular torques that did not agree with measured values. The second, a physically more meaningful model is based on the motion of a molecule which is described by a gaussian distribution of librational frequencies within a molecular cage, gives molecular torque values that agree well with experimental results.