High frequency rotational mode in liquid methyl chloride

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Abstract
Complex permittivity measurements in liquid methyl chloride have been completed over the whole frequency range at temperatures and densities scanning the orthobaric curve, from 176 K up to 363 K. A report on the experimental set-ups and the accuracies obtained is given. Relaxation parameters are derived in the frame of Debye theory, and Hill's distribution of damped resonances is used to construct a model for librational absorption. A new absorption process superimposed upon the main librational and relaxational absorption-dispersion patterns is observed. Coherent rotational features show up in both complex plane representations of permittivities (Cole-Cole) and polarizabilities (Scaife). The spectral density of the new features corresponds to a resonant phenomenon rather than a secondary relaxation process as shown by its insensitivity to temperature and density variations. Its relevance to the dipolaron formalism is examined, pointing to a balance between positive clues, such as dipolar solvation, and inconclusive features, such as the frequency extension of the new process. Other contributions to the absorption pattern are expected to arise from the modulation of reorientational motion by dipole-induced moments and from the dynamics of the intermolecular torques.

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