Re-investigation of the methyl and methoxy group hindered rotations in p-dimethoxybenzene, by comparison of dielectric and far infra-red spectra with ¹³C N.M.R. relaxation data

Abstract

Dielectric relaxation measurements and for infra-red spectra relative to the same para-dimethoxybenzene/tetrachloroethylene solution have been concomitantly analysed with ¹³C N.M.R. (T ₁ and N.O.E.) relaxation data, in order to study the intramolecular dynamics of the methoxy and methyl group internal motions. From a direct comparison between the microscopic dipolar (τ_μ) and N.M.R. (τ_C ^NMR) correlation times, it can be shown that the methoxy group internal rotation significantly contributes to the dielectric relaxation process. A quantitative analysis in terms of a ‘Chemical Relaxation Process’ permitted an estimation of both the kinetic constant k _cis→trans of the dielectrically ‘active’ cis/trans isomerism of the para-dimethoxybenzene molecule, and of the jumping rate of the methyl group from any of its three equivalent positions. The methoxy-torsional modes for the deuterated para-(CD₃O)₂ϕ and normal species have been assigned to features respectively observed at 82 and 92 cm^-1 in the far infra-red spectra of these compounds. Also a speculative assignment of the methyl torsions has been investigated. The coherence of the different results has been carefully discussed.