Overall and internal motions of substituted benzenes studied by means of selective proton magnetic longitudinal relaxation time measurements

Abstract

Selective T ₁ measurements at different temperatures on seven substituted benzenes in CDCl₃ solution show that for these molecules the rotational diffusion model applies. Anisotropic reorientation is important if the molecules contain two polar substituents para to each other. Anomalies in the temperature dependence of the T ₁ values in a 2-methyl substituted phenol are explained by intermolecular hydrogen bonds, which strongly influence the molecular motion. In a 2,6-dimethyl substituted phenol this effect is absent. The correlation times and the energy barriers for methyl rotation of methyl and methoxy groups are determined. The reorientation of methoxy groups around the aryl oxygen bond is slower than the molecular motion. The T ₁ values of ring protons and substituents can in some cases be used for spectral assignment. Expressions are given for the T ₁ value of a ring proton relaxed by an ortho methyl or methoxy group and for the T ₁ value of a reorienting methoxy group in the case of dipolar relaxation and axially symmetric behaviour of the molecule.