Kernresonanzuntersuchung der Molekülrotation in festem Benzol: T1-Spektroskopie / Nuclear Magnetic Resonance Investigation of Molecular Rotation in Solid Benzene: T1-spectroscopy

Abstract

We have extended Haeberlen’s and Maier’s NMR investigation of the molecular rotation in solid polycrystalline benzene (C6H6), performed in 1967 in the Larmor frequency range from 88 MHz to 22 MHz, to lower frequencies down to 4.4 MHz. In some cases monocrystalline samples were also studied. The extensive data show that the presently accepted model of purely uncorrected rotations of the benzene molecules around their sixfold axis has to be rejected. Instead, two results found, namely - a slight increase of the rotational activation energy when approaching the melting point of benzene and - the width of the T1 relaxation minima as a function of the reciprocal temperature suggest an additional relevance of correlated molecular motions, like e.g. the ‘gear-wheel mechanism’ first proposed by Cox. By appropriate modifications of Haeberlen’s theory, i.e. by assuming a rate equation with temperature dependent activation energy and by emphasizing the effect of simultaneous rotations of neighbouring rings compared with the purely statistical, uncorrelated case, we succeeded to describe the considered T1 measurements quantitatively.