Kernrelaxation und behinderte Rotation in organischen Festkörpern bei tiefen Temperaturen

Abstract

The proton magnetic resonance and relaxation of o-, m- and p-xylene has been studied at various temperatures between the melting point and 2°K. Motionally narrowed second moments of the proton lines of about 10 G2 have been observed over the whole temperature range. Spinlattice relaxation is effective at rather low temperatures and can no longer be interpreted in terms of classical hindered rotations of the methyl groups. Quantum mechanical tunneling through the barriers hindering the reorientations is considered instead. Furthermore it is proposed to explain the observed decay of relaxation rates at low temperatures as a consequence of some kind of bottleneck in the energy transfer between torsional motions and other degrees of freedom in the solid. On the basis of a simplified model a quantitative explanation of the experimental data has been possible using elements of the theoretical treatment of Stejskal and Gutowsky. The potential barriers are of sixfold and of threefold symmetry. From the experimental data absolute barrier heights of 2,2 (o-xylene), 0,27 (m-xylene) and 1,1 kcal/Mole (p-xylene) could be calculated.