Kernrelaxation und Quanteneffekte bei der Methylgruppenrotation von festem Toluol und Toluolderivaten

Abstract

Abstract The hindered rotations of methyl groups have been studied in solid toluene, 2-F-toluene and 3-F-toluene. NMR pulse investigations have been carried out between the melting point and liquid helium temperatures at 14.5 MHz and 40.7 MHz. As in the case of earlier studies on the three xylenes, which have been continued in this work, the motional narrowing of the proton resonance lines was found to persist down to very low temperatures. In comparison with their exponential increase at high temperatures, the spin-lattice relaxation rates 1/T1 decay as a function of reciprocal temperature much less rapidly in the low temperature range. In the transition region, 1/T1 exhibits maxima which are more or less broadened or distorted. In effect, the relaxation at low temperatures is much stronger than that predicted by semiclassical theories.An explanation of the observed behaviour takes into acount the quantization of the rotational and torsional motion and the coupling between the spin and rotational degrees of freedom on one hand, and the phonons of the lattice on the other hand. At low temperatures, the motion causing nuclear spin-lattice relaxation is reduced to certain torsional transitions between the lowest states, which are connected with tunneling. At high temperatures, the summation of a great number of rotational transitions in the presence of strong coupling to the lattice leads o the "classical" behaviour.The potential barriers hindering the methyl group reorientations were found to be rather small in all the materials studied: toluene 0.1-0.2 kcal/mole, 2-fluorotoluene 0.50 kcal/mole, 3-fluoro-toluene 0.30 kcal/mole. They are, however, still large in comparison with free molecule data.