Knowledge Bank

For harriers to methyl rotation which have three-fold symmetry a single minimum in the nuclear spin-lattice relaxation time $T1$ is $expected.1$ When both three-fold and six-fold terms make significant contributions to the hindering barrier, two $T1$ minima can $occur.2$ For compounds expected to have three-fold barriers, e.g., $CH3XCl3$ (X = Si, Ge, Sn), the experimental values of $T1$ at the minima are significantly larger than those predicted theoretically. It is shown that the decrease in efficiency of spin relaxation can result from the partial averaging of the spin Hamiltonian by torsional oscillations. A calculation of this effect, based on the harmonic approximation is presented, which improves the agreement with experiment.