Tunneling-Assisted Nuclear Spin-Lattice Relaxation

The tunneling splitting of the torsional-oscillator ground state of a hindered methyl group is taken into account in calculating the temperature dependence of the proton spin-lattice relaxation time. Thermal vibrations cause spin-independent transitions leading to a fluctuation in the sign of the splitting parameter. It is predicted that a second maximum in T₁^-1 may occur at low temperature due to a tunneling-assisted process. This effect is detected in some methyl-substituted benzenes in good agreement with the theory.