Longitudinal nuclear spin relaxation in solids in a triply rotating coordinate system: Selective observation of the multispin dipole contribution

The longitudinal nuclear spin relaxation in an effective magnetic field H _e3 acting in a triply rotating coordinate system is recorded. Rotating and doubly rotating coordinate systems are employed for strong suppression of the secular nuclear dipole interactions in the first two orders and for separation of higher-order interactions (four-and five-spin). Experiments on protons in polycrystalline benzene showed that the contribution of such multispin dipole interactions to this relaxation can be observed selectively as a pronounced local minimum in the temperature dependence of the relaxation time. This contribution correponds to ultraslow molecular motions with rates ≃ γH _e3≃2π(10¹−10³) s⁻¹ and can be employed to study such motions in detail, including for purposes of identification of the form of the motion. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 335–340 (10 September 1996)