Off-resonance rotating-frame spin-lattice relaxation of quadrupolar (spin-1) nuclei

The validity of the formalism for the off-resonance rotating-frame spin-lattice relaxation experiment applicable to spin-1 quadrupolar nuclei was experimentally examined by considering two model systems, deuterated glycerol and deuterated benzene in castor oil, at different temperatures. When appropriately implemented, the deuterium off-resonance rotating-frame spin-lattice relaxation experiment provides spectral-intensity-ratio-dispersion data which agree remarkably well with those predicted by the theoretical formalism. The assumption of quadrupolar relaxation as the dominant relaxation mechanism, and rigid-rotor isotropic tumbling, permits the assessment of rotational diffusion behavior, i.e., the determination of a rotational correlation time, of a variety of molecular systems. With the inclusion of an additional relaxation measurement, T1 or T2, the 2H off-resonance rotating-frame spin-lattice relaxation experiment becomes a convenient method for the estimation of the 2H quadrupolar coupling constant, provided that a realistic reorientational motional model is assumed in the theoretical relaxation expressions used in the analysis. Because the motional window of the 2H off-resonance rotating-frame spin-lattice relaxation experiment includes intermediate molecular motion with correlation times as short as 1 or 2 ns, this experiment is appropriate for the investigation of the rotational diffusion behavior of deuterated molecules varying in size from moderately small to macromolecular. The 2H off-resonance rotating-frame spin-lattice relaxation experiment is applicable to in vitro and in vivo experimental situations.