Nuclear magnetic interactions and molecular motion in liquids by n.m.r. relaxation in the rotating frame

Measurement of 31 P and 1 H nuclear spin relaxation times in the rotating frame, T 1 rho , have been made on the pure liquids PCl 3 , HCl and HBr as a function of the rf field strength H 1 over a wide temperature range. The dominant mechanism responsible for relaxation in each case is modulation of the scalar coupling interaction. From the dependence of T 1 rho on H 1 at each temperature, the scalar coupling constant and the spin-lattice relaxation times of the halogen nuclei at that temperature have been determined. The halogen spin-lattice relaxation times are controlled by modulation of the nuclear quadrupole interaction by molecular reorientation. The quadrupole coupling constants are known and hence the molecular reorientation correlation times are determined as a function of temperature. The activation energy associated with this reorientation is determined for each liquid and these values agree with those obtained by other methods. Application of rotating frame relaxation measurements to measure scalar coupling constants and fast relaxation times of nonresonant spins, which are not measurable by conventional techniques, is discussed and a formula is derived for the relaxation rate, (T 1 rho ) -1 , due to the scalar interaction mechanism.