Rotating frame analog of spin-locking and spin-lattice relaxation in the doubly rotating frame

The present paper reports the achievement of the rotating-frame analog of spin-locking and its application to the precise measurements of the spin-lattice relaxation time T-1DR in the doubly rotating frame. After the magnetization is aligned along the resonant RF field H-1, a pulse sequence of a low-frequency oscillating magnetic field at exact resonance is applied perpendicular to H-1. We have overcome several technical difficulties arising from the fact that the rotating-wave approximation is not valid for the low-frequency field. We have theoretically derived an expression of T-1DR(-1), due to fluctuating magnetic dipole interactions in the weak collision case and found an important relation among the spin-lattice relaxation rates T-1(-1), T-1 rho(-1), and T-1DR(-1). This relation can be used to ascertain whether the relaxation is only due to the fluctuating magnetic dipole interactions between like spins. The experiment was carried out on H-1 nuclei in tetramethylammonium iodide (CH3)(4)NI and the temperature dependence of T-1DR(-1), was measured together with that of T-1(-1) and T-1 rho(-1). The activation energies and the preexponential factors of Arrhenius expressions of the correlation times are newly determined. (C) 2001 Academic Press.