Dynamics of spin I=3/2 under spin-locking conditions in an ordered environment

We have derived approximate analytic solutions to the master equation descr ibing the evolution of the spin I = 3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar i nteractions. Spectra resulting from Fourier transformation of the evolution s of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The centra l line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster rel axing modes. The rates of the fast modes are sensitive to slow molecular mo tion. Neither the amplitude nor the width of the narrow component is affect ed by the magnitude of the static coupling, whereas the corresponding featu res of the broad components depend in a rather complicated manner on the sp in-lock field strength and static quadrupolar interaction. Under certain ex perimental conditions, the dependencies of the amplitudes on the dynamics a re seen to vanish and the relaxation rates reduce to relatively simple expr essions. One of the promising emerging features is the fact that the evolut ions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, thes e coherences can only be excited in the presence of a static coupling and t his makes it possible to discern nuclei in anisotropic from those in isotro pic environment. The feasibility of the spin-lock pulse sequences with limi ted RF power and a nonvanishing average electric field gradient has been de monstrated through experiments on sodium in a dense lyotropic DNA liquid cr ystal. (C) 2001 Academic Press.