Spin-locking of half-integer quadrupolar nuclei in nuclear magnetic resonance of solids: Creation and evolution of coherences

Spin-locking of half-integer quadrupolar nuclei, such as Na-23 (I=3/2) and Al-27 (I=5/2), is of renewed interest owing to the development of variants of the multiple-quantum and satellite-transition magic angle spinning (MAS) nuclear magnetic resonance experiments that either utilize spin-locking directly or offer the possibility that spin-locked states may arise. However, the large magnitude and, under MAS, the time dependence of the quadrupolar interaction often result in complex spin-locking phenomena that are not widely understood. Here we show that, following the application of a spin-locking pulse, a variety of coherence transfer processes occur on a time scale of similar to1/omega(Q) before the spin system settles down into a spin-locked state which may itself be time dependent if MAS is performed. We show theoretically for both spin I=3/2 and 5/2 nuclei that the spin-locked state created by this initial rapid dephasing typically consists of a variety of single- and multiple-quantum coherences and nonequilibrium population states and we discuss the subsequent evolution of these under MAS. In contrast to previous work, we consider spin-locking using a wide range of radio frequency field strengths, i.e., a range that covers both the "strong-field" (omega(1)much greater thanomega(Q)(PAS)) and "weak-field" (omega(1)much less thanomega(Q)(PAS)) limits. Single- and multiple-quantum filtered spin-locking experiments on NaNO2, NaNO3, and Al(acac)(3), under both static and MAS conditions, are used to illustrate and confirm the results of the theoretical discussion. (C) 2004 American Institute of Physics.