Abstract
Spin-locking of half-integer quadrupolar nuclei, such as ²³Na (I = 3/2) and ²⁷Al(I = 5/2), is of renewed interest owing to the development of variants of the multiple-quantum (MQ) and satellite-transition (ST) magic angle spinning (MAS) NMR experiments that either utilise 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 timescale of ~1/ω_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 non-equilibrium 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 radiofrequency field strengths, i.e., a range that covers both the "strong-field" (ω₁ >> ω_Q^PAS) and "weak-field" (ω₁ << ω_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.