The effect of RF power and spinning speed on MQMAS NMR

It is known that the patterns of spinning sidebands observed in the multiple-quantum dimension of MQMAS spectra are often significantly wider than expected from the anisotropies of relevant interactions. It has been recently shown by others that these sidebands are generated due to the rotor-driven reorientations that the quadrupole tensors of the crystallites undergo during the evolution period between the multiple-and single-quantum conversion processes. We present an experimental and theoretical study of the effects of the spinning speed nu(R) and RF field strength nu(RF) on the development of these sideband patterns. The theoretical analysis relies upon numerical simulations and includes propagation of the density matrix during the entire MQMAS experiment. The possibility of additional rotational encoding during the RF pulses is discussed. Both the theoretical and experimental results show the benefits of using the highest available nu(R) and nu(RF). (C) 1998 Academic Press.