THE EFFECT OF RF POWER AND SPINNING SPEED ON MQMAS NMR

It is known that the patterns of spinning sidebands observed in the mu ltiple-quantum dimension of MQMAS spectra are often significantly wide r 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 an d theoretical study of the effects of the spinning speed nu(R) and RF field strength nu(RF) on the development of these sideband patterns. T he 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 pulse s is discussed. Both the theoretical and experimental results show the benefits of using the highest available nu(R) and nu(RF). (C) 1998 Ac ademic Press.