Double-quantum-filtered rotational-resonance MAS NMR in the presence of large chemical shielding anisotropies

Double-quantum filtration under rotational resonance MAS NMR conditions whe re the chemical shielding anisotropies involved exceed the differences in i sotropic chemical shielding is considered by means of numerical simulations and C-13 MAS NMR experiments. The responses of two different pulse sequenc es, suitable for double-quantum filtration specifically under rotational re sonance conditions, to large chemical shielding anisotropies are compared. In the presence of large chemical shielding anisotropies a very recently in troduced pulse sequence (T. Karlsson, M. Eden, H. Luthman, and M. H. Levitt , J. Magn. Reson. 145, 95-107, 2000) suffers losses in double-quantum-filtr ation efficiencies. The double-quantum-filtration efficiency of another pul se sequence (N. C. Nielsen, F. Creuzet, R. G. Griffin, and M. H. Levitt, J. Chem. Phys. 96, 5668-5677, 1992) is less afflicted by the presence of larg e chemical shielding anisotropies. Both sequences deliver double-quantum-fi ltered lineshapes that sensitively reflect chemical shielding tensor orient ations. It is further shown that double-quantum-filtered rotational-resonan ce lineshapes of spin systems composed of more than two spins offer a suita ble experimental approach for determining chemical shielding tensor orienta tions for cases where conventional rotational-resonance experiments are not applicable due to the presence of additional background resonances. (C) 20 01 Academic Press.