MISMATCH-COMPENSATED CROSS-POLARIZATION - W-MOIST, AN IMPROVED PULSE SCHEME

A novel pulse scheme for cross polarization, called Window MOIST or W- MOIST, is presented. W-MOIST improves the insensitivity of the cross-p olarization efficiency to precise matching of the Hartmann-Hahn condit ion. W-MOIST is particularly effective in spin systems where the heter onuclear dipolar interaction is of a strength at least comparable to t he homonuclear dipolar interaction, e.g., for carbon spins with direct ly bound protons. An enhancement was, however, also found for quaterna ry carbons with weak heteronuclear interactions. W-MOIST combines the improved mismatch compensation of MOIST with the good spin-lock effici ency of the standard cross-polarization experiment. It is verified exp erimentally that W-MOIST leads to substantial improvements of the cros s-polarization efficiency under mismatched conditions for static and r otating (MAS) samples. (C) 1994 Academic Press, Inc.