Multiple-quantum cross polarization in quadrupolar spin systems during magic-angle spinning

We describe the concept of multiple-quantum cross polarization (CP) between an I = 3/2 and an I = 1/2 spin during magic-angle spinning. Experimental a nd theoretical results for Na-23-H-1 pairs are presented that elucidate the transfer mechanism and the beneficial effect of adiabatic amplitude modula tions of the CP field. The multiple-quantum CP approach is shown to be bene ficial for improving the sensitivity of CP-MQMAS experiments and for detect ing dipolar correlations. (C) 2000 Academic Press.