BROAD-BAND HARTMANN-HAHN MATCHING IN MAGIC-ANGLE-SPINNING NMR VIA AN ADIABATIC FREQUENCY SWEEP

The cross-polarization condition in high-speed magic-angle spinning NMR is investigated in which the H-1 frequency is swept adiabatically from on-resonance to an off-resonance value during the spin lock, The H-1 frequency sweep dramatically broadens the matching conditions, leading to a nearly flat profile over a wide range of C-13 radiofrequency field strengths, These results can be understood by viewing the frequency-swept H-1 spin-locking field in the tilted rotating frame in which the nutation rate about the effective field, omega(eff) = (omega(1)(2)+Delta omega(2))(1/2), varies over the duration of the spin lock, broadening the matching condition, omega(eff,I) = omega(1S) +/- n omega(r). Experimental results on [1-C-13]glycine, [2-C-13,N-15]glycine, and [3-C-13]D,L-alanine are reported. (C) 1995 Academic Press, Inc.