Rq. Fu et al., FREQUENCY-MODULATED CROSS-POLARIZATION FOR FAST MAGIC-ANGLE-SPINNING NMR AT HIGH FIELDS - RELAXING THE HARTMANN-HAHN CONDITION, Chemical physics letters, 264(1-2), 1997, pp. 63-69
Two novel schemes are described for cross polarization in nuclear magn
etic resonance that make it possible to achieve an efficient transfer
of polarization from abundant I spins to dilute S spins under high-spe
ed magic angle spinning spectroscopy, which is useful for systems with
large chemical shift anisotropies at high fields. The frequency of on
e of the RF carriers is modulated sinusoidally during spin-locking whi
le the RF amplitude is kept constant. it is shown by experiment that f
requency modulation greatly attenuates the critical character of the H
artmann-Hahn matching condition, and allows one to obtain uniform exci
tation over a wide range of offsets. Signal intensities of C-13 resona
nces in alanine were obtained as a function of mismatch at spinning sp
eeds of 12 and 15 kHz and a static field strength of 14 T (600 MHz for
proton NMR) to illustrate the advantages of these schemes.