FREQUENCY-MODULATED CROSS-POLARIZATION FOR FAST MAGIC-ANGLE-SPINNING NMR AT HIGH FIELDS - RELAXING THE HARTMANN-HAHN CONDITION

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.