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 magnetic resonance that make it possible to achieve an efficient transfer of polarization from abundant I spins dilute S spins under high-speed magic angle spinning spectroscopy, which is useful for systems with large chemical shift anisotropies at high fields. The frequency of one of the RF carriers is modulated sinusoidally during spin-locking while the RF amplitude is kept constant. It is shown by experiment that frequency modulation greatly attenuates the critical character of the Hartmann-Hahn matching condition, and allows one to obtain uniform excitation over a wide range of offsets. Signal intensities of 13C resonances in alanine were obtained as a function of mismatch at spinning speeds 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.