Simultaneous adiabatic spin-locking cross polarization in solid-state NMR of paramagnetic complexes

Continuous-wave irradiations cross polarization and its variations had not been a popular choice for sensitivity enhancement in paramagnetic solids due to the large spectral broadening and the effect of fast relaxation on both the source and target spins. Furthermore, with the application of very-fast sample spinning, which is the prerequisite for high-resolution purposes, polarization transfer over a large bandwidth is further degraded. In this work, we report a wideband sensitivity enhancement in paramagnetic solid with our recently developed technique, simultaneous adiabatic spin-locking cross polarization, SADIS CP. Experiments were demonstrated with Cu(II)(d,l-alanine)2 · H2O under high-static field of 14.1 T and very-fast sample spinning of 31 kHz.

13C NMR spectrum of Cu(II)(d,l-alanine)2 · H2O, observed with different techniques, (a) SADIS cross polarization, (b) RAMP cross polarization, and (c) direct excitation of 13C. The signal-to-noise ratios (SNRs) are denoted besides its peaks.