A COMBINATION OF SLOW AND FAST RF FIELD MODULATION FOR IMPROVED CROSS-POLARIZATION IN SOLID-STATE MAS NMR

A method for efficient Hartmann-Hahn cross polarization under rapid ma gic-angle sample spinning is presented. It combines rotor-synchronized amplitude modulation of one of the two spin-lock radio-frequency fiel ds for dipolar recoupling, with a slow adiabatic amplitude variation t o enhance the net polarization transfer. The proposed scheme, called A MAP-CP, allows for cross polarization on the original Hartmann-Hahn ma tching condition omega(1I) = omega(1S). The polarization transfer is i nsensitive to chemical-shift offsets, rotation frequency, and inhomoge neity of the radiofrequency field. (C) 1997 Academic Press.