Improvement of 1H-2H cross polarization under magic-angle spinning by using amplitude/frequency modulation

In 1H-2H cross polarization (CP) under magic-angle spinning (MAS), it has been pointed out that modulation of a H2 resonance frequency caused by MAS acts as adiabatic frequency sweep and efficient CP over the broad H2 powder pattern can be achieved. The adiabaticity, however, does not hold when the MAS frequency becomes faster, leading to insufficient CP enhancement. In this work, it is demonstrated that by applying amplitude/frequency modulation for H2 irradiation during CP, CP efficiency at faster MAS can be improved appreciably. By examining 1H-2H CP spectra taken at off-amplitude or off-resonance conditions, it is suggested that the improvement is ascribed to accumulation of CP signals from various parts of the broad H2 resonance, whose orientational dependence is time-dependent and is partially averaged under MAS.