Application of rotor-synchronized amplitude-modulated cross-polarization in a 13C-1H spin pair under fast magic-angle spinning.

Rotor-synchronized amplitude-modulated Hartmann-Hahn cross-polarization has been applied under fast magic-angle spinning to a powder sample of ferrocene. The influence on the cross-polarization process of the heteronuclear double quantum (flop-flop) transitions induced by the amplitude modulation for low ratios of the radiofrequency-field strength to the spinning speed is studied in details. The experimental data are in good agreement with theoretical calculations for a two-spin 12 system, although the intensity of the double quantum transitions is observed to be significantly smaller than expected. Moreover, it is shown that an efficient polarization transfer at the Hartmann-Hahn centerband matching condition as well as a broadening of the matching conditions is obtained by an appropriate partial scaling of the effective radiofrequency fields which minimizes the destructive effect of double quantum cross-polarization. Copyright 1998 Academic Press.