Recoupling of heteronuclear dipolar interactions in solid state magic-angle spinning NMR by simultaneous frequency and amplitude modulation

A new scheme is proposed for solid-state nuclear magnetic resonance (NMR) which uses simultaneous frequency and amplitude modulation (SFAM) to restore heteronuclear dipolar interactions between two unlike spin-1/2 nuclei such as 13C and 15N under fast magic angle spinning (MAS). The carrier frequenncy of one of the spins is modulated cosinusoidally while its RF amplitude is modulated as a sine wave. It is shown by experiments and simulations that the scheme allows one to reintroduce the heteronuclear dipolar interactions even at very high spinning speeds which are often required for suppression of chemical shift anisotropy (CSA) in high magnetic fields. Unlike the well-known REDOR and TEDOR experiments, the SFAM method is not sensitive to effects of finite pulse lenghts. The advantages of the SFAM technique are illustrated for doubly labeled glycine (15NH213CH2COOH).