13C/14N heteronuclear multiple-quantum correlation with rotary resonance and REDOR dipolar recoupling

A two-dimensional 13C/14N heteronuclear multiple quantum correlation (HMQC) experiment using dipolar recoupling under magic-angle spinning (MAS) is described. The experiment is an extension of the recent indirect 13C detection scheme for measuring 14N quadrupolar coupling under MAS. The recoupling allows the direct use of the much larger dipolar interaction instead of the small J and residual dipolar couplings for establishing 13C/14N correlations. Two recoupling methods are incorporated into the HMQC sequence, both applying rf only to the observed 13C spin. The first one uses the REDOR sequence with two π-pulses per rotor cycle. The second one uses a cw rf field matching the spinning frequency, known as rotary resonance. The effects of CSA, T2′ signal loss, MAS frequency and stability and t1-noise are compared and discussed.