Selective observation of nitrogen-bonded carbons in solid-state NMR by saturation-pulse induced dipolar exchange with recoupling

A solid-state NMR experiment for identifying nitrogen-bonded carbons with good efficiency at standard magic-angle-spinning (MAS) frequencies is presented. In the center of a sequence of 180° pulses recoupling the 14N–13C dipolar interaction, the 14N coherences are saturated by a train of small-angle 14N pulses for 1–2 ms. This saturation-pulse induced dipolar exchange with recoupling (SPIDER) provides efficient partial dephasing of 13C bonded to 14N at 5-kHz MAS and low radio-frequency strengths where REAPDOR and TRAPDOR are inefficient. SPIDER is demonstrated on an amide group with a 14N quadrupole-coupling constant of −3.1 MHz, on a benzamide, and on chitin.