Enhancement of quantum rotor NMR signals by frequency-selective pulses.
Résumé
Quantum-rotor-induced polarisation (QRIP) enhancement is exhibited by substances which contain freely rotating methyl groups in the solid state, provided that the methyl groups contain a (13)C nucleus. Strong signal enhancements are observed in solution NMR when the material is first equilibrated at cryogenic temperatures, then rapidly dissolved with a warm solvent and transferred into an NMR magnet. QRIP leads to strongly-enhanced (13)C NMR signals, but relatively weak enhancements of the (1)H signals. We show that the (1)H signals suffer from a partial cancellation of degenerate contributions, which may be corrected by applying a frequency-selective π pulse to the inner peaks of the (13)C multiplet prior to (1)H observation.