The detection of weak heteronuclear coupling between spin 1 and spin 1/2 nuclei in MAS NMR; nitrogen-14/carbon-13/proton triple resonance experiments.
Magic angle spinning and 14N irradn. were employed to perturb the 14N spin bath, in a new spin-echo triple resonance solid-state NMR expt. Spinning introduces a time dependence in the 14N quadrupolar interaction, permitting transitions between the 3 |I,m>states, 2 or 4 times per rotor cycle; this alters the evolution of a 14N-coupled 13C spin. The redn. in 13C echo intensity, on 14N irradn. during the echo period, is greater for shorter C-N distances. Irradn. at a const. frequency is more effective than sweeping through the 14N double-quantum spectra are obtained indirectly by irradn. of the double-quantum transition.
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