Efficient CH-group selection and identification in 13C solid-state NMR by dipolar DEPT and 1H chemical-shift filtering

A new spectral-editing technique for solid-state nuclear magnetic resonance (NMR), based principally on the different dipolar-dephasing properties of CH and CH2 multiple-quantum (MQ) coherence, yields pure C-H spectra with overall efficiencies of up to 14%. The selection is based on dephasing of methylene heteronuclear MQ coherence by the second proton and can be considered essentially as a solid-state, slow-magic-angle-spinning version of the distortionless enhancement by polarization transfer (DEPT) experiment. A short dipolar transfer and inverse gated decoupling suppress quaternary-carbon resonances, and T-1-filtering reduces methyl signals. Applications to amorphous polymers with long, flexible sidegroups demonstrate excellent suppression of the signals of partially mobile methylene groups, consistent with simulations and superior to existing methods. CH selection in various model compounds and a humic acid confirms the robust nature and good sensitivity of the technique. Distinction of NCH and CCH groups, which have overlapping C-13 chemical-shift ranges, is achieved by combining dipolar DEPT with H-1 isotropic-chemical-shift filtering. In the humic acid, this permits unequivocal assignment of the methine resonance near 53 ppm to NCH groups.