Recoupled long-range C-H dipolar dephasing in solid-state NMR, and its use for spectral selection of fused aromatic rings.

This work introduces a simple new solid-state 13C NMR method for distinguishing various types of aromatic residues, e.g. those of lignin from fused rings of charcoal. It is based on long-range dipolar dephasing, which is achieved by recoupling of long-range C-H dipolar interactions, using two 1H 180 degrees pulses per rotation period. This speeds up dephasing of unprotonated carbon signals approximately threefold compared to standard dipolar dephasing without recoupling and thus provides much more efficient differential dephasing. It also reduces the effects of spinning-speed dependent effective proton-proton dipolar couplings on the heteronuclear dephasing. Signals of unprotonated carbons with two or more protons at a two-bond distance dephase to <3% within less than 0.9 ms, significantly faster than those of aromatic sites separated from the nearest proton by three or more bonds. Differential dephasing among different unprotonated carbons is demonstrated in a substituted anthraquinone and 3-methoxy benzamide. The data yield a calibration curve for converting the dephasing rates into estimated distances from the carbon to the nearest protons. This can be used for peak assignment in heavily substituted or fused aromatic molecules. Compared to lignin, slow dephasing is observed for the aromatic carbons in wood charcoal, and even slower for inorganic carbonate. Direct 13C polarization is used on these structurally complex samples to prevent loss of the signals of interest, which by design originate from carbons that are distant from protons and therefore crosspolarize poorly. In natural organic matter such as humic acids, this combination of recoupled dipolar dephasing and direct polarization at 7-kHz MAS enables selective observation of signals from fused rings that are characteristic of charcoal.