Separation of aromatic-carbon 13C NMR signals from di-oxygenated alkyl bands by a chemical-shift-anisotropy filter

Selection of alkyl-carbon and suppression of aromatic-carbon C-13 nuclear magnetic resonance (NMR) signals has been achieved by exploiting the symmetry-based, systematic difference in their C-13 chemical-shift anisotropies (CSAs). Simple three- or five-pulse CSA-recoupling sequences with gamma-integral cleanly suppress the signals of all sp(2)- and sp-hybridized carbons. The chemicalshift-anisotropy-based dephasing is particularly useful for distinguishing the signals of di-oxygenated alkyl (O-C-O) carbons, found for instance as anomeric carbons in carbohydrates, from bands of aromatic carbons with similar C-13 isotropic chemical shifts. The alkyl signals are detected with an efficiency of > 60%, with little differential dephasing. Combined with C-H dipolar dephasing, the CSA filter can identify ketal (unprotonated O-C-O) carbons unambiguously for the first time. Conversely, after short cross polarization and the CSA filter, O-CH-O (acetal) carbon signals are observed selectively. The methods are demonstrated on various model compounds and applied to a humic acid. (C) 2003 Elsevier Inc. All rights reserved.