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 13C nuclear magnetic resonance (NMR) signals has been achieved by exploiting the symmetry-based, systematic difference in their 13C chemical-shift anisotropies (CSAs). Simple three- or five-pulse CSA-recoupling sequences with "gamma-integral" cleanly suppress the signals of all sp2- and sp-hybridized carbons. The chemical-shift-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 13C 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.
Mao
J-D
JD
Department of Chemistry, Iowa State University, Gilman Hall, Ames 50011, USA.
Schmidt-Rohr
K
K
D000465:Algorithms; D000478:Alkylation; D016880:Anisotropy; D002247:Carbon Isotopes / Q000737:chemistry*; D002686:Chitin / Q000032:analysis / Q000737:chemistry; D006812:Humic Substances / Q000032:analysis; D006841:Hydrocarbons, Aromatic / Q000032:analysis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010100:Oxygen / Q000032:analysis / Q000737:chemistry*; D012815:Signal Processing, Computer-Assisted; D012987:Soil / Q000032:analysis
D002247:Carbon Isotopes; D006812:Humic Substances; D006841:Hydrocarbons, Aromatic; D012987:Soil; D002686:Chitin; D010100:Oxygen