Quick, selective NMR spectra of C-OH moieties in 13C-enriched solids

A convenient one-dimensional magic-angle spinning NMR method is presented that provides selective NMR spectra of COH moieties in uniformly C-13-enriched organic materials. This method, termed hydroxyl-proton selection (HOPS), eliminates the magnetization of protons directly bonded to carbons by recoupling the H-1-C-13 dipolar interaction for a short time (similar to 70 mu s), which also serves as a chemical-shift filter to suppress H-1 magnetization of CH3 groups. After cross polarization to C-13, the signals of C-OH and COOH carbons are observed selectively. This makes it possible to distinguish alcohols from ethers, in particular phenols from aromatic ethers such as the furans often formed by dehydration of glucose, and carboxylic acids from carboxylates and ethers. HOPS NMR reveals that orthodiphenols are often a major component of low-temperature carbon materials. For instance, it forces the reassignment of the 143 ppm C-13 NMR signal of hydrothermal carbon to such catecholic diphenols, while a previous NMR-based structural model had attributed this peak to a central furan-furan linkage. (C) 2019 Elsevier Inc. All rights reserved.