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13 C NMR of humic substances: pH and solvent effects
Author(s) -
PRESTON CAROLINE M.,
SCHNITZER M.
Publication year - 1987
Publication title -
journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 0022-4588
DOI - 10.1111/j.1365-2389.1987.tb02164.x
Subject(s) - chemistry , aqueous solution , solvent , humic acid , dissociation (chemistry) , nuclear magnetic resonance spectroscopy , carbon 13 nmr , radical , organic chemistry , inorganic chemistry , fertilizer
SUMMARY Since the concentration of free radicals in humic subtances increases at high pH the use of basic solutions for 13 C NMR spectroscopy may cause broadening and loss of aromatic signals, with distortion of intensity distributions. No such effects were found in 13 C spectra of soil humic and fulvic acid, an aquatic fulvic acid, and two phenolic polymers run in aqueous solutions at different pH values, and in dimethylsulphoxide. With increasing pH, the peak in the carboxyl region shifted in a manner consistent with greater dissociation of carboxyl and phenolic groups, and also certain features in the aliphatic and carboxyl regions were enhanced under some solution conditions. Elevated solution temperature (70°C) caused only slight improvement in the resolution of some lines. Chemical shifts were determined for some known phenolic and benzenecarboxylic acid compounds in DMSO and NaOD. The range for phenolic carbons extended to 173 ppm in NaOD, while some aromatic carbons occurred around 105 ppm, in the same region as anomeric carbons. Thus, even under quantitative acquisition conditions, relative areas may be used only to estimate proportions of different types of carbons and functional groups.