The ^(13)C chemical shifts of each of the carbons of the simple aliphatic carboxylic acids and their corresponding anions from formic to valeric acid have been measured in aqueous solution. Perhaps surprisingly, ionization of an aliphatic acid results in a downfield shift of its carbon resonances. The results have been correlated with other substituent effects, with ^(13)C and H resonances in corresponding hydrocarbons, and with the distance between carboxylic(ate) group and the carbons undergoing resonance absorption.
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Hagen et al. (1969) studied this question.