A 13C NMR study of the structure of four cinnamic acids and their methyl esters

The 13C NMR spectra, both in DMSO solution and in the solid state of four cinnamic acids (p-methoxy, p-hydroxy, p-methyl, p-chloro) and their corresponding methyl esters have been recorded. The two main results in the solid state are: (i) the only significant difference between acids and esters chemical shifts concerns the C=O group which, on average, appears at 173 ppm in the acids and 168 ppm in the esters; (ii) the signals of the ortho and meta carbons both in the acids and the esters are splitted. The two `anomaliesʹ disappear in DMSO solution. These observations can be rationalized using simple GIAO/B3LYP/6-31G* calculations.