NMR parameters of biologically important aromatic acids. 2. Phenylacetic acid and derivatives

Abstract

In the present study, precise ring proton chemical shifts and coupling constants for phenol acetate, phenylacetic acid, 17 phenylacetic acid derivatives, and four acetoxy substituted benzoic acid derivatives were obtained by computer simulation of the spectra. The additivity of the ring proton and substituent proton chemical shifts was evaluated in detail in order to establish the reliability of the predicted chemical shifts. The additivity of the ring proton coupling constants was also evaluated in detail. The usefulness of empirically predicted chemical shifts and coupling constants as initial approximations in the computer analysis of complex spectra was demonstrated. The magnitudes of the benzylic to ring proton coupling constants for the phenylacetic compounds were also obtained from the computer analysis. The relative signs of the benzylic to ring proton couplings can be established unequivocally only for methyl m-acetoxyphenylacetate, but appear to be the same for the other phenylacetic derivatives, i.e., benzyl to ortho and para proton coupling negative, benzyl to meta proton coupling positive.