Empirical additive parameter and automatic assignment of 13C NMR signals of some aryl and heteroaryl groups. A new criterion for a linear relationship between 13C chemical shifts and charge densities

The ¹³C NMR signals of some mono- and disubstituted aryl compounds were assigned by means of empirical additive substituent parameters and information taken from fully coupled spectra. This assignment was compared with that obtained with the help of the method of automatic assignment of the signals based on the linear relationship between ¹³C chemical shift and charge density. The results show that in the cases of XR substituents (X = O, NH and R = H, CH₃, CH₂CH₂OH) a good correlation is observed and the automatic assignment is correct. In contrast, in the cases of ? CH₂CH₂Y substituents (Y = OH, NH₂, Cl) a worsening of the correlation is observed and the automatic assignment is not correct. It is suggested that for compounds with a preliminarily known assignment, the automatic assignment can serve as an additional criterion for the reliability of the linear relationship between ¹³C chemical shift and charge density.