Studies of substituent effects by carbon‐13 NMR spectroscopy. VI—Application of multivariate data analysis to 13C NMR chemical shifts of protonated chalcones and thiophene chalcone analogues

The 13C NMR chemical shifts of protonated chalcones and thiophene chalcone analogues were analysed by two multivariate data analysis methods, principal components analysis and partial least squares analysis. A general relaying effect of the carbenium centre was manifested by an increased sensitivity to substituent changes at positions conjugated to the charged position. The shift data in both series could be satisfactorily described by the 13C NMR shifts of monosubstituted benzenes, as shown by partial least squares data analysis. This indicates a grouping of substituent effects into those of donors, acceptors, alkyls and halogens.