Correlation of carbon‐13 substituent‐induced chemical shifts: Meta‐ and para‐substituted methyl benzoates

Abstract

Carbon‐13 NMR spectra are reported for 69 substituted methyl benzoates in deuteriochloroform or in its mixture with dimethyl sulphoxide‐d₆. The substituent‐induced chemical shifts (SCS) of the CO carbon correlate poorly with dual substituent parameters (DSP) in all possible modifications, and for meta derivatives in particular this correlation is both overpara meterized and imprecise. A much better correlation was obtained with parameters (designated B^m, B^p and C^p) derived previously by principal component analysis (PCA) from a larger set. The SCS of the CH₃ carbon correlate very well with the original simple Hammett equation, and no DSP treatment is needed. The clustering of substituents is not consequential in such a large set.