Linear free energy relationships applied to the reactivity and the 13C NMR chemical shifts in 4-[[(substituted phenyl)imino]methyl]benzoic acids

Linear free energy relationships (LFER) were applied to the kinetic data and 13C NMR chemical shifts in 4-[[(substituted phenyl)imino]methyl]benzoic acids. The correlation analysis for the kinetic data and substituent-induced chemical shifts (SCS) with σ using single substituent parameter (SSP), as well as inductive (σI) and various resonance (σR) parameters using dual-substituent parameter (DSP), were carried out. The presented calculations account satisfactorily for the polar and resonance substituent effects having similar contributions at all carbons studied. Negative ρ values were found for several correlations (reverse substituent effect). Exceptionally good Hammett correlation of 13C NMR chemical shifts of azomethine carbon with electrophilic substituent constants σ+ indicates a significant resonance interaction in the aniline part of molecules. The conformations of investigated compounds have been studied by the use of DFT method, and together with 13C NMR chemical shifts and kinetic data, give a better insight into the influence of such a structure on the transmission of electronic substituent effects. New σ constants for substituted phenyliminomethyl group have been calculated.