Linear free energy relationships of the 1H and 13C NMR chemical shifts in 3-cyano-4-(substituted phenyl)-6-phenyl-2(1H)pyridones

Linear free energy relationships (LFER) were applied to the 1H and 13C NMR chemical shifts in 3-cyano-4-(substituted phenyl)-6-phenyl-2(1H)pyridones. The correlation analysis for the substituent-induced chemical shifts (SCS) with inductive (σI), and various resonance (σR) parameters were carried out using SSP (single substituent parameter), DSP (dual substituent parameter), and DSP-NLR (dual substituent parameter non-linear resonance) methods, as well as by multiple regression analysis. The presented calculation accounts satisfactorily for the polar and resonance substituent effects operating at pyridone carbon atoms. Negative ρ values were found for several correlations (reverse substituent effect). The conformations of investigated compounds have been studied by the use of semi-empirical MO-PM6 method and B3LYP density functional (DFT) hybrid methods. The twist of the plane of 4-substituted phenyl ring (θ1) is determined by electronic substituent effects, while the angles θ2 are almost constant.