PI-PLUS CONTRIBUTIONS IN DELTA-E AND DELTA-C DUAL-PARAMETER CORRELATIONS - CURVED HAMMETT PLOTS AND F-19 SHIFTS

Structure-reactivity analysis with the dual-parameter Delta E and Delta C substituent constants illustrates deviations that can arise with conjugating substituents 4-X in a para position. Complications arise when the interaction of a 4-X substituent with the reaction site (rates or equilibria) or with a spectroscopic measurement (NMR shift) gives rise to additional pi-contributions thereby leading to a deviation in a single-parameter Hammett linear free energy plot. The deviations for the 4-X groups are referred to in the Delta E and Delta C correlations as pi-plus effects. Their presence in systems that give curved Hammett plots for rates, equilibria, and F-19 chemical shifts is illustrated.