PARA SUBSTITUENT C-13 CHEMICAL-SHIFTS IN SUBSTITUTED BENZENES .1. UPDATING THE SIGMA-OR SCALE AND ANALYSIS OF APROTIC-SOLVENT EFFECTS

The para carbon chemical shifts (CP-SCS) for a selected series of monosubstituted benzenes have been measured in very dilute solutions of the nonpolar solvents cyclohexane and carbon tetrachloride and of the weakly polar solvent deuteriochloroform. These results provide the basis for the redefinition of σ°R values, which measure the ability of a substituent to delocalize π electrons to or from a neutral or unperturbed benzene ring. The new σ°R values are shown to make significant improvements in dual substituent parameter (DSP) correlations of appropriate data sets which are very sensitive to the effects of electron delocalizaion. Applications include a definitive analysis of polar aprotic solvent induced CP-SCS. These results offer strong support for both the new σ°R values and the basic assumptions of DSP analysis. © 1979, American Chemical Society. All rights reserved.