The Substituent Effect. VII. Substituent Chemical Shifts of NH Proton of Acetanilides

NMR chemical shifts of the NH proton of fifty-three m- and p-substituted acetanilides were determined in dilute DMSO solutions. The substituent chemical shifts were correlated with σ0 constants with considerable scatter. Deviations Were particularly significant for −R groups having α-hydrogen. Improved correlation was found for −R substituents with (σ0)DMSO values which had been derived from the phenol hydroxyl chemical shifts in DMSO. Application of the LArSR equation using the DMSO-modified substituent parameters gave a good linear correlation for all substituents; ΔδNHAc=0.686 [(σ0)DMSO+0.277 (ΔσR−)DMSO]. These results indicate that the deviations are caused primarily by solvent-modification of substituents, the modification being essentially the same for both the acetanilide and the phenol systems in DMSO. The lower r− value 0.277 in the chemical shifts for the acetanilides than for the phenols suggests that the π-electronic contribution of +R para substituents is less important in the acetanilide system than in the phenol one. The resonance substituent parameter for these solvent-modified substituents was found to be proportional to both (σpOH−σpNH), the difference between the apparent σp constants of the two systems, and to σp−0.85σm of each series. Estimation of a practical set of inductive and resonance substituent parameters for DMSO-modified substituents is attempted.