Carbon-13 Nuclear Magnetic Resonance Studies of Carbocations. 10. Variation of Cationic Carbon Chemical Shifts with Increasing Electron Demand in 1,1-Diaryl-1-methyl (Benzhydryl) Carbocations

Article abstract of DOI:10.1021/jo00177a003

A series of 28 1-X-phenyl-1-Z-phenyl-1-methyl (benzhydryl) carbocations, where the X and Z substituents have been varied over the range of electron demand (3,4-CH2CH2O, 11; 4-OCH3, 12; 4-CH3, 13; 4-F, 14; 4-H, 1; 4-CF3, 15; 3,5-(CF3)2, 16), have been prepared from the corresponding alcohols by ionization in superacids and their 13C NMR spectra recorded at low temperatures (-70 to -10 deg C).Plots of the substituent chemical shifts of the cationic carbons (ΔδC⁺) at -70 deg C against ?^C+ are linear for the electron donors (Z = 3,4-CH2CH2O to Z = H) of 13 - 16 and 1 but deviate upward from this correlation line (relative shielding) for the electron acceptors (Z = H to Z = 3,5-(CF3)2).The plots of the highly stabilized cations 11 and 12 approximate shallow curves where groups more electron demanding than 4-CH3 cause relative shielding of the cationic carbon.All these plots are interpreted in terms of competing resonance and localized inductive ?-polarization effects.