Carbon-13 N.M.R. studies of carbocations. Determination of σ⁺_ortho constants from ¹³C spectra of benzhydryl cations

Abstract

Carbon spectra of a range of 2- and 4-substituted benzhydryl cations have been measured in super acid at variable (low) temperatures. In the 4-substituted benzhydryl cations the chemical shift of the para carbon in the unsubstituted ring [δ(C4?)] proves to be an excellent probe for changes in charge density at the cationic carbon, as shown by its correlation with σ_p⁺ (r 0.990). Measurement of δ(C4?) for the 2- substituted benzhydryl cations provides a direct measure of σ₀+ for side-chain benzene derivatives; these values are OMe -0.59, Me -0.10, F 0.19, Cl 0.26, Br 0.29 and CF₃ 0.69.