The torsional barrier around the phenyl–C+ bond in phenylcarbenium ions

Abstract

The barrier to rotation around the phenyl–carbenium carbon bond in a series of carbenium ions R¹R²C⁺Ph (with R¹ and/or R²= H, CH₃, cyclopropyl, and C₆H₄X) has been measured by complete line-shape analysis of the n.m.r. spectra or by the spin saturation transfer method. ΔG^‡ Values range from 36 to 78 kJ mol^–1. The results are discussed in terms of steric and electronic contributions and the relationship of the ¹³C chemical shift of the carbenium centre with the σ_c⁺ values of the para-substituent is analysed.