The torsional barrier in aromatic carbonyl compounds. V. Carbon-13 DNMR study of stable protonated p-substituted acetophenones

The barrier to internal rotation around the phenyl–carbonyl bond in a series of stable protonated p-substituted acetophenones has been determined by means of low temperature ¹³C NMR and total bandshape analysis resulting in a value of ΔG = 48.1 kJ mol^?1 for the unsubstituted protonated compound. The energy results and the ¹³C chemical shifts are in good agreement with the quasi-planarity of the ground state in p-substituted protonated acetophenones. The barrier heights are discussed in terms of contributions from steric and resonance effects.