Theoretical study of charge distribution in the benzoyl and alkynoyl cations

SCF-MO calculations, in the lNDO approximation, have been carried out on benzoyl (1), propynoyl (2), but-2-ynoyl (3), 3-phenylpropynoyl (4), and 3-fluoropropynoyl cations (5). In each case the geometries were optimized to minimum energy. In aggreement with 13C n.m.r. observations, significant charge delocalization into the phenyl rings occurs in (1) and (4). From an examination of the charge distributions, π-bond orders, and π- and σ-electron distributions it is shown that extensive delocalization of charge occurs in (2), (3), and (5), contrary to previous interpretations of 13C spectra. Thus all the mesomeric forms RCC–CO+↔RCC–[graphic omitted]O↔R[graphic omitted]z.dbd6;CCO contribute significantly in (2), (3), and (5) as well as in (4), and the charge is not substantially localized on oxygen as previously claimed.