Onium Ions. X: Structural Study of Acyclic and Cyclic Halonium Ions by Carbon-13 Nuclear Magnetic Resonance Spectroscopy. The Question of Intra- and Intermolecular Equilibration of Halonium Ions with Haloalkylcarbenium Ions

A comprehensive study of the structure of acyclic and cyclic halonium ions was carried out by cmr spectroscopy. The proton decoupled carbon-13 nmr spectra of 10 dialkylhalonium ions, 17 arylalkylhalonium ions, 14 cyclic ethylenehalonium ions, and 11 cyclic tetramethylenehalonium ions have been obtained using the Fourier transform method. The structure of the ions is discussed on the basis of the carbon-13 chemical shift data. For the cyclic halonium ions the structures considered were the symmetrically and unsymmetrically bridged ions, an equilibrium mixture of the open chain halocarbenium ions, and of bridged halonium ions with open chain β-haloalkylcarbenium ions. The α- and β-substituent effects of positively charged halogen atoms on carbon-13 chemical shifts are summarized and correlated with similar effects for halogens. Substituent effects of positively charged chlorine, bromine, and iodine are compared.