Journal of the American Chemical Society, Vol.118, No.36, 8669-8676, 1996
Charged Phenyl Radicals
The chemical properties of phenyl radicals with different chemically inert charged substituents in the ortho, meta, and para positions were examined in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The radicals were generated by replacing a chlorine, bromine, or iodine atom in a radical cation of dihalobenzene with a nucleophile and by cleaving the remaining iodine or bromine atom by collision-activated dissociation. The radicals’ structures were characterized by ion-molecule and dissociation reactions and by comparison to the reactivity of isomeric reference ions. Ab initio molecular orbital calculations (ROMP2/6-31G*//ROHF/6-31G* + ZPE) carried out for the 2-, 3-, and 4-dehydrophenylsulfonium ions suggest that these three species are nearly equal in energy and significantly less stable than the isomeric thiophenol radical cation. Most of the charge density is localized on the substituent in the charged phenyl radicals examined computationally. The odd-spin density at the radical site is calculated to be the same as in the neutral phenyl radical. These computational results predict chemical properties drastically different from those typical for conventional organic radical cations, e.g., the radical cation of thiophenol. This was found to be the case. Phenyl radicals with different charged groups in the meta or para position yield the same reaction products as the neutral phenyl radical (the ortho isomers rearrange upon collisions). Further, charged and neutral phenyl radicals show similar reactivity trends toward different substrates. Examination of the reactivity of radicals of various sizes and with the charged group in different locations with respect to the radical site suggests that the reaction efficiency toward a given substrate is predominantly determined by the electron deficiency at the reacting radical site. All these findings parallel those reported earlier for neutral phenyl radicals, and suggest that phenyl radicals with a chemically inert charged substituent in a remote position provide a useful model for the examination of the properties of neutral phenyl radicals in the gas phase.
Keywords:GAS-PHASE REACTIONS;MASS-SPECTROMETRY;4-DEHYDROANILINIUM ION;BIMOLECULAR REACTIONS;DIMETHYL DISULFIDE;DISTONIC ISOMER;CATIONS;REACTIVITY;DISSOCIATION;SUBSTITUTION