Polar effects are demonstrated to be a key factor in controlling the reactivities of related charged phenyl radicals in different exothermic atom and group abstraction reactions in the gas phase. The effects of various meta substituents on the phenyl radicals' reactivity were probed via the measurement of bimolecular reaction rate constants by using Fourier transform ion cyclotron resonance mass spectrometry. This approach requires an additional, charged substituent to be present in the phenyl radical to allow mass spectrometric manipulation. The m-pyridinium group was chosen for this purpose. The substrates studied were allyl iodide, dimethyl disulfide, and tert-butyl isocyanide. Two of the reactions of interest, *I and *SCH(3) transfer, are thought to occur by concerted bimolecular homolytic substitution (S(H)2), and the third one, *CN transfer, by an addition/elimination mechanism. For all three substrates, the reaction rate was found to increase in the following order for the differently substituted phenyl radicals: CH(3) approximately H < Br approximately Cl approximately COOH < NO(2) approximately CN. This trend does not arise from differences in reaction exothermicities or bond dissociation energies but via lowering the reaction barrier by electronic effects. The stabilization of the transition state is attributed to its increased polar character. A semiquantitative measure of the barrier lowering effect for each substituent is obtained from its influence on the electron affinity of the charged radical, as the calculated (B3LYP/6-31+G(d)) adiabatic electron affinities of the radical model systems (ammonium instead of pyridinium charge site) follow the same trend as the reactivities.

In order to investigate competition between radical substitution and addition reactions, the gas-phase reactivity of phenyl radicals bearing a chemically inert, positively charged group and a neutral substituent (CH3, Cl, or Br), both at a meta position with respect to the radical site, was examined toward several aromatic substrates in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. The radicals undergo hydrogen atom abstraction from the substituent and/or addition to the phenyl ring of benzeneselenol, thiophenol, benzaldehyde, toluene, aniline, and phenol. The presence of an electron-withdrawing substituent Cl or Br on the phenyl ring of the radical slightly increases the rates for both hydrogen atom abstraction and addition due to favorable polarization of the reactions’ transition states. The observation of a stable ion-molecule addition product in most reactions was unexpected since in a low-pressure gas-phase environment, adducts are typically unable to release their excess energy before dissociation to products or back to reactants. However, the addition products discussed here are low in energy [addition is exothermic by 24–30 kcal/mol; B3LYP/6-31Gd+ZPVE] and hence are long lived enough to become stabilized by infrared emission. The extent to which the charged radicals are able to abstract a hydrogen atom from the aromatic substrate and form stable products via addition to the aromatic ring was found to vary greatly. The outcome of this competition can be rationalized by reaction exothermicities only in extreme cases, i.e. for benzeneselenol and thiophenol, that predominantly react by hydrogen atom abstraction due to their especially weak heteroatom-hydrogen bonds and aniline that undergoes almost exclusive addition due to particularly stable resonance-stabilized addition products. For the other substrates, competition between the two reaction pathways is controlled by a complex interplay of polar effects that affects the energies of both transition states but to different extents.

The feasibility of generating substituted phenyl radicals and biradicals (with a charged substituent) in the gas phase by laser photolysis was examined by using a Fourier-transform ion cyclotron resonance mass spectrometer. The precursors were generated by ipso-substitution of a halogen atom in the radical cation of a di- or trihalobenzene by various nucleophiles. Photolytic cleavage of the remaining carbon-halogen bond(s) with 266-nm radiation was found to produce many substituted phenyl radicals in greater yields than the earlier employed method, sustained off-resonance irradiated collision-activated dissociation (SORI-CAD). Furthermore, ion generation by photolysis leads to isomerization less often than collisional activation. Finally, not only phenyl-bromine and phenyl-iodine but also certain phenyl-chlorine bonds can be cleaved by photolysis, whereas the synthetic utility of SORI-CAD appears to be largely limited to the cleavage of phenyl-iodine bonds. Hence, laser photolysis greatly expands the variety of substituted phenyl radicals and biradicals that can be synthesized inside a mass spectrometer.