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.

The rate of hydrogen atom abstraction from tributyltin hydride, benzeneselenol, thiophenol, and tetrahydrofuran was measured in the gas phase for charged phenyl radicals with different neutral substituents at the meta-or ortho-position. A charged pyridinium substituent (meta or para) allowed the manipulation of the radicals in the Fourier transform ion cyclotron resonance mass spectrometer that was used to carry out the experiments. All the reaction rates were found to be similarly affected by substituents on the radical: meta, H < Br ∼ Cl < CN (most reactive); ortho, H < CF 3 ∼ Cl ∼ F. The experimental observations parallel the transition-state energies calculated for hydrogen abstraction from methanol. However, the calculated reaction exothermicities do not correlate with the reactivity trends. Instead, a correlation exists between the reactivity and electron affinity of the radicals. We conclude that the electron-withdrawing substituents studied here lower the reaction barrier by increasing the polarity of the transition state, without an associated increase in reaction exothermicity. The increase in the electron affinity (∆EA) of the radical caused by a giVen substituent proVides a sensitiVe probe for the substituent's barrier-lowering effect (in the few cases studied in detail, the barrier is lowered by about 10% of ∆EA v ). Another way to lower the barrier involves lowering the ionization energy of the substrate. Indeed, all the radicals follow the reactivity trend of thiophenol > 4-fluorothiophenol > pentafluorothiophenol. This trend reflects the decreasing ionization energies of the three substrates rather than the decreasing reaction exothermicities or increasing homolytic bond-dissociation energies (4fluorothiophenol > thiophenol > pentafluorothiophenol). Apparently, the polar control overrides the enthalpic control in this case. The results reported for radicals with different distances between the radical site and the charged group suggest that similar substituent effects are expected for neutral phenyl radicals, and that the hydrogen abstraction ability of heteroaromatic radicals is likely to be tunable by pH.

Fourier transform ion cyclotron resonance mass spectrometry has been employed to systematically investigate the intrinsic (solvent-free) reactivity of a 1,3-dehydrobenzene (m-benzyne) with a pyridinium charge site in the 5-position. The m-benzyne was generated by using a combination of ion−molecule reactions and photodissociation and isolated prior to examination of its gas-phase reactions. The ionic reaction products and reaction efficiencies (second-order reaction rate constant/collision rate constant) were compared to those measured for the isomeric o-benzyne and the analogous phenyl monoradical. The m-benzyne yields some of the products formed for the o-benzyne but it also reacts via distinct radical pathways characteristic of the corresponding phenyl radical. These radical pathways are not observed for the o-benzyne. However, the reaction efficiencies measured for the m-benzyne are significantly lower than those measured for the analogous phenyl radical or the isomeric o-benzyne. These findings are partially rationalized by the relatively strong coupling (about 21 kcal mol-1) between the two formally unpaired electrons in the m-benzyne that hinders radical reactions. On the other hand, the greater distance between the reactive sites in the m-benzyne makes alkyne-type addition reactions sterically and energetically less favorable than for the o-benzyne.

The phosphenium ion CH 3 OPOCH 3 + readily attacks hydroxyl groups of neutral substrates in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The electrophilic character of CH 3 OPOCH 3 + is in agreement with molecular orbital calculations (Becke3LYP/6-31G-(d) + ZPE) that predict a singlet electronic ground state for this species. The observed reactions provide a convenient synthetic route to various larger phosphenium ions in the gas phase. Most importantly, however, CH 3 OPOCH 3 + was found to be extremely sensitive to the stereochemical structure of the neutral substrate. The dramatically different reaction product distributions obtained for diastereomeric cyclic vicinal diols suggest that CH 3 OPOCH 3 + provides a powerful chemical ionization reagent for the mass spectrometric determination of the stereochemistry of diols.

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