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 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 similar to Cl < CN (most reactive); ortho, H < CF3 similar to Cl similar to 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 (AEA) 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 Delta 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 (4-fluorothiophenol > 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.

The ability of differently substituted charged phenyl radicals (a class of distonic radical cations) to abstract an iodine atom from allyl iodide was systematically examined in the gas phase by using Fourier transform ion cyclotron resonance mass spectrometry. The reaction products and second-order reaction rate constants were determined for several radicals that differ by the type and/or number of substituents located in the ortho- and/or meta-position with respect to the radical site. All the radicals also carry a para-pyridinium group needed for mass spectrometric manipulation. These electron-deficient phenyl radicals react with allyl iodide by predominant iodine atom abstraction. The reaction is facilitated by the presence of neutral electron-withdrawing substituents, such as F, CF3, Cl, or CN. The extent of rate increase depends on the type and number of the substituents, as well as their location relative to the radical site. Based on molecular orbital calculations (PM3 and Becke3LYP/6-31G(d)+ZPVE), the indicated variations in the transition state energy are not related to enthalpic factors. Instead, the results are rationalized by polar effects arising from a variable contribution of a stabilizing charge transfer resonance structure to the transition state. A semiquantitative measure for the barrier-lowering effect of each substituent is provided by its influence on the electron affinity of the radical (the electron affinities were calculated by Becke3LYP/ 6-31+G(d) and AM1, which were found to produce similar values). Methyl substitution does not significantly affect the electron affinity, and accordingly, does not have a detectable effect on reactivity. Methyl groups located at ortho-positions are an exception, however. o-Methyl-substituted phenyl radicals undergo exothermic rearrangement to a benzyl radical in competition with iodine abstraction from allyl iodide.

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 same 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.

Molecular orbital calculations (at the Becke3LYP/6-31G(d) level of theory) indicate that the distonic ion (CH2C)-C-.(CH2)CO+ is planar and has C-2 upsilon symmetry. Most of the positive charge resides at the carbonyl carbon while the odd spin is delocalized over the allyl group. A similar spin distribution was calculated for the allyl radical. Hence, the reactivity of (CH2C)-C-.(CH2)CO+ toward spin traps may be expected to resemble that of the neutral allyl radical. This issue was examined experimentally by using Fourier transform ion cyclotron resonance mass spectrometry. Similar to other carbon-centered free radicals, the gaseous (CH2C)-C-.(CH2)CO+ abstracts H-. from benzeneselenol, CH3Se. from dimethyl diselenide, and I-. from allyl iodide. However, some of these reactions occur at efficiencies that suggest catalysis by the charged group. Further, the reaction of (CH2C)-C-.(CH2)CO+ with dimethyl disulfide was found to proceed in an unprecedented manner. In addition to the expected formation of CH3SCH2C(CH2)CO+ and CH3S. (via CH3S. abstraction), a new distonic ion, (CH3SCHC)-C-.(CH2)CO+, is generated. This ion is likely produced upon H-. abstraction by CH3S. from CH3SCH2C(CH2)CO+ within the collision complex (net CH2S abstraction). The latter reaction has not been reported for distonic radical cations with localized radical sites. Hence, the reaction must be driven by the regeneration of the delocalized allylic radical. This proposal is supported by the observation of CH2S abstraction for the product ion (CH3SCHC)-C-.(CH2)CO+ that also contains a delocalized radical site.

The reactivity of the dimethylene ketene radical cation (a distonic ion) toward organic disulfides was examined inside a Fourier-transform ion cyclotron resonance mass spectrometer. The radical cation efficiently cleaves the disulfide bond in all the disulfides studied. Hence, this radical cation provides a potentially useful tool for the mass spectrometric characterization and location of disulfide bonds in neutral substrates without the requirement of prior derivatization.

Molecular orbital calculations (Becke3LYP/3-21G(d)) indicate that the distonic acylium ions •CH2CH2CH2CO+ and •CH2CH2CO+ are localized σ-radicals with a spin density of 0.95 e and 0.92 e at the terminal methylene carbon, respectively. Most of the positive charge (+0.88) is localized on the carbonyl carbon. The radical site carries a charge that is only slightly more positive than that in the neutral ethyl radical. Based on these values, the distonic acylium ions may be expected to behave like neutral alkyl radicals. However, an examination of the reactivity of gaseous •CH2CH2CO+ and •CH2CH2CH2CO+ toward a series of alkyl halides in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) revealed that their reactivity is greatly influenced by the charged group. The ions •CH2CH2CO+ and •CH2CH2CH2CO+ are unreactive toward alkyl chlorides but react with alkyl bromides and iodides by electron, halogen atom, alkyl radical and/or hydrogen atom abstraction. While hydrogen and halogen atom abstraction reactions are common for neutral alkyl radicals, electron and alkyl abstractions are not. A mechanism that involves catalysis by the charge site is proposed for the alkyl abstraction reactions.

The phosphenium ion CH3OPOCH3+ readily attacks hydroxyl groups of neutral substrates in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The electrophilic character of CH3OPOCH3+ 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, CH3OPOCH3+ 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 CH3OPOCH3+ provides a powerful chemical ionization reagent for the mass spectrometric determination of the stereochemistry of diols.

The reactivity of the prototypical phosphorus-containing ylidion (alpha-distonic ion) (CH2PH3+)-C-. has been investigated in the gas phase by using a dual cell Fourier-transform ion cyclotron resonance mass spectrometer. The ion (CH2PH3+)-C-. and its more stable conventional isomer CH3PH2.+ show distinctly different reactivities toward neutral reagents. This observation contrasts the facile interconversion of the analogous sulfur- and oxygen-containing distonic ions (CH2SH2+)-C-. and (CH2OH2+)-C-. with their conventional isomers CH3OH.+ and CH3SH.+, respectively, within collision complexes in the gas phase. Bracketing experiments yield a proton affinity of 190.4 +/- 3 kcal mol(-1) for the phosphorus atom in (CH2PH2)-C-.. Together with a calculated heat of formation for (CH2PH2)-C-., this value yields a heat of formation of 217 +/- 3 kcal mol(-1) (at 298 K) for the distonic ion (CH2PH3+)-C-..