No abstract

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.

Reactions of differently substituted phenyl radicals with components of nucleic acids have been investigated in the gas phase. A positively charged group located meta with respect to the radical site was employed to allow manipulation of the radicals in a Fourier-transform ion cyclotron resonance mass spectrometer. All of these electrophilic radicals react with sugars via exclusive hydrogen atom abstraction, with adenine and uracil almost exclusively via addition (likely at the C8 and C5 carbons, respectively), and with the nucleoside thymidine by hydrogen atom abstraction and addition at C5 in the base moiety (followed by elimination of (*)CH(3)). These findings parallel the reactivity of the phenyl radical with components of nucleic acids in solution, except that the selectivity for addition is different. Like HO(*), the electrophilic charged phenyl radicals appear to favor addition to the C5-end of the C5-C6 double bond of thymine and thymidine, whereas the phenyl radical preferentially adds to C6. The charged phenyl radicals do not predominantly add to thymine, as the neutral phenyl radical and HO(*), but mainly react by hydrogen atom abstraction from the methyl group (some addition to C5 in the base followed by loss of (*)CH(3) also occurs). Adenine appears to be the preferred target among the nucleobases, while uracil is the least favored. A systematic increase in the electrophilicity of the radicals by modification of the radicals' structures was found to facilitate all reactions, but the addition even more than hydrogen atom abstraction. Therefore, the least reactive radicals are most selective toward hydrogen atom abstraction, while the most reactive radicals also efficiently add to the base. Traditional enthalpy arguments do not rationalize the rate variations. Instead, the rates reflect the radicals' electron affinities used as a measure for their ability to polarize the transition state of each reaction.

No abstract

Laser-induced acoustic desorption combined with mass spectrometry has been used to demonstrate that phenyl radicals can attack dinucleoside phosphates at both the sugar and base moieties, that purine bases are more susceptible to the attack than pyrimidine bases, and that the more electrophilic the radical, the more efficient the damage to dinucleoside phosphates.