The factors that control the reactivities of aryl radicals toward hydrogen-atom donors were studied by using a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer (FT – ICR). Hydrogen-atom abstraction reaction efficiencies for two substrates, cyclohexane and isopropanol, were measured for twenty-three structurally different, positively-charged aryl radicals, which included dehydrobenzenes, dehydronaphthalenes, dehydropyridines, and dehydro(iso)quinolines. A logarithmic correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) vertical electron affinities (EA) of the aryl radicals. Transition state energies calculated for three of the aryl radicals with isopropanol were found to correlate linearly with their (calculated) EAs. No correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) enthalpy changes for the reactions. Measurement of the reaction efficiencies for the reactions of several different hydrogen-atom donors with a few selected aryl radicals revealed a logarithmic correlation between the hydrogen-atom abstraction reaction efficiencies and the vertical ionization energies (IE) of the hydrogen-atom donors, but not the lowest homolytic X – H (X = heavy atom) bond dissociation energies of the hydrogen-atom donors. Examination of the hydrogen-atom abstraction reactions of twenty-nine different aryl radicals and eighteen different hydrogen-atom donors showed that the reaction efficiency increases (logarithmically) as the difference between the IE of the hydrogen-atom donor and the EA of the aryl radical decreases. This dependence is likely to result from the increasing polarization, and concomitant stabilization, of the transition state as the energy difference between the neutral and ionic reactants decreases. Thus, the hydrogen-atom abstraction reaction efficiency for an aryl radical can be “tuned” by structural changes that influence either the vertical EA of the aryl radical or the vertical IE of the hydrogen atom donor.

An experimental method is described for obtaining quantitative selectivity information for H-atom abstraction by organic radicals from different sites of a substrate in the gas phase. The method is used to determine the selectivities of five different phenyl radicals toward the three different types of hydrogen atoms in ethanol. This experimental method involves studying the reactivities and selectivities of derivatives of the radicals that contain a chemically inert, charged group (distonic ions), which allows them to be manipulated in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. A large number of studies suggest that the action of some potential anti-tumor antibiotics is based on H-atom abstraction by aromatic, ,-biradical intermediates from a sugar moiety in DNA (which ultimately leads to cell death) [1][2][3][4][5][6][7][8][9]. These compounds, however, are also destructive to healthy cells. An understanding of the factors that control the selectivity of aryl monoradicals and biradicals in H-atom abstraction reactions could aid the development of drugs that are more selective and, thus, less cytotoxic [10]. Here, we report the results for the first of a series of gas-phase studies on the selectivity of aryl radicals in H-atom abstraction reactions.An examination of H-versus D-atom abstraction from a partially labeled substrate by a phenyl radical yields semi-quantitative information about the selectivity of the radical. The difficulty, however, is that the selectivity observed in such reactions does not exactly correspond to the selectivity that is observed for the same site in an unlabeled compound. If a kinetic isotope effect (KIE) exists, the presence of a heavy isotope can affect the relative abundances of the products. That is, D-atom abstraction is likely to be slower, relative to H-atom abstraction, from the same site in the same molecule. Thus, in order to use the results obtained from selectivity studies of reactions of deuterium labeled compounds to understand the selectivities of the reactions of unlabeled analogs, it is necessary to develop a method that accounts for any potential KIE.Dunlop and Tully have previously reported a method for obtaining site-specific rate constants for H-atom abstraction by HO⅐ from 2-propanol in the gas phase by using a laser photolysis/laser-induced fluorescence technique [11]. However, this approach is based on the assumption that the KIE measured for HO⅐ for abstraction of a primary (methyl) H-/D-atom from ethane, or neopentane, represents the relative reactivity of HO⅐ toward CH 3 -and CD 3 -groups in 2-propanol. Herein, we report an approach that can be used to obtain quantitative selectivity information for radicals in H-atom abstraction reactions that involve partially deuterium-labeled substrates without the assumption made by Dunlop and Tully, and the application of this method for the determination of the selectivities of five substituted phenyl radicals toward the three different types of hydrogen atoms in ethanol. Our method is ...

The vertical electron affinity is demonstrated to be a key factor in controlling the selectivity of charged phenyl radicals in hydrogen atom abstraction from isopropanol in the gas phase. The measurement of the total reaction efficiencies (hydrogen and/or deuterium atom abstraction) for unlabeled and partially deuterium-labeled isopropanol, and the branching ratios of hydrogen and deuterium atom abstraction, by using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, allowed the determination of the selectivity for each site in the unlabeled isopropanol. Examination of hydrogen atom abstraction from isopropanol by eight structurally different radicals revealed that the preferred site is the CH group. The selectivity of the charged phenyl radicals correlates with the radical's vertical electron affinity and the reaction efficiency. The smaller the vertical electron affinity of a radical, the lower its reactivity, and the greater the preference for the thermodynamically favored CH group over the CH3 group or the OH group. As the vertical electron affinity increases from 4.87 to 6.28 eV, the primary kinetic isotope effects decrease from 2.9 to 1.3 for the CD group, and the mixture of primary and alpha-secondary kinetic isotopes decreases from 6.0 to 2.4 for the CD3 group.

Oxidation of benzene, toluene, ethylbenzene, and xylenes (BTEX) in air, of significance due to, for example, the potential for O 3 formation, is believed to be initiated by OH attack on the ring (addition) or on the alkyl side chain (H abstraction). A series of ring-breaking reactions follows, with major products predicted to be α-dicarbonyls, simple aldehydes, and organic acids. To test this prediction, ambient air mixing ratios of aldehydes (formaldehyde, acetaldehyde, benzaldehyde, glyoxal, and pyruvaldehyde), along with some supporting BTEX data, were measured at an urban site in Las Vegas, NV. Samples were collected on sorbents and determined by chromatographic methods; mixing ratios were compared to ambient levels of CO, O 3 , and NO x . A meteorological analysis (temperature, wind speed, and wind direction) was also included. Statistically significant relationships were noted among the BTEX hydrocarbons (HCs) and among the photochemically derived species (e.g., O 3 , NO 2 , and some of the aldehydes), although there was seasonal variation. The observations are consistent with a common primary source (i.e., vehicular exhaust or fuel evaporation) for the BTEX compounds and a common secondary source (e.g., OH attack) for glyoxal and pyruvaldehyde.

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An experimental method is described for obtaining quantitative selectivity information for H-atom abstraction by organic radicals from different sites of a substrate in the gas phase. The method is used to determine the selectivities of five different phenyl radicals toward the three different types of hydrogen atoms in ethanol. This experimental method involves studying the reactivities and selectivities of derivatives of the radicals that contain a chemically inert, charged group (distonic ions), which allows them to be manipulated in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. A large number of studies suggest that the action of some potential anti-tumor antibiotics is based on H-atom abstraction by aromatic, ,-biradical intermediates from a sugar moiety in DNA (which ultimately leads to cell death) [1][2][3][4][5][6][7][8][9]. These compounds, however, are also destructive to healthy cells. An understanding of the factors that control the selectivity of aryl monoradicals and biradicals in H-atom abstraction reactions could aid the development of drugs that are more selective and, thus, less cytotoxic [10]. Here, we report the results for the first of a series of gas-phase studies on the selectivity of aryl radicals in H-atom abstraction reactions.An examination of H-versus D-atom abstraction from a partially labeled substrate by a phenyl radical yields semi-quantitative information about the selectivity of the radical. The difficulty, however, is that the selectivity observed in such reactions does not exactly correspond to the selectivity that is observed for the same site in an unlabeled compound. If a kinetic isotope effect (KIE) exists, the presence of a heavy isotope can affect the relative abundances of the products. That is, D-atom abstraction is likely to be slower, relative to H-atom abstraction, from the same site in the same molecule. Thus, in order to use the results obtained from selectivity studies of reactions of deuterium labeled compounds to understand the selectivities of the reactions of unlabeled analogs, it is necessary to develop a method that accounts for any potential KIE.Dunlop and Tully have previously reported a method for obtaining site-specific rate constants for H-atom abstraction by HO⅐ from 2-propanol in the gas phase by using a laser photolysis/laser-induced fluorescence technique [11]. However, this approach is based on the assumption that the KIE measured for HO⅐ for abstraction of a primary (methyl) H-/D-atom from ethane, or neopentane, represents the relative reactivity of HO⅐ toward CH 3 -and CD 3 -groups in 2-propanol. Herein, we report an approach that can be used to obtain quantitative selectivity information for radicals in H-atom abstraction reactions that involve partially deuterium-labeled substrates without the assumption made by Dunlop and Tully, and the application of this method for the determination of the selectivities of five substituted phenyl radicals toward the three different types of hydrogen atoms in ethanol. Our method is ...

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