Correlation of Hydrogen-Atom Abstraction Reaction Efficiencies for Aryl Radicals with their Vertical Electron Affinities and the Vertical Ionization Energies of the Hydrogen-Atom Donors

Abstract

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.

Tracking radical migration in large hydrogen deficient peptides with covalent labels: Facile movement does not equal indiscriminate fragmentation

“…7.1 and 15.9 kJ/ mol, respectively. These results agree with previous conformationally unconstrained calculations on related aromatic molecules [39]. Therefore, for a large variety of possible sites, the only substantial barriers to radical migration in peptides will originate from conformational constraints.…”

Hydroxyl radical formation in the gas phase oxidation of distonic 2-methylphenyl radical cations

“…Rate coefficients for m/z 149 from PD of 3Br4Me and 4Br3Me reveal a slight, but reproducible, difference in reaction rate where the radical is at the 3-over the 4-position relative to the charge-tag, i.e., k 3Br4Me > k 4Br3Me . This effect could be due to small differences in the electrophilicity of the two positive distonic radical ions 24,52 but overall the reaction efficiencies are very similar to each other and to previous measurements for phenyl-type radicals. For example, the measured reaction efficiencies in Table 2 are similar to those reported by Kirk et al 31 for positively charge-tagged distonic phenyl radicals (about 5%), suggesting, intriguingly, that the methyl substituent, although presenting a new reaction pathway, does not drastically alter the overall reaction efficiency.…”

Ion–molecule reactions reveal facile radical migration in peptides

“…12 In contrast, other computational studies have estimated significantly smaller barriers. [13][14][15] In addition, a body of indirect experimental evidence also suggests that radical migration occurs in peptides prior to fragmentation. [16][17][18] However, experiments requiring collisional activation or other fragmentation methods are an imperfect probe of the structure of the nascent radical ions given that the possibility of radical migration (over substantial activation barriers), immediately prior to fragmentation, cannot be excluded.…”