Electron spin resonance studies of phenyl and pyridyl radicals in aqueous solution

Abstract

Publication costs assisted by Carnegie-Meilon University and the U. S. Energy Research and Development AdministrationESR spectra of a number of phenyl and 2-pyridyl radicals have been detected in aqueous solution. Two methods of radical production were used, namely, reaction of SO4.-produced by photolysis of &Os2-with aromatic carboxylate ions and reaction of eaq-produced by radiolysis with aromatic bromides. Most radicals had only carboxyl groups as further substituents. The proton hyperfine constants of the phenyl radicals, a, -17, um -6, and up -2 G, are readily assigned on the basis of effects of the various substitutions and agree well with previous determinations for radicals in the solid state. With four phenyl radicals containing two to four carboxyl groups it was also possible to detect all of the possible 13C containing isomers at natural abundance. A hyperfine constant of -135 G for the carbon at the radical site confirms the u nature of phenyl radical. The other ring 13C hyperfine constants are uoc -7.5, urnc -13, and upc -1.5 G.Both 14N and proton hyperfine constants were determined for the 2-pyridyl radicals. Only the 14N value (-27 G) was known previously. The g factors of phenyl and 2-pyridyl radicals are low (near or below the free electron value) reflecting their u nature. The presence of carboxyl groups in a phenyl radical affects the g factor in an additive fashion with increments of 13, -2, and -3 units in the fifth decimal place for ortho, meta, and para substituents, respectively. The observation of phenyl and 2-pyridyl radicals demonstrates in a direct way that these radicals are formed in the two above-mentioned reactions. (One example each of a 4-pyridyl and a pyrazyl radical were also produced and studied.) Phenyl and 2-pyridyl radicals with no ortho carboxyl group were found to be very reactive toward addition to another aromatic molecule and were best detected using low concentrations (5 X lov4 M ) of aromatic bromide. Spectra of the unsubstituted phenyl and 2-pyridyl radicals were obtained in this way from the corresponding bromides. At higher solute concentrations such as used for the decarboxylation reactions these radicals react further to form adducts of the cyclohexadienyl type. Radicals with an ortho carboxyl group (for example, 2-carboxylphenyl radical produced from phthalate) are much less reactive toward addition and could be studied at higher concentrations of the starting compound. Reaction of SO4--with acids in which three (or more) adjacent carboxyl groups are present leads preferentially to loss of a central or interior carboxyl group. To provide data for comparison with earlier indirect studies, a number of adducts of phenyl and pyridyl radicals to CH2=N02-and trimesate were studied and their hyperfine constants determined. In the latter cases it was found that the ortho and meta protons in the phenyl radical which has added (i.e., the side group in the final cyclohexadienyl radical) can produce splittings of -0.05 and -0.3 G, respectively. The radicals detected in exp...

Initiators Based on Benzaldoximes: Bimolecular and Covalently Bound Systems

“…From the associated time traces it can be inferred that 6 produces 7 via decarboxylation within about 500 ns. The extracted hyperfine coupling constants (hfcs) for 7 are in agreement with published data 36 and calculations (see Figure 8). No signals stemming from (reduced) BP or related radicals could be identified, again indicating that BP only acts as a sensitizer.…”

Radical reactions in aqueous solution: use of the aci-anion of nitromethane as a spin trap

“…(10) and PhCH2N02-' (11) are correct stems from the observation that these same two signals are detected (32) when peroxybenzoic acid is reduced by Ti"' under identical conditions. As expected if reactions [27]- [30] are involved, the intensity of the signal assigned to 10 increased at the expense of 11 as [CH2:N02-] was increased (precisely as found for the PhC02-I SO4-. oxidation).…”

“…As mentioned earlier, oxidation of benzoate ion with SO4-' in the presence of CH2:N02-at pH 9 led to the detection of signals assigned to PhC02CH2N02-' and PhCH2N02-' (evidently formed as in reactions [9] and ['IQ]), as well as to those of 02NCH2CH2N02-• and -0,-0SCH2N02-' formed by direct reaction between SO,-' and the trap (32). The assignment PhC02-CH2N02-' has been challenged (27,48) on the grounds that an (unassigned) spectrum with these parameters is also detected from the reaction between photolytically generated SO,--and CH2: NO2-in the absence of benzoate (48). However, important structural evidence (which also provides interesting mechanistic insight) that the assignments to PhC02CH2N02-.…”

Semi-Empirical Evaluation of the Probability of Structural Rearrangements in Nitrone Spin Traps

“…Calculation suggests that cycle opening should be expected at the bond between the two methylene carbon atoms, this process being most probable for the oxidized form. We can find some experimental evidence in literature: various complications in interpreting the EPR spectra of DMPO spin adducts were reported (for example, see [12,13,14]). We again evidence a contradiction between the desired high sensitivity of a spin trap and the requirement for its stability [15].…”