Investigation of perfluoroperoxy radicals self-coupling reactions by kinetic EPR spectroscopy

The rate constants and activation parameters for the self-coupling of perfluoroperoxy radicals of structure A and B: C₇F₁₅OO (A) and R_FOCF₂OO (B) have been determined in perfluorohexane solution in the temperature range 228-258 K. The magnitude of the rate constants obtained ranks between 6.6×10⁸ and 2.5×10⁹ l mol⁻¹ s⁻¹ and are therefore, among the largest rate values so far reported in the literature for primary peroxy radicals couplings. The activation energy is positive and lower for the peroxy radicals (A) with respect to the peroxy radicals (B) (10.5 and 23.0 kJ mol⁻¹, respectively).Analysis by kinetic modeling has shown that the peroxy radicals decay curves are compatible with the participation of peroxy radicals↔tetroxide equilibria to the reaction mechanism.Upper limit values of k_bs<10 and <20 s⁻¹ were inferred for the β-scission reactions of the perfluoroalkylperoxy radicals at 228 and 258 K, respectively.     

Muon level-crossing spectroscopy of organic free radicals

Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.     

Monolayer-protected gold nanoparticle coalescence induced by photogenerated radicals

Acyl and alkyl radicals generated photochemically in a solution containing monolayer-protected gold nanoparticles are shown to efficiently liberate the alkylthiolate ligands into the solution as the thioacetyl or alkyl sulfide, respectively. This radical-induced reaction initiates a coalescence of the gold cores to facilitate an increase in core size. The photoinitiated radical reaction also liberates monolayers from two-dimensional gold surfaces.     

Characterization of titanium dioxide photoactivity following the formation of radicals by EPR spectroscopy

In order to find ways to characterize oxygen-saturated aqueous TiO₂ suspensions, the formation of photo-induced free radicals was followed by EPR spectroscopy, using as indicators N-oxide and nitrone spin trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POB N), 4-(N-methylpyridyl)-N-tert-butylnitrone (MePyBN), as well as semi-stable free radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL), cation radical of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). DMPO and TMPO are efficiently oxidized to the EPR-silent products via radical in termediates. Conversely, the nitrone spin traps (POBN and MePyBN) showed selective formation of hydroxyl radical spin adducts upon continuous irradiation of oxygenated TiO₂ suspensions. Their concentrations increased proportionally with the amount of photocatalyst and irradiation time. The EPR spectrum of the semi-stable free radicals TEMPOL, ABTS^·+ or DPPH is gradually eliminated during irradiation, and this system represents a simple technique for the evaluation of TiO₂ activity.     

EPR spectroscopy of tooth enamel: the tooth radicals and the microcrystal alignment

The paramagnetic radicals induced by radiation in dental enamel are very important because they can be related to the crystalline structure of hydroxyapatite. The R-value, that is, the ratio of the amplitude of the lines of the EPR signal due to radiation, is a measure of the degree of microcrystal alignment in human tooth enamel. The aim of this study is to underline the importance of a correct evaluation of the R-value, by using the current method to increase reproducibility in EPR spectroscopy of tooth enamel. Data with and without correction show a significant difference and, consequently, they give rise to a different valuation of microcrystal alignment.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.     

Generation of radicals by the photolysis of branched fluoroolefins and their study by EPR spectroscopy

The UV irradiation of branched fluoroolefins leads to the formation of free radicals due to the addition of CF₃ to the starting olefin. Branched fluoroolefins containing a fluorine atom at the radical site are incapable of recombination but react with oxygen.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1926–1929, August, 1989.     

The numerical calculation of ESR spectra of randomly oriented quartet radicals

The ESR spectra of randomly oriented quartet radicals were calculated with the aid of an analytical formula. A line broadening through Lorentz lines was explicity taken into consideration. The experimental spectrum of an alternant hydrocarbon radical was simulated with sufficiently good agreement.     

EPR spectra of halogenonitromethane anion radicals

Conclusions It was deduced that fluorine is in a -combined state in the anion radicals we studied; this was based on an analysis of the EPR spectra of a series of fluoronitroalkane anion radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 458–460, February, 1969.In conclusion, the authors express their appreciation to A. A. Fainzil'berg for his interest in our work.     

Transient magnetic resonance of a photoexcited quartet state

Wide-band detection following pulsed laser excitation of the decacyclene anion has yielded transient election paramagnetic resonance signals which we assign to an excited quartet state. The spin-spin coupling constant D and the path of intersystem crossing from doublet to quartet have been determined.     

EPR study of free radicals in bread

The features of the recorded EPR spectra of paramagnetic species formed in bread and rusk are reported. The appearance of free radicals in them is only connected with their thermal treatment since the starting materials (flour and grains) exhibit very weak EPR signal. The obtained EPR spectra are complex and indicate that: (i) the relative number of paramagnetic species depends on the temperature and treating time of the raw product; (ii) the g-values are strongly temperature dependent with a tendency to coincide at t > or = 220 degrees C. Because of the relatively low (150-220 degrees C) temperature of thermal treatment, the studied free radicals can be assumed to appear in the course of the browning (Maillard) reaction and not to the carbonization of the material.     

EPR spectra of halo-substituted 2,4,6-triphenylpyranyl radicals

The hyperfine structure of three halo-substituted pyranyl radicals, 2,6-bis(p-chlorphenyl)-4-phenylpyranyl, 2,6-bis(perfluorophenyl)-4-phenylpyranyl, and 2,6-bis(perfluorophenyl)-4-(o,p-dichlorophenyl)pyranyl, was investigated by EPR. The EPR spectra contained splitting from the protons of the aromatic rings and magnetic fluorine nuclei. An ortho-effect phenomenon was detected, the ratio of the constants of splitting on the ortho and para fluorine atoms a _F ⁿ /a _F ^o =2, unlike in the case of unsubstituted pyranyl radicals, where a _H ⁿ a _H ^O .Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 741–743, November–December, 1986.     

W-band time-resolved electron paramagnetic resonance spectroscopy on transient organic radicals in solution

Time-resolved electron paramagnetic resonance (TREPR) spectra of spin-polarized transient radicals in liquid solution, generated in a continuous flow-system of a W-band (95 GHz) high-field (3.4 T) EPR spectrometer, are reported. The organic free radicals are created by laser flash photolysis of ω,ω-dimethoxy-ω-phenylacetophenone (DMPA) and diphenyl-2,4,6-trimethylbenzoil phosphine oxide (TMDPO) inside the microwave cavity, and are observed at 10 ns to 20 μs delay times after the laser pulse. The analysis of the positions of the well-separated EPR signals at W-band yields the g-values of the observable transients with high accuracy. The chemically induced dynamic electron polarization (CIDEP) patterns are different from those in conventional X-band (9.5 GHz) EPR. This is due to different spin relaxation times at different magnetic fields, to field-dependent CIDEP mechanisms operating in the studied systems, and to the increased Boltzmann polarization at high fields.     

EPR characterization of new phosphavinyl σ radicals

A number of radicals have been added to 1,3-bis(2,4,6-tri-tert-butylphenyl)diphospha-allene at low temperature in cyclopropane solution. Addition of thiyl and alkoxyl radicals occurs regiospecifically to one of the phosphorus atoms leading to new phosphavinyl radicals, isoelectronic with vinyl radicals, which have been characterised by means of EPR spectroscopy. The results of ab initio calculations are in agreement with the experimentally determined spectral parameters and suggest that these radicals are bent, with a PCP bond angle of ca. 150°. In contrast, silyl and germyl radicals appear to add to the carbon atom of the phosphaallene.