Free radicals and their electron spin relaxation in cellobiose. X-band and W-band ESR and electron spin echo studies

By use of 9.7 GHz and 94 GHz ESR spectra and electron spin echo (ESE)-detected spectra the six radical centres produced by γ-irradiation of cellobiose were identified. The radicals are localized on different carbon atoms. Use of high-frequency ESR spectra with computer resolution enhancement methods enabled unique radical identification and determination of g-factors and proton hyperfine splitting, A, with high accuracy. For radiation doses below 20 kGy three radicals dominate: on C₁ with isotropic doublet A = 1.8 mT; on C₂, C₃ and C₄ with triplet A = 2.9 mT; and localized on CH₂ with anisotropic triplet. For doses above 100 kGy the radical on C₁ dominates, because of cleavage of the glycosidic bonds. Electron spin–lattice relaxation shows that radiation damage of the cellulose structure around the radical centres is significant and radical molecules do not participate in phonon dynamics of the host lattice. The relaxation is because of tunnelling motions of the ring or OH-groups, with tunnelling splitting 2.4 cm⁻¹. Electron spin echo dephasing results identify cellobiose ring torsions with activation energy 117 cm⁻¹.     

Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical

UV irradiation of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the ^·CF₃ radical and perfluoroolefin molecule. About 90% of the ^·CF₃ radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF₃)₂CF]₃C^·. Unlike the ^·CF₃ radicals produced by the photodecomposition of radicals 1, the ^·CF₃ radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K.     

An EPR investigation of acetonitrile reactivity with superoxide radicals on polycrystalline TiO<Subscript>2</Subscript>

The interaction of acetonitrile with superoxide radicals over a polycrystalline TiO₂ (Degussa P25) surface was investigated using continuous-wave electron paramagnetic resonance (cw-EPR) spectroscopy. For the first time, a thermally unstable radical intermediate has been observed following the low-temperature exposure of acetonitrile to surface-adsorbed O₂⁻ radicals. The radical intermediate has been identified as an [O₂⁻···CH₃CN] type surface complex characterised by the g values of g ₁ = 2.031, g ₂ = 2.010 and g ₃ = 2.003. This surface complex is thermally unstable and decomposes at temperatures of T > 240 K. A second oxygen-centred species was also observed following acetonitrile adsorption, characterised by the spin Hamiltonian parameters of g ₁ = 2.028, g ₂ = 2.010, g ₃ = 2.004, A ₁ = 1.2 mT, A ₂ = 1.0 mT and A ₃ = 1.0 mT, and was assigned to a hydroperoxy radical (HO₂^•).     

New complexes of fullerences C60 and C70 with CoII and MnII tetraphenylporphyrinates and their ESR study

Complexes of fullerenes C₆₀ and C₇₀ with cobalt(II) and manganese(II) tetraphenylporphyrinates of compositions Mn(TPP)·(C₆₀)₂(CS₂)_1.5 (1), Mn(TPP)·C₇₀(CS₂) _x , wherex<=1.25 (2), Co(TPP)·C₆₀(CS₂)_0.5 (3), and Co(TPP)·C₇₀(CS₂) _x , wherex<0.25 (4), were synthesized and studied by ESR spectroscopy. At 77 K, complexes1 and2 have singlet ESR spectra characteristic of the low-spin (S=1/2) state of Mn^II, withg=2.002 and linewidths of 250 G and 300 G, respectively, and differing significantly from that of the initial Mn^II(TPP) (g ₁=5.9 andg=2.0,S=5/2). The spectra of complexes1 and2 exposed to oxygen exhibit hyperfine structure due to interaction with⁵⁵Mn and¹⁴N nuclei. The ESR spectra of complexes3 and4 are asymmetric (<g>=2.4, ΔH _pp=(500–600) G), which is due to the overlap of parallel and perpendicular spectral components. The absence of ESR signals from C₆₀ ^.− and C₇₀ ^.− radical anions makes it possible to conclude that the formation of complexes1–4 is not accompanied by electron transfer from Co(TPP) and Mn(TPP) to fullerences C₆₀ and C₇₀. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 722–725, April, 1999.     

Yield and enrichment studies of C-13 isotope by multi-photon dissociation of Freon-22 at low temperatures

Multi-photon dissociation of Freon-22 (CF₂HC1) at low temperatures has been carried out to separate the C-13 isotope using a TEA CO₂ laser. Yield and enrichment of C-13 isotope in the product C₂F₄ are studied at 9P(22) laser line as a function of temperature (−50°C to 30°C). It is observed that at a given fluence when the temperature is lowered the yield decreases and the enrichment factor of C-13 increases. Room temperature irradiation of CF₂HC1 towards the blue edge of C-13 absorption (i.e. at 9P(20) laser line) gives low yield of the product (C₂F₄) at a fluence, which produces the desired enrichment factor of 100. An increase in fluence gives very high yield of C₂F₄ but the enrichment factor is very low. Irradiating CF₂HC1 at a temperature of −10°C enhances the enrichment factor to 100 and the yield obtained is comparable to that towards the red edge of C-13 absorption (i.e. at 9P(26) laser line). At a given enrichment factor higher enrichment efficiency is achieved when CF₂HC1 is irradiated at lower temperature.     

Magnetic properties of fullerene salts containing d- and f-metal cations (Co2+, Ni2+, Fe2+, Mn2+, Eu2+, Cd2+). Specific features of the interaction between C60·? and the metal cations

The interaction between the radical anions C₆₀ ^·− and divalent d- and f-metal (Co, Fe, Ni, Mn, Eu, Cd) cations in DMF and acetonitrile-benzonitrile (AN-BN) mixture was studied. Black solid polycrystalline salts (C₆₀ ^·−)₂{(M²⁺)(DMF) _x } (x = 2.4–4, 1–6) containing the radical anions C₆₀ ^·− and metal(ii) cations solvated by DMF were prepared for the first time and their optical and magnetic properties were studied. The salts containing Co²⁺, Fe²⁺, and Ni²⁺ are characterized by antiferromagnetic interactions between the radical anions C₆₀ ^·−, which result in unusually large broadening of the EPR signal of C₆₀ ^·− upon lowering the temperature (from 5.55–12.6 mT at room temperature to 35–40 mT at 6 K for Co²⁺ and Ni²⁺). The salts containing Mn²⁺ and Eu²⁺ form diamagnetic dimers (C₆₀ ⁻)₂, which causes a jumpwise decrease in the magnetic moment of the complexes and disappearance of the EPR signal of C₆₀ ^·− in the temperature range 210–130 K. A feature of salt 6 is magnetic isolation of the radical anions C₆₀ ^·− due to the presence of diamagnetic cation Cd²⁺. The salts prepared are unstable in air and decompose in o-dichlorobenzene or AN. Reactions of C₆₀ ^·− with metal(ii) cations in AN-BN mixture result in decomposition products of the salts that contain neutral fullerene dimers and metals solvated by BN. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1909–1919, September, 2008.     

New EPR signals of endohedral metallofullerenes

The EPR spectra of endohedral metallofullerenes (EMF), La-EMF and Y-EMF, which were free of admixtures of C₆₀ and of other empty fullerenes, were examined. Endohedral metallofullerenes were prepared by extraction of fullerene-containing soots with DMF. New signals withg factors close to those of fullerene radical anions were observed in the EPR spectra of solutions of EMF in DMF and DMSO. At −20 °C, these signals are observed as a doublet (ΔH _pp ≈0.04 mT) and singlet (ΔH _pp ≈0.01 mT) in solutions of La-EMF and Y-EMF, respectively. These EPR signals belong to solvated La@C₈₂ and Y@C₈₂ molecules and are characterized by small hyperfine interaction constantsa _M due to a substantial decrease in the spin density of the unpaired electron at the metal atom. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1765–1769, October, 2000.     

Analysis of the ESR spectra of the ~ CF2CFCF2~ macroradical trapped in a γ-irradiated polytetrafluoroethylene matrix at 77 K

The Β-fluorine atoms in the ~CF₂CFCF₂~ radical trapped in γ-irradiated polytetrafluoroethylene (PTFE) were found to be nonequivalent and, hence, responsible for a doublet-triplet-triplet ESR spectral pattern. The conformational angle between the axis of the unpaired-electron orbital and the projection of C_Β-C_γ bond is equal to 60‡. The hyperfine coupling constants for α- and Β-fluorine atoms were determined. The hyper-fine splitting constants for two equivalent Β-fluorine atoms were found to be 7.2 mT. The corresponding value for the two other equivalent fluorine atoms is 1.8 mT, whereas the hyperfine splitting due to α-fluorine atom is 23.8 mT.     

Electron spin resonance study of copolymerization of hexafluoroisobutylene with vinylidene fluoride

The Copolymerization reaction of hexafluoroisobutylene (HFIB) and vinylidene fluoride (VF₂) was studied using X-band electron spin resonance (ESR) by applying a photo-in situ method. Owing to the toxicity of HFIB monomer, the flow ESR method could not be directly applied without extensive modification. The observed ESR signal with 21-line hyperfine structure was assigned to the copolymer radical with head-to-head configuration. Although the HFIB monomer radical possesses the same hyperfine pattern, the ambiguity has been removed by using CF₃ as an initiator radical. Owing to the high steady-state concentration of the observed copolymer radical with head-to-head configuration as well as the nature of the static photo-in situ ESR method, we believe the actual molecular configuration for HFIB/VF₂ copolymer must be head-to-tail. The observed hyperfine constant for A_βF = 1.74 mT suggests that the geometry for HFIB/VF₂ copolymer radicals with both head-to-head or head-to-tail configurations is possibly similar to that of (CF₃)₃C· radical. The small value for A_βH indicates steric hindrance to rotation about the C_α? C_β bond, and this is also supported by the experimental results of nonalteration in linewidth during the temperature dependence study from ?40 to about 90°C. Attempts to measure directly the monomer reactivities have been unsuccessful owing to the fact that not all the possible radicals were detected, but nevertheless the relative reactivities of VF₂ and HFIB could be estimated. The relative reactivities of VF₂ and HFIB monomers and the steric hindrance effect indicate that the conformation of the copolymer is head-to-tail; this has been further confirmed by infrared analysis.     

Rate constant for the reaction of bromine atoms with ethane: Kinetic and thermochemical implications

Relative-rate kinetic experiments were carried-out at T = 310 ± 3 K to determine rate constant ratios for the reactions of Br atoms with C₂H₆(1), CH₂ClBr(2) and neo-C₅H₁₂(3). Br atoms were produced by stationary photolysis of Br₂ and the consumption of the reactants was determined by gas-chromatography. k ₂/k ₁ = 1.174 ± 0.053 and k ₃/k ₁ = 0.458 ± 0.027 were determined (with 1σ precision given). The rate constant ratios were resolved to absolute k ₁ values, and k ₁(310 K) = (2.27 ± 0.30) × 10⁵ cm³ mol⁻¹ s⁻¹ was recommended. The recommended k ₁ was applied in a third law analysis providing Δ_f H ^o ₂₉₈(C₂H₅) = (122.0 ± 1.9) kJ mol⁻¹.     

The ESR spectra of the spin-adducts of branched perfluorinated σ-radicals with fullerene-60

The ESR spectra of the radical adducts of the^.CF=CFC(CF₃)₃ and^.C(O)CF(CF₃)₂ radicals with C₆₀ are characterized by hyperfine interaction with the nucleus of the δ-fluorine atom located above the five-membered cycle in the fullerenyl radical. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1222–1224, June, 1997.     

Isomerism in OBe3F3 + cation: anab initio study

Ab initio MP2/6-31G^*//HF/6-31G^*+ZPE(HF/6-31G^*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe₃F₃ ⁺ cation detected in the mass spectra of μ₄-Be₄O(CF₃COO)₆ were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol⁻¹ and those of the remaining four structures lie in the range 20–40 kcal mol⁻¹. Two most favorable isomers, aC _2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC _3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol⁻¹, except for the pyramidalC _3v isomer (∼16 kcal mol⁻¹). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol⁻¹). A planarD _3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe₃H₃ ⁺ cation, a hydride analog of the OBe₃F₃ ⁺ ion; the energies of the remaining five isomers are more than 25 kcal mol⁻¹ higher. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999.     

ESR spectra of trifluoromethyl radical formed during the solid-phase photodecomposition of long-lived perfluoro-2,4-dimethyl-3-ethylpentyl-3 radicals

The products of photolysis of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ at 77 K were studied by ESR. The mechanism of photodecomposition to form ^·CF₃ radicals was proposed. The ESR spectra of the trifluoromethyl radicals stabilized at 77 K in a glassy hexafluoropropylene trimer matrix were simulated, and the parameters À = 25.15 mT, À = 9.1 mT, and g = l.9996, g = 2.0056 were determined.     

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na₃FeF₆ and for the Na₃FeF₆ melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δ_trans H(Na₃FeF₆, 920 K) = (19 ± 3) kJ mol⁻¹ and enthalpy of fusion at the temperature of fusion 1255 K, δ_fusH(Na₃FeF₆, 1255 K) = (89 ± 3) kJ mol⁻¹ have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: C _p(Na₃FeF₆, cr, α) = (294 ± 14) J (mol K)⁻¹, for 723 = T/K ≤ 920, C _p(Na₃FeF₆, cr, β) = (300 ± 11) J (mol K)⁻¹ for 920 ≤ T/K = 1233 and C _p(Na₃FeF₆, melt) = (275 ± 22) J (mol K)⁻¹ for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.      

ESR study of radicals formed by the reduction of α-diketones C(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with several group I–III metals

The ESR spectra of radical anions formed by reduction of α-diketones RC(O)C(O)CF₃ (R=(CF₃)₂CF, C₆F₅, (CF₃)₃C) with metals (Li, Na, K, Mg, Cd, Zn, Hg, In, and TI) in THF were studied. For R=(CF₃)₂CF and C₆F₅, the radical anions are formed ascis-isomers, whereas for R=(CF₃)₃C,trans-isomers are obtained. Line broadening due to solvation and desolvation of the cation is observed in the latter case. The reduction of α-diketone (CF₃)₂CFC(O)C(O)CF₃ with Group II metals (Mg, Cd, Zn) results in the formation of radical pairs. Translated fromIzvestiya Akadmii Nauk. Seriya Khimicheskaya, No. 11, pp. 2228–2231, November, 1998.     

Investigation of CF2 carbene on the surface of activated charcoal in the synthesis of trifluoroiodomethane via vapor-phase catalytic reaction

This paper investigates the synthetic mechanism of trifluoroiodomethane (CF₃I) in the reaction of trifluoromethane and iodine via vapor-phase catalytic reaction. It is suggested that CF₂ carbene is the key intermediate and is formed in the pyrolysis process of CHF₃ at high temperature. However, in pyrolysis of CHF₃ under activated charcoal (AC) existing conditions, no C₂F₄ was detected. H₂ and 2-methyl-2-butene could not trap the CF₂ carbene. When treating the remained compounds on the used AC with H₂, CH₄ is formed on the process. It is proposed that CF₂ carbene combines with AC strongly and transfers into CF₃ radical on heat. In addition, it is found that the AC is not only the catalyst supporter to form CF₃I, but also a co-catalyst to promote the formation of CF₂ carbene and CF₃ radical.     

Searching blue-shifted O–H···Y hydrogen bond in CH<Subscript>3</Subscript>OH complexes with CF<Subscript>4</Subscript>, C<Subscript>2</Subscript>F<Subscript>2</Subscript>, OC,Ne, and He: a theoretical study

The hydrogen-bonded structures of the CH₃OH complexes with CF₄, C₂F₂, OC, Ne, and He are designated as the starting points for geometry optimizations without and with counterpoise (CP) correction at MP2 level of theory with the basis sets 6-31+G*, 6-31++G**, and 6-311++G**, respectively. Tight convergence criteria are applied throughout all geometry optimizations in order to reduce the computational errors. According to the optimizations without CP correction, a blue-shifted O–H···Y (where Y = F, O, Ne, or He) hydrogen bond exists in all these five complexes. The magnitudes of blue shifts of ν(O–H) of the former four complexes with respect to that of CH₃OH are reduced greatly when the polarization and diffuse functions of the hydrogen atoms are added (results from 6-31+G* versus those from 6-31++G**). However, for the complexes CH₃OH–CF₄ and CH₃OH–C₂F₂, our optimizations using the CP corrections did not find the hydrogen-bonded structure to be a stationary point. The energy minimum of both the complexes corresponds to a non-hydrogen-bonded structure.     

Temperature dependence of EPR spectrum of 2-trifluoromethylnitrobenzene radical anion in acetonitrile

Temperature dependence of the EPR spectrum of 2-trifluoromethylnitrobenzene radical anion in anhydrous acetonitrile in the temperature range 217 K ≤ T ≤ 296 K was studied and simulated. Temperature-dependent dynamic modulation of the fluorine isotropic hyperfine structure is caused by slow hindered rotation of CF₃ group with an activation energy of E _F* = 36.5±0.5 kJ mol⁻¹, which is the highest value for motions in π-type free radicals studied to date. Dedicated to the memory of Academician V. A. Koptyug on the occasion of the 75th anniversary of his birth. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 926–931, June, 2006.     

Spin-spin coupling constant 3 J H,F as a reliable criterion for recognition of individual regioisomeric and tautomeric pairs of H(CF2)2-containing isoxazoles and pyrazoles

The spin-spin coupling constant ³ J _H,F of the H(CF₂)₂ group varies within 1.6—3.5 Hz for 5-R^F- and 3.8—4.5 Hz for 3-R^F-isoxazoles and pyrazoles in CDCl₃ and can serve as a reliable criterion for recognition of regioisomeric and tautomeric structures of H(CF₂)₂-containing heterocyclic compounds.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.