The role of triplet repulsion in alkyl radical addition to a 7π-C-O bond and alkoxy radical addition to a π-C-C bond

The experimental activation energies of the R^• + O = CR¹R² and RO^• + CH₂ = CHR¹ addition reactions are analyzed within the framework of the parabolic model of the bimolecular addition reaction. The activation energy also depends on the dissociation energy of the forming C-O bond and on the reaction enthalpy: the higher the dissociation energy, the higher the activation energy. The empirical relationshipr _e J..D _e = 0.97 x 10-¹³ m kJ.-¹ mol is found for H^•, Cl^•, Br^{• •} and RO^• radical addition to multiple C=C and C=O bonds (r_e is the distance between the peaks of the intersecting parabolic curves). This is due to the effect of the triplet repulsion on radical addition. The interaction of polar groups and the steric effect also influence the activation energy.     

Kinetics of radical decomposition of di(tert-butyl) trioxide

The kinetics of radical decomposition of di(tert-butyl) trioxide was studied by spectrophotometry from the consumption of an acceptor of free radicals, 2,6-di(tert-butyl)-4-methylphenol, in CFCl₃ and CH₂Cl₂ (in the latter case, in the presence of 0.1M Bu^tOOH). The activation parameters of the reaction (log(A/s ⁻¹)=14.8±1.2 and 14.1±1.6,E _a=21.6±1.4 and 20.1±1.9 kcal mol⁻¹ in CFCl₃ and CH₂Cl₂, respectively) and the probability of radical escape to the bulk (e=0.9±0.1) were determined. The known experimental and calculated values of the O−OO bond strength in trioxides were analyzed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 61–65, January, 1999.     

Kinetics of the reaction between dimethyldioxirane and 2-methylbutane

The kinetics of the reaction between dimethyldioxirane and 2-methylbutane in acetone solutions were studied spectrophotometrically at 25 °C. The radical-chain induced decomposition of dioxirane proceeding with the participation of the carbon-centered radicals follows the first-order kinetic law. The reaction is inhibited by dioxygen. In the presence of O₂, the dimethyldioxirane consumption is due to the homolysis of the O−O bond (at a rate constant of 6.3·10⁻⁴ s⁻¹) followed by attack of the C−H bond of 2-methylbutane by the biradical formed. The rate constant of the reaction between the alkyl radical and dimethyldioxirane was estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1785–1788, October, 1997.     

Kinetic parameters and geometry of the transition state of acyl radical decarbonylation

Experimental data on acyl radical decomposition reactions (RC^·O → R^· + CO, where R = alkyl or aryl) are analyzed in terms of the intersecting parabolas method. Kinetic parameters characterizing these reactions are calculated. The transition state of methyl radical addition to CO at the C atoms is calculated using the DFT method. A semiempirical algorithm is constructed for calculating the transition state geometry for the decomposition of acyl radicals and for the reverse reactions of R^· addition to CO. Kinetic parameters (activation energy and rate constant) and geometry (interatomic distances in the transition state) are calculated for 18 decomposition reactions of structurally different acyl radicals. A linear correlation between the interatomic distance r ^#(C…C) (or r ^#(C…O)) in the transition state the enthalpy of the reaction (δH _e) is established for acyl decomposition reactions (at br _e = const). A comparative analysis of the enthalpies, activation energies, and interatomic distances in the transition state is carried out for the decomposition and formation of acyl, carboxyl, and formyl radicals.     

Determination of dissociation energies of N-H bond in the 4-anilinodiphenylaminyl radical and O-H bond in the 2,5-dichloro-4-hydroxyphenoxyl radical from the equilibrium constants of chain reactions in quinoneimine-hydroquinone systems

The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)—2,5-dichlorohydroquinone systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine was estimated from these data (ΔH = − 14.4±1.6 kJ mol⁻¹) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (D _NH = 278.6±3.0 kJ mol⁻¹). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation energy in 2,5-dichlorohydroquinone: D _OH = 362.4±0.9 kJ mol⁻¹. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: D _OH = 253.6±1.9 kJ mol⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1661–1666, October, 2006.     

Reduced parabolic model for radical addition reactions

The transition state of addition of free radicals and atoms to multiple bonds is considered as a result of intersecting of two parabolic potential curves. One of them characterizes the stretching vibration of the attacked multiple bond, and another curve characterizes the stretching vibration of the bond formed in the transition state. The force constant of the latter is calculated by an empirical equation that correlates the force constant with the bond dissociation energy. In the framework of this model, the thermally neutral activation energy (E _e0) and the elongation of the attacked and formed bonds (r _e) in the transition state were calculated from the experimental data (activation energy (E _e) and enthalpy of reaction (H _e)) for the addition of an H atom and methyl, alkoxyl, aminyl, triethylsilyl, and peroxyl radicals to the C=C bond and the addition of H and CH₃ to the C=O and CC bonds. Analysis of the data obtained showed that E _e0 depends linearly on the |H _e| + E_e sum, i.e., E_e0/kJ mol^–1 = 14.2 + 0.61 · (E_e – H _e), and the bond elongation in the transition state for addition of the most part of radicals to ethylene and acetylene vary within (0.65–0.87)·10_–10 m. The factors affecting the activation energy of the radical addition reactions are discussed.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1542–August, 2004.     

Radical addition reactions: factors determining the transition-state geometry

Interatomic distances in the reaction centers of the addition reactions of (i) H^· to the C=C, C=O, N≡C, and C≡C bonds, (ii) ^·CH₃ radical to the C=C, C=O, and C≡C bonds, and (iii) alkyl, aminyl, and alkoxyl radicals to olefin C=C bonds were determined using a new semiempirical method for calculating transition-state geometries of radical reactions. For all reactions of the type X^· + Y=Z → X— Y—Z^· the r ^# _X...Y distance in the transition state is a linear function of the enthalpy of reaction. Parameters of this dependence were determined for seventeen classes of radical addition reactions. The bond elongation, Δr ^# _X...Y, in the transition state decreases as the triplet repulsion, electronegativity difference between the atoms X and Y in the reaction center, and the force constant of the attacked multiple bond increase. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 894–902, April, 2005.     

Determination of enthalpies of formation of organic free radicals from bond dissociation energies

The values of C−H and C−I bond dissociation energies were used to calculate the enthalpies of formation (δH _f ^o of 20 cyclic and conjugated hydrocarbon radicals (R′). The values of δH _f ^o (R′) were analyzed in terms of the quantitative structure-property correlation based on the additive-group model, and the reliability of these data was shown. Based on the correlation, several strain energies of cycles and energies of conjugation of a lone electron with a ρ-system were calculated. The additive-group method for calculation of δH _f ^o can be extended for radicals of the naphthalyl type. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 286–288, February, 1999.     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Pulse radiolysis study of dithio-oxamide in aqueous solutions

The reactions of e⁻ _aq, H-atoms, OH radicals and some one electron oxidants and reductants were studied with dithio-oxamide (DTO) in aqueous solutions using pulse radiolysis technique. The transient species formed by the reaction of e⁻ _aq with DTO at pH 6.8 has an absorption band with λ _max at 380 nm and is reducing in nature. H-atom reaction with DTO at pH 6.8 also produced the same transient species. The semi-reduced species was found to be neutral indicating that the electron adduct gets protonated quickly. However at pH 1, the species produced by H-atom reaction had a different spectrum with λ _max at 360 and 520 nm. Reaction of acetone ketyl radicals and CO₂ ⁻ radicals with DTO at pH 6.8 gave transient spectra which were identical to that obtained by e⁻ _aq reaction. However at pH 1, the spectrum obtained by the reaction of acetone ketyl radicals with DTO was similar to that obtained by H-atom reaction at that pH. The transient species formed by OH radical reaction with DTO in the pH range 1–9.2 also has two absorption maxima at 360 and 520 nm. This spectrum was identical with the spectrum obtained by H-atom reaction at pH 1. This means that all these radicals viz. OH, H-atom and (CH₃)₂COH radicals react with DTO at pH 1 by H-abstraction mechanism. The transient species produced was found to be sensitive to the presence of oxygen. One-electron oxidizing radicals such as Br₂ ^−· and SO₄ ^−· radicals reacted with DTO at neutral pH to give the same species as produced by OH radical reaction having absorption maxima at 360 to 520 nm. At acidic pHs, only Br₂ ^−· and Cl₂ ^−· radicals were able to oxidize DTO to give the same species as produced by OH radical reaction. The semioxidized species is a resonance stabilized species with the electron delocalized over the-N-C-S bond. This species was found to be neutral and non-oxidizing in nature.     

Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

The β bond dissociation of alkyl radicals and their reverse reactions, the addition of alkyl radicals to olefins were studied by G3MP2 level of theory to obtain a consistent kinetic data set. Both reaction families can be classified depending on the type of radical formed by β bond scission, namely the CH₃, primary, secondary tertiary radical formed. The kinetics of the reaction classes were described by only a limited number of Arrhenius parameters. The unified A factor of 10^13.7 s⁻¹ was found for all β bond dissociations. The Arrhenius activation energies are 125, 121, 113 and 103 kJ mol⁻¹, for methyl, primary, secondary, and tertiary radicals, respectively. The activation energies of 32, 25 and 18 kJ mol⁻¹ are calculated for the terminal addition of primary (including methyl), secondary, and tertiary radicals to olefins, respectively. The biologically important nonterminal radical additions to olefins have higher barriers of 37, 31 and 35 kJ mol⁻¹, respectively. At room temperature both strongly exothermic additions can compete with H-atom abstraction. New groups for Benson’s group additivity rules were defined to describe activation parameters for the β bond dissociation reactions. The group values were calculated by using the ab initio heats of formation of transition state structures.     

Accurate calculations of dissociation energies of weakly bonded He<Subscript>2</Subscript> and Be<Subscript>2</Subscript> molecules by MRCI method

The He₂ and Be₂ ground state potential curves have been calculated by extrapolating to an infinite basis BSSE corrected MRCI total energies obtained with large Gaussian basis sets, large reference configuration spaces, and pseudo-natural molecular orbitals. The calculated D _e = 11.0031 K and R _e = 5.607 a.u. of He₂ are in an excellent agreement with D _e = 11.006 ± 0.004 K and R _e = 5.608 ± 0.012 a.u. obtained recently by SAPT with SM energy correction. The obtained Be₂ non-relativistic D _e = 822 cm⁻¹ and relativistically corrected D _e = 818 cm⁻¹ are in a good agreement with experimental D _e = 790 ± 30 cm⁻¹ and the value of 829 ± 64 cm⁻¹ obtained recently by a quantum Monte Carlo method.     

A study of alkyl radicals in the matrix of polycrystallinen-alkane γ-irradiated at 77 K

The composition of alkyl radicals (AR) formed by γ-radiolysis (T=77 K) of polycrystallinen-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(11), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiatedn-alkanes are superpositions of the signals from the H₃CC^.HCH₂− and −CH₂C^.HCH₂− radicals, whose HFS constants with α and β protons as well as the equilibrium conformation are independent of the chain length of then-alkane molecule. A dependence of the concentration of the radicals on the chain length ofn-alkane was found. The absence of the −CH₂C^.H₂ radicals that may arise upon H atom elimination from the Me fragments of then-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the −CH₂C^.H₂ radicals into H₃CC^.HCH₂− radicals. With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the corresponding C atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1034–1037, June, 2000.     

Pulsed KrF excimer laser induced degradation of cellulose based insulating paper

We report on the accelerated ageing of cellulose based insulating paper by means of pulsed UV laser irradiation (λ = 248 nm) under various experimental conditions including paper composition, background gas (He, N₂ and air) and moisture content of the paper. The temperature reached by the paper samples during their laser irradiation was monitored by means of real-time IR imaging. It is shown that the equilibrium temperature (T _eq) reached by the paper increases from ~30 to ~270 °C when the laser energy density was raised from 15 to 550 mJ cm⁻². The laser irradiated samples were systematically characterized by means of scanning electron microscopy (SEM) observations and degree of polymerization (DP_v) measurements. Interestingly, it is found that, for a given moisture content, the degradation level of the cellulose is mainly triggered by the T _eq value reached during the laser irradiation. Moreover, their moisture content was found to influence significantly the number of laser produced bond scissions (it doubles when the moisture content is increased from 0.5 to 6%); the paper degradation is apparently not affected by the presence of oxygen as the background gas. These results suggest that the laser induced cellulose degradation occurs through a direct photolysis (i.e. direct breakage of C–C, C–O and C–H bonds), leading to radicals formation, which, in turn, are believed to induce the acid hydrolysis degradation mechanism, the latter being moisture dependent. The activation energy (E _a) of each gaseous species collected after the laser degradation was estimated. Their E _a values were found to be in good agreement with the one associated to the laser depolymerisation of cellulose (i.e. ~56 kJ mol⁻¹), suggesting thereby a direct correlation between the cellulose degradation and the formation of the detected gaseous species. Finally, the pulsed laser irradiation can be seen as an attractive tool to identify primarily generated molecules, on a very short time scale, that can be used as relevant chemical markers for the monitoring of the ageing of transformers materials with cellulose.     

Rate constants for reactions of Cl abstraction from CCl4 by CCl3CH2·CHR radicals and Br abstraction from CCl3CH2CHBrR (R=Bun, AcO, OCNC4H8, CN) by ·Re(CO)5 radicals

The rate constants for reactions of Cl abstraction from CCl₄ by CCl₃CH₂·CHR radicals and Br abstraction from CCl₃CH₂CHBrR (R=Buⁿ, AcO, OCNC₄H₈, CN) by·Re(CO)₅ radicals were determined by ESR spectroscopy using spin trapping technique. Replacement of H atoms at the C(β) atom by O or N atoms reduces the reactivity of the radicals in the reactions of Cl abstraction from CCl₄ by approximately an order of magnitude. The presence of two polar groups at the C(β) atom results in appreciable decrease in the strength of the C−Br bond in CCl₃CH₂CHBrR adducts. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 45–48, January, 2000.     

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.     

Cyclopropane ring opening in the reaction of methylenecyclopropane with silyl radicals stabilized on an activated Aerosil surface

Alkyl type radicals stable at room temperature and incorporating a double bond not conjugated with the free valence, ≡Si−C(=CH₂)−CH₂−CH₂, are formed in the reaction of methylenecyclopropane with silyl radicals (≡SiO)₃Si on an activated Aerosil surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1065–1066, May, 1997.     

Reactivity of the silyl, germanyl, and stannyl radicals in addition reactions: The effect of the atomic radius on the activation energy

Experimental data on the addition of silyl, germanyl, and stannyl radicals to olefins are analyzed in the framework of a three intersecting parabolas model. The parameters characterizing these reactions are calculated. The activation energy of the thermally neutral reaction for this class of reactions depends on both the strength of the formed bond and the radius of the atom bearing the free valence. The dependence is the following: E _{e, 0} ^1/2 ～ αD _e + bD _e ^3/2 r _C-X ^1/2 , where D _e is the strength of the formed bond and r _C-X is its length. Steric repulsion is observed in the reactions of the silyl radicals with symmetrically substituted ethylene derivatives. The presence of a π-bond or aromatic ring near the attacked double bond increases E _{e, 0}. The increments are calculated that characterize the contribution to the activation energy from the following factors: the enthalpy of the reaction, triplet repulsion, steric hindrance, and effect of adjacent π electrons.     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000