Disproportionation reactions between alkyl and fluoroalkyl radicals. V. Perfluoro-n-propyl and ethyl radicals revisited

A redetermination of the disproportionation/combination ratio for n–C₃F₇ and C₂H₅ radicals gives a value of Δ(n–C₃F₇, C₂H₅) = 0.13 ± 0.01, independent of the temperature. The radicals were produced by the photolysis of n–C₃F₇COC₂H₅. The previous determinations of this ratio are discussed and are found to be largely incorrect. The values for Δ(CF₃, C₂H₅) and Δ(C₂F₅, C₂H₅) are also re-evaluated, and the recommended values are 0.10 ± 0.02 and 0.12 ± 0.02, respectively. Systems involving perfluoroalkyl and ethyl radicals are complicated due to rapid perfluororadical addition to the ethylene formed in the disproportionation process. The extent of this reaction, and its consequences, are discussed and evaluated. The role of the propionyl (C₂H₅CO) radical in the room temperature photolysis is also assessed. However, it is found that the Δ values determined by the intercept method used in this work are not affected by the secondary reactions that occur. It is concluded that high cross-combination ratios are general to perfluoroalkyl-alkyl radical interactions. For C₃F₇ and C₂H₅ radicals the ratio is 2.7–2.8. Above 100°C ratios exceed 3 due to secondary reactions.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. III. A reassessment of Δ values for CFyH(3−y) radicals (y = 1,2,3) with C2H5 radicals

New determinations of the disproportionation and combination ratios between CF₂H and C₂H₅ radicals yield (the hydrogen acceptor radical is given first) Δ(CF₂H, C₂H₅) = 0.068 ± 0.008, and Δ(C₂H₅, CF₂H) = 0.37 ± 0.01. A reevaluation of the existing data on CFH₂ and CF₃ radicals leads to the following recommended values, Δ(CFH₂, C₂H₅) = 0.038 ± 0.006, and Δ(CF₃, C₂H₅) = 0.11 = ± 0.02.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. VII. Difluoromethyl with perfluoro-n-propyl and methyl radicals

Mixtures of 1,1,3,3-tetrafluoroacetone and perfluorodi-n-propyl ketone have been photolyzed together over the temperature range 50° to 200°C, and the disproportionation/combination ratio for n-C₃F₇ and CF₂H radicals has been determined to be Δ(n-C₃F₇, CF₂H) = 0.072 ± 0.003. A reevaluation of existing data on CH₃ and CF₂H radicals leads to a value of Δ(CH₃, CF₂H) = 0.35. The large variations in Δ for the reactions of alkyl and perfluoroalkyl radicals with CF₂H radicals are discussed. © John Wiley & Sons, Inc.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. II. Fluoro- and trifluoromethyl with ethyl radicals

Cross-disproportionation/combination ratios for CFH₂ and CF₃ with C₂H₅ radicals have been determined to be Δ = 0.032 ± 0.012 and δ = 0.098 ± 0.020, respectively, over the temperature range 25–75°C. For the pathway that yields CFH and C₂H₆, δ = 0.020 ± 0.005 at 25°C.     

The reaction of C2F5 radicals with H2S

The reaction of C₂F₅ radicals with H₂S was studied over the range 1°?123°C using C₂F₅ radicals generated by photolysis of perfluoropropionic anhydride. The rate constant k_H for reaction (2) is given by where θ = 2.303RT/cal mole^?1. The relevance of this result to conflicting published data on the analogous reaction between CF₃ radicals and H₂S is discussed. It is concluded that there is little difference in the Arrhenius parameters for reaction of CF₃ and C₂F₅ radicals with H₂S.     

Hydrocarbon radicals and nitroxy complexes on the surface of HZSM-5 and CuZSM-5 zeolites: An ESR study

The conditions favorable for the formation of oligomer and benzene cation radicals upon C₂H₄, C₆H₆, and C₃H₆ adsorption on thermally activated HZSM-5 and CuZSM-5 zeolites were studied. In the case of isolated copper ions, the radicals were shown to be formed from C₃H₆ but not from C₂H₄ and C₆H₆. Paramagnetic nitroxy complexes, which were formed as a result of the interaction of adsorbed hydrocarbons C₂H₄, C₆H₆, and C₃H₆ with NO and NO₂, were revealed on HZSM-5 zeolite and Al₂O₃. Hydrocarbon radicals and nitroxy complexes are localized on different zeolite sites. Nitroxy complexes are formed on aluminum cations. The role of radicals and nitroxy complexes in the process of NO _x selective catalytic reduction by hydrocarbons on the zeolites was discussed on the basis of the temperature dependences of the concentrations of these species.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. VI. Difluoromethyl and n-propyl radicals

Cross-disproportionation to combination ratios for CF₂H and n-C₃H₇ radicals have been determined (the hydrogen acceptor radical is given first) to be Δ(n-C₃H₇, CF₂H) = 0.30 ± 0.01 and Δ(CF₂H, n-C₃H₇) = 0.057 ± 0.006.     

Disproportionation reactions between CF2H and C2H5 radicals in the gas phase

The disproportionation and combination reactions between CF₂H and C₂H₅ radicals have been studied in the gas phase from room temperature to 70°C. For the pathway which yields CF₂H₂ + C₂H₄, relative to CF₂HC₂H₅, Δ = 0.072 ± 0.019. The competing reaction channel that produces CF₂ + C₂H₆ is approximately four times as efficient, with Δ = 0.265 ± 0.038. With CF₂D radicals an isotope effect for the CF₂ + C₂H₅D reaction pathway was observed with Δ ? 0.2.     

Addition of ethyl radicals to carbon monoxide. Kinetic and thermochemical properties of the propionyl radical

Azoethane was irradiated in the presence of carbon monoxide in the temperature range of 238 to 378 K. Kinetic parameters for the addition of ethyl radicals to carbon monoxide and for the decomposition of propionyl radicals were determined. The rate constants were found to be log k(cm³ mol^?1 sec^?1) = 11.19 - 4.8/θ and log k(sec^?1) = 12.77 - 14.4/θ, respectively. Estimated thermochemical properties of the propionyl radical are ΔH_f⁰ = -10.6 ± 1.0 kcal mol^?1, S⁰ = 77.3 ± 1.0 cal K^?1 mol^?1, and D(C₂H₅CO? H) = 87.4 kcal mol^?1.     

Determination of enthalpies of formation of organic free radicals from bond dissociation energies 1. Hydrocarbon radicals

The enthalpies of formation (ΔH _f ^o) for 24 hydrocarbon radicals (R^?), mainly polycyclic aromatic radicals with the complex structure, were determined from the published data on bond dissociation energies. The ΔH _f ^o values of the corresponding molecules were calculated, in the majority of cases, by the macroincrement method. Calculations by the group contribution method were performed. Some ΔH _f ^o(R^?) values were compared to those calculated by the additive-group method. Calculations were performed, and the conjugation energies of the radicals were discussed. The errors of determination of the ΔH _f ^o(R^?) values found were estimated. Due to this work, the database for ΔH _f ^o values of hydrocarbon radicals was increased more than by 25%.     

Reactions of primary and secondary butoxy radicals in oxygen at atmospheric pressure

The reaction pathways of n-butoxy and s-butoxy radicals have been investigated by TLC and HPLC analysis of end products, particularly peroxides and carbonyl compounds. The butoxy radicals were produced by the pyrolysis of very low concentrations of the corresponding dibutylperoxide in an atmosphere of oxygen and nitrogen, at atmospheric pressure. The decomposition reaction (3) s-BuO → C₂H₅ + CH₃CHO and the reaction (2) s-BuO + O₂ → HO₂ + CH₃COC₂H₅ have been studied, and the ratio k₃/k₂ has been determined in the temperature range 363–503 K by kinetic modeling of the formation of the observed acetaldehyde and methylethylketone. The rate constant k₃ obtained was: A good agreement was observed between experimental data and RRKM theory. The implications of the results for atmospheric chemistry and combustion are discussed. At room temperature, the reaction with O₂, yielding HO₂ radicals and methylethylketone is, by far, the main channel for s-BuO radicals. In the field of low temperature combustion, the decomposition of s-BuO radicals producing C₂H₅ and CH₃CHO is the main pathway; the route s-BuO + O₂ decreases tremendously in importance as the temperature is raised above 393 K.     

Enthalpies of formation for alkylcarbonyl radicals

The data on enthalpies of formation (Δ_f H ^○) of alkylcarbonyl radicals are expanded to 22 items. The reference value of Δ_f H ^○ for diacetylperoxide in the gas phase (?485.5 kJ/mol) is determined via recalculation from evaporation enthalpy (n-C₅H₁₁C(O)O)₂. The Δ_f H ^○ values of 22 diacylperoxides (gaseous) are calculated and used in combination with the literature data on dissociation energies of D(O-O) bonds in them to determine the Δ_f H ^○ of corresponding radicals. The interrelation between structure and properties (the enthalpy of formation) is considered, and the parameters for the calculated prediction of Δ_f H ^○ are found.     

Disproportionation-combination rate constant ratios for haloalkyl radicals: Evidence for heterogeneous disproportionation

Disproportionation-combination rate constant ratios, k_d/k_c, have been determined for R + RCH₂CHCl and for the auto disproportionation-combination of RCH₂CHCl radicals, R = CF₃, C₂F₅, and C₃F₇. The k_d/k_c for R = CF₃ and to a lesser degree for R = C₂F₅ and C₃F₇ were very sensitive to the surface/volume ratio of the reaction vessel suggesting a heterogeneous component for disproportionation.     

Conformations and electronic properties of alkyl peroxides,alkylperoxy radicals and alkoxy radicals

The equilibrium geometric parameters have been determined for peroxides (H₂O₂, CH₂O₂H, and CH₃O₂CH₃), alkylperoxy radicals (HO₂, CH₃O₂), and C₂H₅O₂), and alkoxy radicals (HO, CH₃O, and C₂H₅O) by means of the semiempirical MINDO/2' method. The predicted bond lengths were found to be more reasonable than those estimated by the CNDO/2 and INDO methods, but the bond angles were predicted too large because of the insufficient parametrizations for such heteroatoms as oxygen. The electronic properties of the above species are also discussed in connection with their physicochemical constants.     

Kinetic and thermochemical parameters of chlorine atom transfer reactions from CF3Cl,CF3CF2Cl,CF2ClCF2Cl,and CF2ClCFCl2 in gas phase

The following reactions: (1) were studied over the temperature ranges 533–687 K, 563–663 K, and 503–613 K for the forward reactions respectively and over 683–763 K, for the back reaction. Arrhenius parameters for chlorine atom transfer were determined relative to the combination of the attacking radicals. The ΔH_r°(1) = ?3.95 ± 0.45 kcal mol^?1 was calculated and from this value the ΔH∮(C₂F₅Cl) = ?2.66.3 ± 2.5 kcal mol^?1 and D(C₂F₅-Cl) = 82.0 ± 1.2 kcal mol^?1 were obtained. Besides, the ΔH_r°(2) was estimated leading to D(CF₂ClCF₂Cl) = 79.2 ± 5 Kcal mol^?1. The bond dissociation energies and the heat of formation are compared with those of the literature. The effect of the halogen substitutents as well as the importance of the polar effects for halogen transfer processes are discussed.     

Isotope and temperature effects on the lifetime of excited diphenyl ketyl radicals

The lifetime of excited diphenyl ketyl radicals is lengthened by deuterium substitution. The largest effect is observed by substitution at the hydroxylic position; for example, the lifetimes are 3.9, 4.2, 8.7 and 10.5 ns for (C₆H₅)₂COH, (C₆D₅)₂COH, (C₆H₅)₂COD and (C₆D₅)₂COD, respectively, in toluene or toluene-d₈ at room temperature. Deuterium substitution in the solvent has no effect other than providing a different atom for the hydroxylic position in the ketyl radical.     

Isomerization reactions of the n-C4H9O and n-OOC4H8OH radicals in oxygen

Reactions of n-C₄H₉O radicals have been investigated in the temperature range 343–503 K in mixtures of O₂/N₂ at atmospheric pressure. Flow and static experiments have been performed in quartz and Pyrex vessels of different diameters, walls passivated or not towards reactions of radicals, and products were analyzed by GC/MS. The main products formed are butyraldehyde, hydroperoxide C₄H₈O₃ of MW 104, 1-butanol, butyrolactone, and n-propyl hydroperoxide. It is shown that transformation of these RO radicals occurs through two reaction pathways, H shift isomerization (forming C₄H₈OH radicals) and decomposition. A difference of activation energies ΔE = (7.7 ± 0.1 (σ)) kcal/mol between these reactions and in favor of the H-shift is found, leading to an isomerization rate constant k_isom (n-C₄H₉O) = 1.3 × 10¹² exp(− 9,700/RT). Oxidation, producing butyraldehyde, is proposed to occur after isomerization, in parallel with an association reaction of C₄H₈OH radicals with O₂ producing OOC₄H₈OH radicals which, after further isomerization lead to an hydroperoxide of molecular weight 104 as a main product. Butyraldehyde is mainly formed from the isomerized radical HOCCCC˙ + O₂ ··· → O (DOUBLE BOND) CCCC + HO₂, since (i) the ratio butyraldehyde/(butyraldehyde + isomerization products) = 0.290 ± 0.035 (σ) is independent of oxygen concentration from 448 to 496 K, and (ii) the addition of small quantities of NO has no influence on butyraldehyde formation, but decreases concentration of the hydroperoxides (that of MW 104 and n-propyl hydroperoxide). By measuring the decay of [MW 104] in function of [NO] added (0–22.5 ppm) at 487 K, an estimation of the isomerization rate constant OOC₄H₈OH → HOOC₄H₇OH, κ₅ ≅ 10¹¹exp(−17,600/RT) is made. Implications of these results for atmospheric chemistry and combustion are discussed. © 1996 John Wiley & Sons, Inc.     

Addition of sulfur radicals to fullerenes

The addition of oxygen‐centered radicals to fullerenes has been intensively studied due to their role in cell protection against against hydrogen peroxide induced oxidative damage. However, the analogous reaction of sulfur‐centered radicals has been largely overlooked. Herein, we investigate the addition of S‐centered radicals to C₅₀, C₆₀, C₇₀, and C₁₀₀ fullerenes by means of DFT calculations. The radicals assayed were: S, SH, SCH₃, SCH₂CH₃, SC₆H₅, SCH₂C₆H₅, and the open‐disulfide SCH₂CH₂CH₂CH₂S. Sulfur, the most reactive species, prefers to be attached to a 66‐bond of C₆₀ with a binding energy (E_bind) of 2.4 eV. For the SR radicals the electronic binding energies to C₆₀ are 0.77, 0.74, 0.58, 0.67, and 0.35 eV for SH, SCH₃, SCH₂CH₃, SCH₂C₆H₅, and SC₆H₅, respectively. The reactivity of C₆₀ toward SR radicals can be increased by lithium doping. For Li@C₆₀, the E_bind is increased by 0.65 eV with respect to C₆₀, but only by 0.33 eV for the exohedral doping. Fullerenes act like free radical sponges. Indeed, the C₆₀‐SR E_bind can be duplicated if two radicals are added in ortho or para positions. The enhanced reactivity because of multiple additions is mostly a local effect, although the addition of one radical makes the whole cage more reactive. Therefore, as observed for hydroxylated fullerenes, they should protect cells from oxidative damage. However, the thiolated fullerenes have one advantage, they can be easily attached to gold nanoparticles. For the addition on pentagon junctions smaller fullerenes like C₅₀ are more reactive than C₆₀. Interestingly, C₇₀ is as reactive as C₆₀, even for the addition on the equatorial belt. For larger fullerenes like C₁₀₀, reactivity decreases for the carbon atoms belonging to hexagon junctions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Photolysis of 1,1,1-trifluoromethylazomethane as a source of methyl and trifluoromethyl radicals

The photochemistry of 1,1,1-trifluoromethylazomethane has been partially characterized. The quantum yield for N₂ formation from photolysis at 366 nm and room temperature was unity at low pressure and decreased to 0.5 at 630 torr. At room temperature the principal products were C₂H₆, C₂F₆, CH₃CF₃ (or CH₂CF₂ + HF at reduced pressures), plus substituted hydrazines, which mainly arise from addition of CF₃ to the parent followed by combination of these radicals with CH₃ or CF_3. These fluorinated methyl hydrazine products detract from the general utility of CF₃-N₂-R compounds as sources for simultaneous study of the chemistry of CF₃ and R radicals. At room temperature the hydrazine products accounted for more than 50% of the total yield; however, these products can be reduced by lowering the temperature and at 195°K their yields are negligible. The quantum yield for intramolecular (direct) formation of CH₃CF₃ + N₂ was shown to be ≤0.002.     

Self- and cross-disproportionation-to-combination rations for cyclopentyl and cyclohexyl radicals and their deuterated analogues

Hydrogen, cycloalkene, and bicycloalkyl were found to be the principal products which account for ≈?97% of all products formed in the gas-phase radiolysis of water vapor containing low concentrations of cycloalkanes. From the ratios of cycloalkene-to-bicycloalkyl yields extrapolated to the zero dose, the self- and cross-disproportionation-to-recombination rate constant ratios Δ = k_d/k_c were determined for the following 12 reactions: Δ(c-C₅H₉, c-C₅H₉) = 0.73; Δ(c-C₅D₉, c-C₅D₉) = 0.58; Δ(c-C₆H₁₁, cC₆H₁₁) = 0.59; Δ(c-C₆D₁₁, c-C₆D₁₁) = 0.46; Δ(c-C₅H₉, c-C₆H₁₁) = 0.28; Δ(c-C₅D₉, c-C₆H₁₁) = 0.28; Δ(c-C₅H₉, c-C₆D₁₁) = 0.24; Δ(c-C₅D₉, c-C₆D₁₁) = 0.24; Δ(c-C₆H₁₁, c-C₅H₉) = 0.33; Δ(c-C₆H₁₁, c-C₅D₉) = 0.25; Δ(c-C₆D₁₁, c-C₅H₉) = 0.35; and Δ(c-C₆D₁₁, c-C₅D₉) = 0.28, where in the case of the cross-disproportionation the symbol Δ(R₁,R₂) is used to represent k_d/k_c for the disproportionation in which radical R₁ captures a hydrogen (deuterium) atom from radial R₂. The geometrical mean rule holds in the cross-combination reactions of cyclopentyl and cyclohexyl radicals. The kinetic isotope effect in the disproportionation reaction was determined as 1.24 ± 0.06.