Reaction Of Phenyl Radicals Research Articles

A theoretical and experimental kinetic study of phenyl radical addition to butadiene

The reactions of phenyl radical (C 6H 5) are of growing technological and scientific interest. A better understanding of phenyl radical addition to unsaturated hydrocarbons is of great practical interest because it is believed to be an essential component in both soot and fullerene formation. In this study, the rate of phenyl radical addition to butadiene was measured, and the potential surface of the reaction C 6H 5 + C 4H 6 was explored using quantum chemistry with the B3LYP density functional. Vibrational analysis allowed the determination of thermodynamic data and deduction of high-pressure-limit rate constants via transition-state theory. The pressure and temperature dependences of this chemically activated reaction were computed using a weak collision master equation analysis. The comparison of the predictions for the C 6H 5 + C 4H 6 system with experimental data showed good agreement. The rate constant for disappearance of phenyl radical was found to be (3.16 ± 0.29) × 10 12 cm 3/mol-s exp [−(870 ± 30)/ T] over the temperature range 298–450 K. The dominant product at low temperature is the initial adduct, 4-phenyl-buten-3-yl. Around 1000 K, the dominant product is phenyl butadiene formed from the chemically activated adduct, even at 10 atm. Above about 1400 K, bimolecular H-abstraction to form benzene is the most important process. Other products such as 1,4-dihydronaphthalene are much less important.

Combined Quantum Chemical/RRKM-ME Computational Study of the Phenyl + Ethylene, Vinyl + Benzene, and H + Styrene Reactions

Reactions of phenyl radicals with ethylene (R1), vinyl radicals with benzene (R2), and H-atoms with styrene (R3) are important prototype processes pertinent to the formation and degradation of aromatic hydrocarbons in high-temperature environments. Detailed mechanisms for these reactions are elucidated with the help of quantum chemical calculations at the G2M level of theory. Reactions R1−R3 initially produce chemically activated intermediates interconnected by isomerization pathways on the extended [C8H9] potential energy surface. All kinetically important transformations of these isomeric C8H9 radicals are explicitly characterized and utilized in the construction of multichannel kinetic models for reactions R1−R3. Accurate thermochemistry is evaluated for the key intermediates from detailed conformational and isodesmic analyses. An examination of the G2M energetic parameters for reactions R1−R3 and for briefly revisited C6H5 + C2H2 and C6H6 + H addition reactions reveals common theoretical deficiencies ...

Kinetics and Mechanisms for the Reactions of Phenyl Radical with Ketene and its Deuterated Isotopomer: An Experimental and Theoretical Study

Kinetics and mechanism for the reaction of phenyl radical (C6H5) with ketene (H2C beta=C alpha=O) were studied by the cavity ring-down spectrometric (CRDS) technique and hybrid DFT and ab initio molecular orbital calculations. The C6H5 transition at 504.8 nm was used to detect the consumption of the phenyl radical in the reaction. The absolute overall rate constants measured, including those for the reaction with CD2CO, can be expressed by the Arrhenius equation k = (5.9 +/- 1.8) x 10(11) exp[-(1160 +/- 100)/T] cm3 mol-1 s-1 over a temperature range of 301-474 K. The absence of a kinetic isotope effect suggests that direct hydrogen abstraction forming benzene and ketenyl radical is kinetically less favorable, in good agreement with the results of quantum chemical calculations at the G2MS//B3LYP6-31G(d) level of theory for all accessible product channels, including the above abstraction and additions to the C alpha, C beta, and O sites. For application to combustion, the rate constants were extrapolated over the temperature range of 298-2500 K under atmospheric pressure by using the predicted transition-state parameters and the adjusted entrance reaction barriers E alpha = E beta = 1.2 kcal mol-1; they can be represented by the following expression in units of cm3 mol-1 s-1: k alpha = 6.2 x 10(19)T-23 exp[-7590/T] and k beta = 3.2 x 10(4)T2.4 exp[-246/T].

Kinetics of phenyl radical reactions with propane, n‐butane, n‐hexane, and n‐octane: Reactivity of C6H5 toward the secondary CH bond of alkanes

AbstractThe kinetics of C6H5 reactions with n‐CnH2n+2 (n = 3, 4, 6, 8) have been studied by the pulsed laser photolysis/mass spectrometric method using C6H5COCH3 as the phenyl precursor at temperatures between 494 and 1051 K. The rate constants were determined by kinetic modeling of the absolute yields of C6H6 at each temperature. Another major product C6H5CH3 formed by the recombination of C6H5 and CH3 could also be quantitatively modeled using the known rate constant for the reaction. A weighted least‐squares analysis of the four sets of data gave k (C3H8) = (1.96 ± 0.15) × 1011 exp[−(1938 ± 56)/T], and k (n‐C4H10) = (2.65 ± 0.23) × 1011 exp[−(1950 ± 55)/T] k (n‐C6H14) = (4.56 ± 0.21) × 1011 exp[−(1735 ± 55)/T], and k (n−C8H18) = (4.31 ± 0.39) × 1011 exp[−(1415 ± 65)T] cm3 mol−1 s−1 for the temperature range studied. For the butane and hexane reactions, we have also applied the CRDS technique to extend our temperature range down to 297 K; the results obtained by the decay of C6H5 with CRDS agree fully with those determined by absolute product yield measurements with PLP/MS. Weighted least‐squares analyses of these two sets of data gave rise to k (n−C4H10) = (2.70 ± 0.15) × 1011 exp[−(1880 ± 127)/T] and k (n−C6H14) = (4.81 ± 0.30) × 1011 exp[−(1780 ± 133)/T] cm3 mol−1 s−1 for the temperature range 297‐‐1046 K. From the absolute rate constants for the two larger molecular reactions (C6H5 + n‐C6H14 and n‐C8H18), we derived the rate constant for H‐abstraction from a secondary CH bond, ks−CH = (4.19 ± 0.24) × 1010 exp[−(1770 ± 48)/T] cm3 mol−1 s−1. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 49–56, 2004

Reaction of Phenyl Radicals with Acetylene: Quantum Chemical Investigation of the Mechanism and Master Equation Analysis of the Kinetics

The mechanism of the C(6)H(5) + C(2)H(2) reaction has been investigated by various quantum chemical methods. Electrophilic addition to the CC triple bond is found to be the only important mode of phenyl radical attack on acetylene. The initially formed chemically activated C(6)H(5)C(2)H(2) adducts may follow several isomerization pathways in competition with collisional stabilization and H-elimination. Thermochemistry of various decomposition and isomerization channels is evaluated by the G2M method. For key intermediates, the following standard enthalpies of formation have been deduced from isodesmic reactions: 94.2 +/- 2.0 kcal/mol (C(6)H(5)CHCH), 86.4 +/- 2.0 kcal/mol (C(6)H(5)CCH(2)), and 95.5 +/- 1.8 kcal/ mol (o-C(6)H(4)C(2)H(3)). The accuracy of theoretical predictions was examined through extensive comparisons with available experimental and theoretical data. The kinetics and product branching of the C(6)H(5) + C(2)H(2) reaction have been evaluated by weak collision master equation/Rice-Ramsperger-Kassel-Marcus (RRKM) analysis of the truncated kinetic model including only kinetically important transformations of the isomeric C(8)H(7) radicals. Available experimental kinetic data can be quantitatively reproduced by calculation with a minor adjustment of the C(6)H(5) addition barrier from 3.7 to 4.1 kcal/mol. Our predicted total rate constant, k(R1) = (1.29 x 10(10))T(0.834) exp(-2320/T) cm(3) mol(-)(1) s(-)(1), is weakly dependent on P and corresponds to the phenylation process under combustion conditions (T > 1000 K).

Elementary reactions of the phenyl radical, C$_\mathsf{6}$H$_\mathsf{5}$, with C$_\mathsf{3}$H$_\mathsf{4}$ isomers, and of benzene, C$_\mathsf{6}$H$_\mathsf{6}$, with atomic carbon in extraterrestrial environments

Binary collisions of ground state carbon atoms, C( 3 Pj), with benzene, C6H6(X 1 A1g), and of phenyl radicals, C6H5(X 2 A1), with methylacetylene, CH3CCH(X 1 A1), were investigated in crossed beam experiments, ab initio calculations, and via RRKM theory to elucidate the underlying mechanisms of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The reactions of phenyl radicals with allene, H2CCCH2, and with cyclopropene, cyc-C3H4, as well as the reaction of benzyl radicals, C6H5CH2, with acetylene, HCCH, were also inves- tigated theoretically. The C( 3 Pj) atom reacts with benzene via complex formation to a cyclic, seven membered C7H5 doublet radical plus atomic hydrogen. Since this pathway has neither an entrance nor an exit barrier and is exoergic, the benzene molecule can be destroyed by carbon atoms even in the coldest molecular clouds. On the other hand, the reaction of phenyl radicals with methylacetylene has an entrance barrier; at high collision energies, the dynamics are at the boundary between an osculating complex and a direct pathway. Statistical calculations on the phenyl plus methylacetylene reaction demonstrate dramatic energy/temperature dependencies: at lower temperatures, the bicyclic indene isomer is the sole reaction product. But as the temperature increases to 2000 K, formation of indene diminishes in favor of substituted acetylenes and allenes, such as PhCCH, PhCCCH3, PhCHCCH2, and PhCH2CCH. Also, direct H-abstraction channels become accessible, forming benzene and C3H3 radicals, including propargyl. Similar conclusions were reached for the reactions of phenyl radicals with the other C3H4 isomers, as well as for the benzyl + acetylene reaction. The strong temperature dependence emphasizes that distinct product isomers must be included in reaction networks modeling PAH formation in extraterrestrial environments.

Reactions of chemically activated C9H9 species II: The reaction of phenyl radicals with allene and cyclopropene, and of benzyl radicals with acetyleneElectronic supplementary information (ESI) available: Full [C9H9] reaction scheme. Additional quantum chemical calculations not reported yet in earlier publications. Full product distribution and over-all rate coefficient listings, both uncorrected and effective data, for all reaction conditions examined. For the

The rate coefficient and product distributions of the reactions of phenyl radicals with allene and cyclopropene, and of the reaction with benzyl radicals with acetylene, have been studied by transition state theory and RRKM-master equation analyses over an extended temperature and pressure range, using data from our earlier quantum chemical characterization of the [C9H9] potential energy surface. The reaction of phenyl radicals with C3H4 are found to form indene + H as principal product at low temperatures and pressures. At higher temperatures, formation of benzene + propargyl, and of phenyl-substituted acetylenes and allenes dominates. The reaction of benzyl radicals with acetylene produces indene + H as dominant product for temperatures up to 2500 K, making this reaction a potential source of indene in combustion systems. The potential role of phenyl + C3HX reactions in the formation and growth of PAHs is discussed. Extensive electronic supplementary information (ESI) is available.

Reactions of chemically activated C9H9 species.

The rate coefficient and product distribution of the reaction of phenyl radicals with propyne has been studied by transition state theory and RRKM–master equation analyses over an extended temperature and pressure range, using data from our earlier quantum chemical characterization of the potential energy surface. At very low pressures and low temperatures (<500 K), indene + H was found to be the principal products, while for combustion systems yields of 30% PhCHCCH2 + H, 30% PhCCH + CH3, 15% PhCCCH3 + H and 20% benzene + 2-propynyl (propargyl) are expected. These results indicate that the title reaction might contribute to the formation of (polycyclic) unsaturated hydrocarbons with an aromatic ring in a wide variety of reaction conditions, and could therefore play a role in the formation of PAHs in reaction conditions ranging from the low-temperature, low-pressure regime found in the Interstellar Medium, to the high-temperature conditions found in combustion systems. Extensive supporting information is available.

Reaction of phenyl radicals with propyne.

The potential energy surface (PES) for the phenyl + propyne reaction, which might contribute to the growth of polycyclic aromatic hydrocarbons (PAHs) under a wide variety of reaction conditions, is described. The PES was characterized at the B3LYP-DFT/6-31G(d) and B3LYP-DFT/6-311+G(d,p) levels of theory. The energies of the entrance transition states, a direct hydrogen-transfer channel and two addition reactions leading to chemically activated C(9)H(9) intermediates, were also evaluated at the QCISD(T)/ 6-311G(d,p) and CCSD(T)/6-311G(d,p) levels of theory. An extensive set of unimolecular reactions was examined for these activated C(9)H(9)(dagger) intermediates, comprising 70 equilibrium structures and over 150 transition states, and product formation channels leading to substituted acetylenes and allenes such as PhCCH, PhCCCH(3), and PhCHCCH(2) were identified. The lowest energy pathway leads to indene, a prototype PAH molecule containing a five-membered ring. The title reaction thus is an example of possible direct formation of a PAH containing a five-membered ring, necessary to explain formation of nonplanar PAH structures, from an aromatic radical unit and an unsaturated hydrocarbon bearing an odd number of carbons. Extensive Supporting Information is available.

Kinetics of phenyl radical reactions with ethane and neopentane: Reactivity of C6H5 toward the primary C‐H bond of alkanes

The kinetics of C6H5 reactions with C2H6 (1) and neo-C5H12 (2) have been studied by the pulsed laser photolysis/mass spectrometric method using C6H5COCH3 as the phenyl precursor at temperatures between 565 and 1000 K. The rate constants were determined by kinetic modeling of the absolute yields of C6H6 at each temperature. Another major product, C6H5CH3, formed by the recombination of C6H5 and CH3, could also be quantitatively modeled using the known rate constant for the reaction. A weighted least-squares analysis of the two sets of data gave k1 = 1011.32±0.05 exp[−(2236 ± 91)/T] cm3 mol−1 s−1 and k2 = 1011.37±0.03 exp[−(1925 ± 48)/T] cm3 mol−1 s−1 for the temperature range studied. The result of our sensitivity analysis clearly supports that the yields of C6H6 and C6H5CH3 depend primarily on the abstraction reactions and C6H5 + CH3, respectively. From the absolute rate constants for the two reactions we obtained the value for the H-abstraction from a primary C-H bond, k-CH = 1010.40±0.06 exp(−1790 ± 102/T) cm3 mol−1 s−1. This result is utilized for analysis of other kinetic data measured for C6H5 reactions with alkanes in solution as well as in the gas phase. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 64–69, 2001

Kinetics of phenyl radical reactions with ethane and neopentane: Reactivity of C 6 H 5 toward the primary C‐H bond of alkanes

The kinetics of C6H5 reactions with C2H6 (1) and neo-C5H12 (2) have been studied by the pulsed laser photolysis/mass spectrometric method using C6H5COCH3 as the phenyl precursor at temperatures between 565 and 1000 K. The rate constants were determined by kinetic modeling of the absolute yields of C6H6 at each temperature. Another major product, C6H5CH3, formed by the recombination of C6H5 and CH3, could also be quantitatively modeled using the known rate constant for the reaction. A weighted least-squares analysis of the two sets of data gave k1 1011.32 0.05 exp[ (2236 91)/T] cm3 mol 1 s 1 and k2 1011.37 0.03 exp[ (1925 48)/T] cm3 mol 1 s 1 for the temperature range studied. The result of our sensitivity analysis clearly supports that the yields of C6H6 and C6H5CH3 depend primarily on the abstraction reactions and C6H5 CH3, respectively. From the absolute rate constants for the two reactions we obtained the value for the H-abstraction from a primary C-H bond, kp-CH 1010.40 0.06 exp( 1790 102/T) cm3 mol 1 s 1. This result is utilized for analysis of other kinetic data measured for C6H5 reactions with alkanes in solution as well as in the gas phase. 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 64–69, 2001

Kinetics and Mechanism for the Reaction of Phenyl Radical with Formaldehyde

The kinetics and mechanism for the C6H5 + CH2O reaction were investigated by the cavity ringdown spectrometric (CRDS) and pulsed laser photolysis/mass spectrometric (PLP/MS) methods at temperatures between 298 and 1083 K. With the CRDS method, the rate constant was measured by monitoring the decay times of injected probing photons in the absence (tc0) and presence (tc) of the C6H5 radical. In the PLP/MS experiment at higher temperatures, the rate constant was determined by kinetic modeling of the absolute yields of C6H6. The values of the rate constants obtained by the two different methods agree closely, suggesting that the C6H5 + CH2O → C6H6 + CHO reaction 1 is the dominant channel. A weighted least-squares analysis of the two sets of data gave k1 = (8.55 ± 0.25) × 104 T2.19±0.25 exp[−(19 ± 13)/T] cm3 mol-1 s-1 for the temperature range studied. The mechanism for the C6H5 + CH2O reaction was also elucidated with a quantum-chemical calculation employing a hybrid density functional theory (B3LYP) using the aug-cc-PVTZ basis set. The theory predicts the barriers for the abstraction producing C6H6 and the addition giving C6H5CH2O and C6H5OCH2 to be 0.8, 1.4, and 9.1 kcal/mol, respectively. The rate constant calculated for the H-abstraction process using the canonical variational transition-state theory with a 1.1 kcal/mol barrier agrees closely with the experimental result over the entire range of temperatures studied.

Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X 1A′) formation from reaction of C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1)

The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X 1A′), via reactive collisions of the phenyl radical C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X 1A1), was used to identify the site of the carbon–hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol−1. Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the π electron density of the C1 carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C6H5)HCCCH3 intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the A′2 surface. The phenylallene isomer channel was not observed experimentally. The dynamics of the title reaction and the identification of the phenyl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photospheres of carbon stars and oxygen poor combustion systems.

Kinetics of phenyl radical reactions with ethane and neopentane: Reactivity of C6H5 toward the primary C-H bond of alkanes

Kinetics of phenyl radical reactions with ethane and neopentane: Reactivity of C6H5 toward the primary C-H bond of alkanes

Computational Study of the Mechanisms for the Reaction of O2(3Σg) with Aromatic Radicals

The potential energy surface (PES) of the C6H5• + O2(3Σg) reaction has been studied using the B3LYP method. Several pathways were considered following the formation of the phenylperoxy (C6H5OO•) radical. At low temperatures (T < 432 K), the lowest energy pathway was found to go through a dioxiranyl radical. Scission of the O−O bond to form the phenoxy (C6H5O•) radical and O(3P) atom is more favorable at higher temperatures. Transition state structures for several steps in the decomposition of the phenylperoxy radical are presented to augment the C6H5• + O2 PES. For the heteroatomic aromatic hydrocarbon radicals, such as pyridine, furan, and thiophene, only minima on the PES are calculated in analogy with the intermediates obtained for the reaction of phenyl radical with O2. One important result of the proposed decomposition mechanism is that subsequent rearrangements of the heteroatomic aromatic hydrocarbon peroxy radicals (Ar−OO•) are likely to yield intermediates that are of atmospheric interest.

Experimental and Theoretical Studies of the Reaction of the Phenyl Radical with Methane

The kinetics of the metathetical reaction of phenyl radical with methane has been studied theoretically and experimentally. The rate constants determined by two complementary methods, pyrolysis/Fourier transform infrared spectrometry and pulsed laser photolysis/mass spectrometry in the temperature range 600−980 K, give the Arrhenius equation: k1 = 1012.78 ± 0.13 exp[(−6201 ± 225)/T] cm3/(mol s). At the best theoretical level employed (G2M(CC,MP2)), the barrier for the reaction at 0 K is E10 = 9.3 kcal/mol. The rate constant k1 calculated from theoretical molecular parameters fits experimental data if the barrier height is increased to 10.5 kcal/mol. The fitted barrier is well within the 2−3 kcal/mol accuracy of the G2M method for the present open-shell, seven-heavy-atom system. Because of the relatively high reaction barrier and the predicted high imaginary frequency (1551 cm-1), tunneling corrections resulted in a significant enhancement in the calculated rate constant, 150% at 500 K and 7% at 2000 K. T...

Kinetics of C6H5 Radical Reactions with Toluene and Xylenes by Cavity Ringdown Spectrometry

The kinetics for the metathetical reactions of phenyl radical with toluene and xylenes have been studied experimentally and theoretically. The absolute bimolecular rate constants for the reactions of C6H5 with toluenes (C7H8 and C7D8) and xylenes (three C8H10 isomers) were measured by cavity ringdown spectrometry at temperatures between 295 and 483 K. For the reaction with toluene, a strong isotope effect was observed, whereas for xylene reactions no structural preference was noticed among the three isomers. The weighted least-squares analysis of each reaction gave rise to the following rate constant expressions in units of cm3/(mol s): k(C7H8) = (2.08 ± 0.11) × 1011 exp[−(1027 ± 35)/T]; k(C7D8) = (2.27 ± 0.43) × 1011 exp[−(1340 ± 64)/T]; k(C8H10) = (1.48 ± 0.11) × 1011 exp[−(526 ± 27)/T]. Additionally, we have carried out hybrid density functional theory (B3LYP) calculations for the reactions of C7H8 and C7D8 using the 6-31G(d,p) basis set. The predicted rate constants using the conventional transition state theory with the calculated vibrational frequencies and moments of inertia fit well to the experimental results with only minor adjustments in the calculated reaction barriers. Combination of our low-temperature C7H8 kinetic data with those obtained at high temperatures in shock waves (ref 13) gave the expression k(C7H8) = (4.15 × 10-3)T4.5 exp(800/T) cm3/(mol s) for the temperature range 300−1450 K.

Hydrogen Transfer Induced Cleavage of Biaryl Bonds

Biaryl bonds are the strongest carbon−carbon single bonds in fossil fuels. This paper examines hydrogenolysis and alkane cohydrogenolysis of biphenyl and dimethylbiphenyls, in detail. Biphenyl cleavage was found to be enhanced by copyrolysis and cohydrogenolysis with small amounts of 2,2,3,3-tetramethylbutane (hexamethylethane, HME). Much smaller enhancements were found for cohydrogenolysis with other alkanes. Increased biaryl cleavage rates, in HME cohydrogenolysis, were found to be a direct consequence of initiation by hydrogen atom generated during HME decomposition. Both biphenyl pyrolysis and hydrogenolysis mechanisms involve ipso hydrogen atom attack, followed by ejection of phenyl radical and the formation of benzene. Hydrogen atom is regenerated either through the phenylation of starting biphenyl or through the direct reaction of phenyl radical with H2. Both propagation reactions are very fast, leading to highly efficient chain transfer. Biaryl bond hydrogenolysis was found to be first-order in bi...

High-temperature reactions and thermodynamic properties of phenyl radicals

Reactions of phenyl radicals were studied in reflected shock waves in the temperature range 1050–1450 K using UV absorption spectroscopy. Rate constants for the following reactions were derived: 2 C 6 H 5 → C 12 H 10 ( k 1 =5.7×10 12 cm 3 mol −1 s −1 ), C 6 H 5 +H 2 →C 6 H 6 +H ( k 3 =10 12.6 exp(−33 kJ mol −1 / RT ) cm 3 mol −1 s −1 ): C 6 H 5 +C 6 H 5 CH 3 →C 6 H 6 +C 6 H 5 CH 2 ( k 4 =10 13.9 exp(−50 kJ mol −1 / RT ) cm 3 mol −1 s −1 ), C 6 H 5 +CH 4 →C 6 H 6 +CH 3 ( k 5 =10 12.3 exp(−36 kJ mol −1 / RT ) cm 3 mol −1 s −1 ), and C 6 H 5 +C 2 H 2 →C 6 H 5 C 2 H+H ( k 8 =10 13.0 exp(−32 kJ mol −1 / RT ) cm 3 mol −1 s −1 ). Phenyl iodide dissociation equilibrium was also investigated. In combination with results from other laboratories, equilibrium constants for five different equilibria involving phenyl radicals were determined. A third-law analysis of these data leads to an enthalpy of formation of phenyl radicals of ΔH f 298 o =(338±3) kJ mol −1 .

Temperature Dependence of the Reactions of Phenyl Radicals with 1,1-Diphenylethylene, Carbon Tetrachloride, and Cyclohexene

The reaction of phenyl radicals with 1,1-diphenylethylene (DPE), carbon tetrachloride, and cyclohexene has been examined over a range of temperatures using laser flash photolysis techniques. The activation energies are 0.71 ± 0.04, 3.53 ± 0.11 and 2.32 ± 0.33 kcal/mol, and the preexponential factors, expressed as log (A/M-1s-1), are 9.19 ± 0.03, 8.96 ± 0.09, and 9.02 ± 0.26, for DPE, CCl4, and c-C6H10, respectively. In particular the CCl4 data provide a much needed reference for competitive studies.