The gas-phase reaction of Cl atoms with benzene has been studied using both experimental and computational methods. The bulk of the kinetic data were obtained using steady-state photolysis of mixtures containing Cl2, C6H6, and a reference compound in 120−700 Torr of N2 diluent at 296 K. Reaction of Cl atoms with C6H6 proceeds via two pathways; (a) H-atom abstraction and (b) adduct formation. At 296 K the rate constant for the abstraction channel is k 1a = (1.3 ± 1.0) × 10-16 cm3 molecule-1 s-1. Phenyl radicals produced via H-atom abstraction from C6H6 react with Cl2 to give chlorobenzene. The main fate of the C6H6−Cl adduct is decomposition to reform C6H6 and Cl atoms. A small fraction of the C6H6−Cl adduct undergoes reaction with Cl atoms via a mechanism which does not lead to the production of C6H5Cl, or the reformation of C6H6. As the steady-state Cl atom concentration is increased, the fraction of the C6H6−Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for benzene removal and a decrease in the chlorobenzene yield. Thermodynamic calculations show that a rapid equilibrium is established between Cl atoms, C6H6, and the C6H6−Cl adduct, and it is estimated that at 296 K the equilibrium constant is K c,1b = [C6H6−Cl]/[C6H6][Cl] and lies in the range (1−2) × 10-18 cm3 molecule. Flash photolysis experiments conducted using C6H6/Cl2 mixtures in 760 Torr of either N2 or O2 diluent at 296 K did not reveal any significant transient UV absorption; this is entirely consistent with results from the steady-state experiments and the thermodynamic calculations. The C6H6−Cl adduct reacts slowly (if at all) with O2 and an upper limit of k(C6H6−Cl + O2) < 8 × 10-17 cm3 molecule-1 s-1 was established. As part of this work a value of k(Cl + CF2ClH) = (1.7 ± 0.1) × 10-15 cm3 molecule-1 s-1 was measured. These results are discussed with respect to the available literature concerning the reaction of Cl atoms with benzene.

The UV spectrum, self-reaction kinetics, and stability of the cyclohexadienyl radical (C6H7) were investigated by flash photolysis, using a novel method to generate the radical. The absolute UV spectrum was obtained for the first time. It exhibits an intense peak of absorption at 302 nm, similar to that of other cyclohexadienyl-type radicals, with σmax = (2.55 ± 0.45) × 10-17 cm2 molecule-1 at 302 nm (total uncertainty). As the radical was generated in the absence of any other reactive species, the kinetics of the self-reaction could be investigated, leading to k(C6H7+C6H7) = (3.1 ± 1.0) × 10-11 cm3 molecule-1 s-1 at 298 K. In addition, the equilibrium constant of reaction 1, H + C6H6 ⇌ C6H7 (1, −1), was measured at 628 and 670 K, and the enthalpy of reaction was derived using the third law method of analysis. The result is ΔH° 298(H + C6H6 → C6H7) = −(88.4 ± 12.0) kJ mol-1 using the calculated value ΔS°298(H + C6H6 → C6H7) = −(80.5 ± 4.0) J mol-1 K-1 (derived from DFT and BAC-MP4 type quantum calculations), corresponding to ΔH°f, 298(C6H7) = 212 ± 12 kJ mol-1. The experimental work was complemented by theoretical calculations with the objective of establishing a scale of stability of a series of cyclohexadienyl-type radicals XC6H6. Calculations were performed for X = F, Cl, Br, H, OH, and CH3 and the few experimental data available to date were used to validate the results of calculations. The following sequence, from the more to the less stable radical, was established: FC6H6 > HC6H6 > HOC6H6 > CH3C6H6 > ClC6H6 > BrC6H6. The latter three radicals of this series are too unstable for having a chance to be observed in laboratory. The important factors influencing the stability of the XC6H6 radical according to the nature of X are discussed.

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The kinetics of the association reaction of the phenoxy radical with NO were investigated using a flash photolysis technique coupled to UV absorption spectrometry. This yielded k(C6H5O + NO) = (1.65 ± 0.10) × 10-12 cm3 molecule-1 s-1, with no significant temperature effect over the temperature range 280−328 K. Experiments were performed at atmospheric pressure, and theoretical calculations using the RRKM method showed that the rate constant is at the high-pressure limit above ≈50 Torr for temperatures below 400 K. Upon increasing the temperature, the reaction was found to be reversible, and the equilibrium kinetics have been studied at seven temperatures between 310 and 423 K. The equilibrium constant can be expressed as ln(K c/cm3 molecule-1) = −(63.3 ± 1.0) + (10 140 ± 1000)K/T. Thermodynamic treatment of the data by the Third Law method of analysis yielded ΔH°298 = (−87.3 ± 8.0) kJ mol-1 (yielding ΔH°0 = (−83.8 ± 8.0) kJ mol-1 and ΔH°f,298(C6H5O(NO)) = (51.5 ± 8.0) kJ mol-1), corresponding to the calculated ΔS°298 = (−164.9 ± 8.0) J mol-1 K-1. All spectroscopic parameters necessary for RRKM calculations and for the entropy determination using statistical thermodynamics were calculated using both semiempirical (MNDO) and DFT methods. Influence of the resonance stabilization energy of radicals on R−NO bond dissociation energy is discussed.