UV Absorption Spectrum and Self-Reaction Kinetics of the Cyclohexadienyl Radical, and Stability of a Series of Cyclohexadienyl-Type Radicals

Abstract

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.

Formation of Peroxy Radicals from OH−Toluene Adducts and O2

“…A recently measured rate constant of the selfreaction of the H-benzene adduct (C 6 H 7 ) by Berho et al 34 is of similar magnitude: (3.1 ( 1.0) × 10 -11 cm 3 s -1 , based on σ(C 6 H 7 ) ) (2.55 ( 0.45) × 10 -17 cm 2 at 302 nm. 34 The rate constant of the self-reaction of the OH-benzene adduct was reevaluated in this work from the data of ref 16 by accounting for the adduct + HO 2 reaction. However, compared with the earlier result 16 the present rate constant is only slightly smaller by about 15% (see Table 3).…”

Investigation of InP etching mechanisms in a Cl2/H2 inductively coupled plasma by optical emission spectroscopy

“…͑R7͒ is a fast reaction since values of the second-order rate constant k 1 around of 2 ϫ 10 −11 cm 3 molecule −1 s −1 have generally been reported in literature. 20 Berho et al 21 measured a rate constant of 2 ϫ 10 −11 cm 3 molecule −1 s −1 at 300 K, and have derived an estimation of k 1 from 2 ϫ 10 −11 to 4 ϫ 10 −11 cm 3 molecule −1 s −1 for a gas temperature varying from 300 to 600 K. In an older paper, Albright et al, 22 reported measured k 1 values of 3 ϫ 10 −11 to 10ϫ 10 −11 cm 3 molecule −1 s −1 for temperatures from 300 to 500 K, and a value of 13 ϫ 10 −11 cm 3 molecule −1 s −1 could be extrapolated for a temperature of 600 K. The rate constants k 2 and kЈ 2 for reaction ͑R8͒ and reverse reaction ͑R9͒ are lower than k 1 by more than one order of magnitude. 20,23 Typical values for k 2 lie in the range from 1 ϫ 10 −14 to 2.5ϫ 10 −14 cm 3 molecule −1 s −1 at 300 K, and from 7 ϫ 10 −13 to 15ϫ 10 −13 cm 3 molecule −1 s −1 at 600 K. The rate constant kЈ 2 is typically of the same order of k 2 , since typical values lie in the range from 5 to 13ϫ 10 −13 cm 3 molecule −1 s −1 at 600 K. The first reaction to be considered in a kinetic model is consequently reaction ͑R7͒.…”

A new method for investigating infrared spectra of protonated benzene (C6H7+) and cyclohexadienyl radical (c-C6H7) using para-hydrogen

“…The UV-visible spectra of c-C 6 H 7 have been studied in the condensed phase [25][26][27][28][29][30] and in the gaseous phase. [31][32][33] The dispersed laser-induced fluorescence spectra of c-C 6 H 7 radical, produced via H-abstraction of 1, 4cyclohexadiene by Cl atom, yield wavenumbers for six vibrational modes of the ground electronic state. 34 However, except for the ν 10 mode near 981 cm −1 , these modes do not match with those identified for c-C 6 H 7 according to IR absorption lines of a C 6 H 6 /Xe matrix that was irradiated by fast electrons followed by annealing at 45 K. 35 Hence, it is important to obtain an IR spectrum of C 6 H 7 with improved signal-to-noise ratio and spectral resolution.…”