UV absorption spectrum and self-reaction kinetics of the cyclohexadienyl radical, and stability of a series of cyclohexadienyl-type radicals

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 sigma(max) = (2.55 +/- 0.45) x 10(-17) cm(2) 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) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K. In addition, the equilibrium constant of reaction 1, H + C6H6 reversible arrow 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 Delta H degrees (298)(H + C6H6 --> C6H7) = -(88.4 +/- 12.0) kJ mol(-1) using the calculated value Delta S degrees(298)(H + C6H6 --> C6H7) = -(80.5 +/- 4.0) J mol(-1) K-1 (derived from DFT and BAC-MP I type quantum calculations), corresponding to Delta H degrees(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 > BTC6H6. 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.