On the thermal unimolecular decomposition of the cyclohexoxy radical--an experimental and theoretical study.

The kinetics of the thermal unimolecular decomposition of the cyclohexoxy radical (c-C(6)H(11)O) was experimentally studied, and the results were analyzed in terms of statistical rate theory with molecular and transition state data from quantum chemical calculations. Laser flash photolysis of cyclohexylnitrite at 351 nm was used to produce c-C(6)H(11)O radicals, and their concentration was monitored by laser-induced fluorescence after excitation at 356.2 or 365.2 nm. The experiments were performed at temperatures ranging from 293 to 341 K and pressures between 5 and 55 bar with helium as the bath gas. Over the whole temperature range, biexponential profiles were observed, which is an indication of a consecutive reaction with a pre-equilibrium. From our quantum chemical calculations, it follows that this pre-equilibrium corresponds to the reversible ring-opening via beta-C-C bond fission to form the 6-oxo-1-hexyl radical (l-C(6)H(11)O), c-C(6)H(11)O <--> l-C(6)H(11)O (1,-1). The following temperature-dependent rate coefficients were deduced with an estimated uncertainty of +/-30%: k(1)(T) = 3.80 x 10(13) exp(-50.1 kJ mol(-1)/RT) s(-1) and k(-1)(T) = 3.02 x 10(8) exp(-23.8 kJ mol(-1)/RT) s(-1); a pressure dependence was not observed. In our theoretical analysis, the different conformers of c-C(6)H(11)O were explicitly taken into account, and the C-C torsional motions in l-C(6)H(11)O were treated as hindered internal rotators using a recently suggested approach. This explicit consideration of the hindered internal rotators significantly improved the agreement between the experimentally determined rate coefficients and the results from the quantum chemical computations.