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Issue 2, 2008
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On the thermal unimolecular decomposition of the cyclohexoxy radical—an experimental and theoretical study

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Abstract

The kinetics of the thermal unimolecular decomposition of the cyclohexoxy radical (c-C6H11O) 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-C6H11O 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 β-C–C bond fission to form the 6-oxo-1-hexyl radical (l-C6H11O), c-C6H11O ⇔ l-C6H11O (1,–1). The following temperature-dependent rate coefficients were deduced with an estimated uncertainty of ±30%: k1(T) = 3.80 × 1013 exp(–50.1 kJ mol–1/RT) s–1 and k–1(T) = 3.02 × 108 exp(–23.8 kJ mol–1/RT) s–1; a pressure dependence was not observed. In our theoretical analysis, the different conformers of c-C6H11O were explicitly taken into account, and the C–C torsional motions in l-C6H11O 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.

Graphical abstract: On the thermal unimolecular decomposition of the cyclohexoxy radical—an experimental and theoretical study

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Supplementary files

Article information


Submitted
29 Aug 2007
Accepted
17 Oct 2007
First published
05 Nov 2007

Phys. Chem. Chem. Phys., 2008,10, 320-329
Article type
Paper

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

O. Welz, F. Striebel and M. Olzmann, Phys. Chem. Chem. Phys., 2008, 10, 320
DOI: 10.1039/B713286G

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