Methyl substitution effect on the jet-cooled laser-induced fluorescence spectrum of cyclohexoxy radical.

Understanding the structure and properties of cyclohexoxy radical and its substitutes is important because of their presence in combustion processes, in atmospheric chemistry, and as intermediates in the hydrocarbon reactions. In this work, jet-cooled laser-induced fluorescence (LIF) spectra of five dimethyl substituted cyclohexoxy radicals are obtained for the first time. The correlation between the spectral variations and the radical structural changes is studied with the assistance of theoretical calculations at the B3LYP/6-31+G(d) and CASSCF/6-31+G(d) levels. The results show that the spectral characters of the dimethylcyclohexoxy radicals and their dissociation kinetics are predominantly affected by the methyl substitution position related to the C-O group. The spectral effect of the two methyl groups will add up if they locate on asymmetric carbons of the cyclohexoxy ring. Methyl substitution on β carbon weakens the six-member ring of cyclohexoxy and results in unimolecular dissociation via β C-C bond cleavage on the methyl group side and forms vinoxy variants. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmospheric and combustion processes.