Methyl Substitution Effect on the Jet-Cooled Laser-Induced Fluorescence Spectrum of Cyclohexoxy Radical

Abstract

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.

Revealing Long-Range Substituent Effects in the Laser-Induced Fluorescence and Dispersed Fluorescence Spectra of Jet-Cooled CHxF3–xCH2O (x = 1, 2, 3) Radicals

“…Jet-cooled laser-induced fluorescence (LIF) has been used to study alkoxy radicals in their second excited electronic (B̃) state. − While moderate-resolution scans provide information on the vibrational levels, high-resolution LIF spectra are suitable to determine experimental rotational and spin-rotational constants. Similarly, dispersed jet-cooled laser-induced fluorescence (DF) spectroscopy has been used to explore the vibrational levels in the ground state (X̃) and in the low-lying first excited electronic (Ã) state, ,− while stimulated-emission pumping (SEP) and microwave spectroscopy (MW) can provide high enough resolution to obtain rotational constants in the electronic ground state. − In most of these studies, alkyl nitrites (RONO) have been used as precursors, and the alkoxy radicals were produced by laser photolysis typically at 308, 351, or 355 nm.…”