CH₃O Substitution Effect Revisited in the Vibrationally Resolved Laser-Induced Fluorescence Spectra of...

The oxy-substituted alkoxy radicals have attracted wide attention due to the increasing application of oxygenated volatile organic compounds as fuel additives and solvents. Direct detection of these intermediate radicals is desired for measuring the reaction rate and investigating the oxidation mechanism of organic compounds in the atmosphere. A charge-transfer excited state induced by CH₃O substitution was identified in our previous study of 3-methoxy-1-propoxy radical [Xue, J. Phys. Chem. Chem. Phys. 2021, 23, 2586]. As the C-C bonds of chain alkoxy radicals can freely rotate, further studies are needed to understand the mechanism of this long-range charge-transfer effect. In this work, vibrational-resolved laser-induced fluorescence (LIF) spectra of 3- and 4-methoxycyclohexoxy radicals were obtained under jet-cooled conditions. A large red-shift of ∼454 cm^-1 of the origin band was observed when the CH₃O substituent moved from the δ site to the γ site of the cyclohexoxy radical. The LIF spectra are assigned to 3-cis (e, e) and 4-trans (e, e) conformers, respectively, with the assistance of structural optimization and electron excitation studies conducted at the CAM-B3LYP/6-311++G(d,p) level of theory. Natural transition orbital analysis reveals that the intramolecular charge transfer from the C-O-C p orbital to the radical O p orbital in 3-methoxycyclohexoxy has a strong effect on the radical CO σ → O p excitation and hence results in a spectral change. On the other hand, the spectral effect of CH₃O substitution almost vanishes at δ carbon. The results propose a through-bond interaction between CH₃O and radical CO groups.