Revealing Long-Range Substituent Effects in the Laser-Induced Fluorescence and Dispersed Fluorescence Spectra of Jet-Cooled CH_xF_3–xCH₂O (x = 1, 2, 3) Radicals

Abstract

The B ̃−X ̃laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important β-monofluoro ethoxy (MFEO), β,β-difluoro ethoxy (DFEO), and β,β,β-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck−Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected longrange field effects of the CH x F 3−x group on electronic transition energies and bond strengths, especially in the excited electronic (B ̃) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the −CF 3 internal rotation.

Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part I. Spectral Overviews and Vibronic Analysis

“…Alkoxy radicals (RO • ), where R stands for an alkyl group, are key reaction intermediates in the oxidation of hydrocarbons in both the combustion of fossil fuels and degradation of organic molecules in the atmosphere. − In our labs, they are studied comprehensively by laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectroscopy. − LIF spectra of both primary and secondary alkoxy radicals have been recorded with two different resolutions: the moderate-resolution spectra reveal the vibronic structure, and the high-resolution ones show the rotational and fine structures. The methoxy radical (CH 3 O), the smallest alkoxy radical, is a prototypical Jahn–Teller (JT) molecule and has been subject to extensive theoretical and experimental studies. − In its ground electronic state ( X̃ 2 E), CH 3 O demonstrates both linear and quadratic JT effects, although its dynamic geometry on the zero-point level retains its 3-fold symmetry and still belongs to the C 3 v point group.…”

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

“…For better predicting the long-range effect between the CH 3 O substituent and the CO radical group, the Coulomb-attenuating method (CAM) was applied in the structural optimization of the ground state at the B3LYP/6-311++G­(d,p) level of theory, and the time-dependent TD-CAM-B3LYP/6-311++G­(d,p) method was used for electronic excited states. CAM computations include long-range corrections and provide more accurate results for the charge-transfer excitation than normal DFT calculations. , It has been successfully used to reproduce the LIF spectra of free radicals. , Natural transition orbital (NTO) analysis was carried out to identify the character of the electronic transition of the excited states.…”

“…In recent years, experimental and theoretical investigations of hydroxy-substituted and halogenated alkoxy radicals have been reported. − To understand the OH-initiated oxidation mechanism of isoprene in the presence of O 2 and NO in the atmosphere, the intermediate δ-hydroxyalkoxy channel was studied using the laser photolysis-laser-induced fluorescence (LP-LIF) and isotopic labeling techniques . Smog chamber/Fourier transform infrared (FTIR) and theoretical methods were applied in studies of β, γ, and δ-hydroxyalkoxy radicals, and the results show that decomposition was the atmospheric fate of β- and γ-hydroxyalkoxy, while for the δ-hydroxyalkoxy radicals, the decomposition pathway was unlikely to be important. , Chhantyal-Pun et al obtained the LIF spectra of FCH 2 CH 2 O but only observed fragments HCHO and CH 2 CHO in the attempt to record the LIF spectra of ClCH 2 CH 2 O, BrCH 2 CH 2 O, and HOCH 2 CH 2 O via photolysis of their corresponding nitrite precursors. , The long-range substitution effects in the LIF spectra of CH x F 3– x CH 2 O radicals were further revealed by Koncz and colleagues . However, despite multiple efforts, the unimolecular dissociation mechanism of XCH 2 CH 2 O (X = F, Cl, Br) is still not fully resolved. , …”

“…21,22 radicals were further revealed by Koncz and colleagues. 25 However, despite multiple efforts, the unimolecular dissociation mechanism of XCH 2 CH 2 O (X = F, Cl, Br) is still not fully resolved. 26,27 As for oxy-substituted alkoxy radicals (other than hydroxyalkoxy), related research works are sparse despite the fact that they are important intermediates of the atmospheric oxidation of ketones, ethers, and esters.…”

“…42,43 It has been successfully used to reproduce the LIF spectra of free radicals. 25,28 Natural transition orbital (NTO) analysis was carried out to identify the character of the electronic transition of the excited states. As it shows, no spectrum was observed for 2-methoxycyclohexoxy, while rich spectral bands were obtained for 3-and 4methoxycyclohexoxy.…”