Dispersed Fluorescence Spectroscopy of Jet-Cooled Isobutoxy and 2-Methyl-1-butoxy Radicals

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

We report vibronically resolved laser-induced fluorescence (LIF) spectra of jet-cooled C5-C10 secondary alkoxy radicals. The LIF spectra demonstrate vibronic structures similar to smaller (C3-C4) secondary alkoxies. For 2-pentoxy and 2-hexoxy, rotationally resolved LIF spectra have also been recorded. Two types of rotational structures have been observed in vibronic bands of each molecule. Extensive quantum chemistry calculations have been performed on 2-pentoxy and 2-hexoxy. The computed results include the relative energies of conformers, their geometries, and the energy separations between the nearly degenerate à and X̃ electronic states (ΔE^Ã-X̃). Based on the similarity between the vibronic structures of different secondary alkoxies and calculated molecular parameters, including the relative energies of conformers, the B̃ ← X̃ transition frequencies, and the vibrational frequencies, strong vibronic bands in the LIF spectra are assigned to the origin bands and CO stretch bands of the two lowest-energy conformers of each secondary alkoxy radical. The distinct rotational structures of the two different conformations of 2-pentoxy and 2-hexoxy will be simulated and analyzed in Part II of this series ( J. Phys. Chem. A 2021, DOI: 10.1021/acs.jpca.0c10663).

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

The B̃-X̃ 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 long-range field effects of the CH_xF_3-x group on electronic transition energies and bond strengths, especially in the excited electronic (B̃) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the -CF₃ internal rotation.

Laser-Induced Fluorescence and Dispersed Fluorescence Spectroscopy of Jet-Cooled Isopentoxy Radicals

The B̃-X̃ laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled isopentoxy radicals have been obtained. The LIF spectrum of isopentoxy lacks strong transitions to the CO-stretch levels that are typical for alkoxy radicals. Instead, it contains two low-frequency vibrational progressions due to large-amplitude motions of the GG' and GG conformers involving torsion of the C₁C₂C₃H dihedral angle. Other vibronic bands observed in the LIF spectrum are attributed to the TG conformer. Molecular carriers of the vibronic transitions in the LIF spectrum are identified by comparing the experimentally obtained spectrum and the simulated one. DF spectra of the GG and TG conformers are dominated by strong vibrational progressions of the CO-stretch mode when the origin or the CO-stretch band is pumped. When non-CO-stretch bands are pumped, the DF spectra are dominated by combination bands of the CO stretch and the pumped mode. Ã-X̃ separations of the GG and TG conformers were also determined from the DF spectra.