Dispersed Fluorescence Spectroscopy of Jet-Cooled 2-, 3-, and 4-Methylcyclohexoxy Radicals.

Abstract

Vibrational structures of the nearly degenerate X̃ and à states of the 2-, 3-, and 4-methylcyclohexoxy (MCHO) radicals were studied by jet-cooled dispersed fluorescence (DF) spectroscopy. The observed transitions were assigned on the basis of vibrational frequencies and Franck-Condon factors predicted by quantum chemical calculations. Intensities of vibronic transitions in the DF spectra are dependent on the laser-induced fluorescence (LIF) bands pumped in the experiment, which can be explained by the difference in geometry and symmetry between the lower X̃/à states and the highly excited B̃ state. All three studied isomers of MCHO have close-lying X̃ and à states although their energy separations are affected by the position of the methyl group. It is suggested by quantum chemical calculations that the lowest-energy conformers of all three isomers have the half-filled orbital oriented perpendicular to the OCH plane, which is consistent with the observed relative intensities of the B̃ → X̃ and B̃ → à origin bands. When the origin and the CO-stretch bands of the B̃ ← X̃ LIF excitation spectra were pumped, the DF spectra were dominated by CO-stretch progressions. When non-CO-stretch vibrational levels of the B̃ state were pumped, progressions of CO-stretch modes combined with the pumped vibrational mode were observed. Excited-state vibrational population relaxation from the CO stretch level to the vibrational ground level and from combination levels of the CO stretch mode and other vibrational modes to the non-CO stretch modes was observed. Analysis of the DF spectra confirms the previous conclusion that all strong LIF bands observed under jet-cooled conditions belong to a single conformer of each positional isomer (Lin et al. RSC Adv. 2012, 2, 583-589).