Rotational and fine structure of open-shell molecules in nearly degenerate electronic states. II. Interpretation of experimentally determined interstate coupling parameters of alkoxy radicals

Abstract

Rotationally and fine-structure resolved B̃←X̃ laser-induced fluorescence (LIF) spectra of alkoxy radicals have been simulated with a “coupled two-states model” [J. Liu, J. Chem. Phys. 148, 124112 (2018)], in which the nearly degenerate X̃ and à states are considered together. These two electronic states are separated by the “difference potential” and coupled by the spin–orbit (SO) interaction and the Coriolis interaction. Molecular constants determined in fitting the LIF spectra using the coupled two-states model provide quantitative insight into the SO and Coriolis interactions, as well as other intramolecular dynamics, including the pseudo-Jahn–Teller effect. The spectroscopic model also allows semi-quantitative prediction of effective spin-rotation constants using molecular geometry and SO constants, which can be calculated ab initio with considerable accuracy. The dependence of fit values of molecular constants on the size and conformation of alkoxy radicals is discussed.

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

“…The obstacle has been overcome in recent years by implementing a newly developed “coupled-states” Hamiltonian and the use of extensive electronic structure calculations (see Part II of this series) . Rotational and fine structures in the high-resolution LIF spectra of iso-propoxy, 2-butoxy, , as well as cyclohexoxy, , have been simulated and fit, which serves as a good starting point to understanding the spectra of larger secondary alkoxies. In the present paper (called Part I), we report the moderate-resolution spectra of straight-chain secondary alkoxies with 5–10 carbon atoms.…”

Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part II. Rotational and Fine Structure

“…As Table 5 shows, the interstate-coupling constants (aζ e d and ΔE 0 ) of the G+T conformer of 2-pentoxy and the G+TT conformer of 2-hexoxy are close to those of the G+ conformer of 2-butoxy (aζ e d = −49.79 cm −1 and ΔE 0 = 115 cm −1 ). 18 The effective SO constants (aζ e d) of the TT conformer of 2pentoxy and the TTT conformer of 2-hexoxy are close to each other (−28.8 and −25.6 cm −1 , respectively) but significantly smaller than the T conformer of 2-butoxy (aζ e d = −43.96 cm −1 ). As a result, ΔE 0 of the TT and TTT conformers are larger than the T conformer of 2-hexoxy (ΔE 0 = 33 cm −1 ).…”

“…As a result, ΔE 0 of the TT and TTT conformers are larger than the T conformer of 2-hexoxy (ΔE 0 = 33 cm −1 ). 18 It is worth noting that in the present work, the A ̃−X ̃separations of 2-pentoxy and 2-hexoxy are fixed to those of corresponding 2-butoxy conformers because of the lack of experimental…”

“…Details of the coupled-states model can be found in previous publications of our groups. 16,18 The Hamiltonian includes the following terms…”

“…where a is the atomic-like SO constant, ζ e is the expectation value of the z′-axis projection of the total orbital angular momentum of the electrons, and d is the vibronic quenching factor, viz., the Ham factor. Following previous works, 3,15,18,29,30 we assume that a is the same for the A ̃/X ̃state alkoxy radicals and the ground-state OH radical [a OH (X ̃2Π) = −139.050877 (41) cm −1 ] 31,32 because in all cases, the unpaired electron is largely localized on the oxygen atom.…”