Analyzing the rotational and fine structure of the two lowest electronic states of asymmetrically substituted alkoxy radicals

Yan, Yi

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https://hdl.handle.net/2142/107623 Copy

Description

Title

Analyzing the rotational and fine structure of the two lowest electronic states of asymmetrically substituted alkoxy radicals

Author(s)

Yan, Yi

Contributor(s)

• Liu, Jinjun
• Miller, Terry A.
• Sharma, Ketan

Issue Date

2020-06-23

Keyword(s)

Radicals

Abstract

Alkoxy radicals are vital in the process of oxidation and have been well studied spectroscopically. The simplest species, CH$_3$O, has a degenerate $\tilde{X}^2$E ground electronic state, which has a near-UV transition to a non-degenerate electronic state, $\tilde{B}^2$A$_1$. Larger alkoxy radicals are formed by substitution of the H atom(s) with the $\tilde{B}$-$\tilde{X}$ electronic transition shifting to the red as the size of the alkyl group increases. Most importantly, when the H atom(s) substitution is asymmetric, the degeneracy of the $\tilde{X}$ state is resolved into two non-degenerate electronic states, $\tilde{X}$ and $\tilde{A}$. Typically the spin-orbit-free energy separation, $\Delta$E$_0$, between these two states, is small ($\leq$1000 cm$^{-1}$ and even $\leq$100 cm$^{-1}$ in some cases). Historically, the approach to the analysis of spectra has been to treat the rotational structure in the $\tilde{X}$ and $\tilde{A}$ states separately via a Hamiltonian including an asymmetric top rotational term and a spin-rotation interaction term. Recently Liu\footnote{J. Liu, J. Chem. Phys. 148, 124112 (16 pages) (2018).} suggested that, as is done with the $\tilde{X}^2$E state of methoxy, the structure of both the $\tilde{X}$ and $\tilde{A}$ states, now separated by $\Delta$E$_0$ and coupled by the spin-orbit interaction and the Coriolis interaction, should be better considered together. This “coupled two-state model” also allows semi-quantitative prediction of effective spin-rotation constants using molecular geometry and spin-orbit constants, which can be calculated with considerable accuracy. In the present work, we have simulated rotationally and fine-structure resolved laser-induced fluorescence (LIF) spectra of alkoxy radicals with the Liu model and fit the rotational constants, as well as the spin-orbit and Coriolis coupling parameters between the $\tilde{A}$ and $\tilde{X}$ states. The Coriolis coupling constant ($\zeta_t$) was held equal to the quenched electronic orbital angular momentum ($\zeta_e d$) of the spin-orbit constant. For these fits the spin-rotation parameters are held zero. The fits have been carried out for isomers and conformers of alkoxy radicals with four or less carbon atoms for which high-resolution LIF spectra have been obtained. Dependence of fit values of molecular constants ($\zeta_t$, $\zeta_e d$, and $\Delta$E$_0$) on the size and conformation of alkoxy radicals will be discussed.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

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http://hdl.handle.net/2142/107623

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Copyright 2020 is held by the Author(s)

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