Rotationally resolved ${\tilde{B}} \leftarrow {\tilde{X}}$B̃←X̃ electronic spectra of the isopropoxy radical: A comparative study

The \documentclass[12pt]{minimal}\begin{document}$\tilde{B} - \tilde{X}$\end{document}B̃−X̃ laser-induced-fluorescence spectrum of jet-cooled isopropoxy radical (i-C3H7O·) has been recorded. Using an isolated state model the observed rotational and fine structure of the origin band has been well simulated to determine rotational constants for both the \documentclass[12pt]{minimal}\begin{document}$\tilde{X}$\end{document}X̃ and \documentclass[12pt]{minimal}\begin{document}$\tilde{B}$\end{document}B̃ states and the electron spin-rotation constants of the \documentclass[12pt]{minimal}\begin{document}$\tilde{X}$\end{document}X̃ state. The line intensities are well simulated with a parallel transition type, requiring the same symmetry for the levels involved of each the \documentclass[12pt]{minimal}\begin{document}$\tilde{X}$\end{document}X̃ and \documentclass[12pt]{minimal}\begin{document}$\tilde{B}$\end{document}B̃ state, which confirms the previous suggestion that going from ethoxy (C2H5O·) to isopropoxy, the energy ordering of the electron configurations with in- and out-of-plane half-filled p-orbitals of the oxygen atom is reversed and the ground vibronic symmetry changes from a″ to a′. However, the observed spin-rotation coupling constants are not consistent with their predication from either semi-empirical theory or quantum chemical calculations. Additionally, the lack of observed transitions involving the out-of-plane transition moment component is not consistent with high level electronic structure calculations suggesting mixing of vibronic levels by strong spin-orbit coupling. A new twofold model has been developed that explicitly includes Coriolis and spin-orbit coupling between different vibronic levels. This model renders the discrepancy between theoretical and experimental spin-rotation constants moot. Moreover, it determines independently the contributions to the observed splitting between the lowest two levels, resulting from non-relativistic kinetic and Coulombic effects, and that due to the relativistic spin-orbit interaction. The experimental values show that these effects are comparable, but that the vibronic one is slightly more important. This result is at variance with state-of-the-art electronic structure calculations which otherwise do a remarkably good job of describing the ground state of isopropoxy.