Rotationally resolved (B)over-tilde ← (X)over-tilde electronic spectra of the isopropoxy radical: A comparative study

The (B) over tilde - (X) over tilde laser-induced-fluorescence spectrum of jet-cooled isopropoxy radical (i-C3H7O center dot) 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 (X) over tilde and (B) over tilde states and the electron spin-rotation constants of the (X) over tilde state. The line intensities are well simulated with a parallel transition type, requiring the same symmetry for the levels involved of each the (X) over tilde and (B) over tilde state, which confirms the previous suggestion that going from ethoxy (C2H5O center dot) 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. (C) 2013 AIP Publishing LLC.