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

Abstract

The B̃-X̃ laser-induced-fluorescence spectrum of jet-cooled isopropoxy radical (i-C3H7O[middle 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 X̃ and B̃ states and the electron spin-rotation constants of the X̃ state. The line intensities are well simulated with a parallel transition type, requiring the same symmetry for the levels involved of each the X̃ and B̃ state, which confirms the previous suggestion that going from ethoxy (C2H5O[middle 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.

Laster spectroscopic investigations of organic alkoxy and peroxy radicals.

“…Recent high-resolution spectroscopy investigations revealed both similarities and differences between the rotational and fine structure of the JT molecules (e.g., t-butoxy) and pseudo-JT molecules (e.g., isobutoxy). 33,132 It would be interesting to compare the vibronic structures of these two types of molecules. Such investigation, however, is out of the scope of the present dissertation.…”

“…33,35 Analysis of the rotational and fine structure of the X̃ / Ã state decomposes the Ã−X̃ separation into two components: the spin−orbit (SO) splitting and the nonrelativistic separation because of difference in both the electrostatic energy and the zero-point energy (ZPE) between the two states. 33,132 It is of both theoretical and practical interests to extend the spectroscopic investigation of cyclohexoxy to the methylcyclohexoxy (MCHO) radicals. Especially important is the effect of the methyl group position at the six-membered ring.…”

“…Simulation of DF spectra of isopentoxy was not attempted for two reasons: (i) severe broadening of transitions in the DF spectra, and (ii) the expected complexity of the vibronic structure of the quasi-degenerate Xand Ã-states. In a CAM-B3LYP calculation in which the HUMO and LUMO of the β electron are swapped, 132 the energy separation between the X̃ 2 A'' and the low-lying à 2 A' states of the TT (Cs) conformer of isopentoxy was determined to be 208 cm −1 . Ã−X̃ separations of other conformers, which have C1 symmetry, cannot be calculated accurately but are expected to be comparable to that of the TT conformer.…”

Laser spectroscopy investigations of jet-cooled metal-containing free radicals.

“…In the C s symmetry case, the 2 A E  state of organometallic monomethyl radicals is split into two states with A′ and A′′ symmetries. The energy separation between these two states is determined by SO and vibronic interactions, as well as the difference between their zeropoint energies (ZPEs) 96,97 . The vibronic structures of the pJT molecules may also be significantly different from those of the JT ones.…”

“…In the 1 A X →   spectrum, one peak is observable at 93 cm -1 that is absent in the 2 A X →   spectrum. This difference can be explained by the fact that the 1 A  and A  states have different symmetries ( A′ and A′′ ) and are coupled by the pJT effect, which distorts the PESs 96,97 . Although the A′ state PES retains its C s symmetry, the A′′ state PES is reduced to C 1 at its minima.…”