Axis-switching in the vibrationless (A)over-tilde ← (X)over-tilde transition of the jet-cooled deuterated methyl peroxy radical CD3O2

The jet-cooled high resolution spectrum of the vibrationless (A) over tilde <- (X) over tilde transition of the deuterated species of the methyl peroxy radical has been recently published in this journal (S. Wu, P. Dupre, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007)). The spectrum was analyzed using a rigid-rotor model with quadratic spin-rotation coupling. The analysis was based on the fit of similar to 350 partially resolved line positions and was quite satisfactory. However, the full simulation of the spectral intensity clearly identifies a lack of ability to reproduce relatively small line clumps (extra lines) located between the two main central Q branches. This is indicating of an incomplete initial analysis. In the present paper we reanalyze this electronic transition by considering a reference-frame axis-switching resulting from the nuclear rearrangement associated to the electronic transition (spectra obtained at two different temperatures are considered). The potential energy hypersurfaces of the two electronic states are sufficiently dissimilar to induce changes in the molecule geometry, particularly, the angle (COO) over cap, which induces a rotation (similar to 1.7 degrees) of the principal axes of inertia located in the molecule symmetry plane. The present analysis is supported by a global fitting of the spectrum intensity and gives rise to a slightly different set of molecular constants. Attention is paid to the wavefunction symmetry assignment of a non-orthorhombic molecule. Couplings due to the torsion of the methyl group are discussed in the following paper (P. Dupre, J. Chem. Phys. 134, 244309 (2011)). (C) 2011 American Institute of Physics. [doi: 10.1063/1.3599953]