TD DFT insights into unusual properties of excited sandwich complexes: structural transformations and vibronic interactions in Rydberg-state bis(η6-benzene)chromium.

Sandwich compounds represent the only class of organometallics revealing vibronic structures of Rydberg transitions in their gas-phase absorption and ionization spectra. This provides rare possibilities of verifying computational results for Rydberg-state metal complexes by comparison with experimental spectroscopic data. In this work, the lowest Ryberg p state of bis(η6-benzene)chromium (1) corresponding to the 3dz2→ R4px,y transition has been modeled for the first time by TD DFT. The calculations were found to be able not only to estimate the energy of the Rydberg excitation in the 1 molecule but also to simulate its vibronic structure on the basis of the Rydberg-state optimized geometries and vibrational frequencies. The structural transformations caused by the Jahn-Teller effect in the excited 1 molecule appear to differ strongly from those in the degenerate-state benzene ion, cobaltocene or other metal-benzene complexes. The in-plane CH bending mode provides the main contribution to the JT distortion of the 1 excited-state D6h structure resulting in splitting of the R4px,y state into the R4px and R4py components belonging to the D2h point group. The calculations predict, however, a fluxional 1 behavior described by the D6h symmetry. Nevertheless, the JT effect leads to additional allowed vibronic components of the 3dz2→ R4px,y transition which is clearly revealed by the TD DFT simulation. The computational results correlate surprisingly well with the known experimental spectroscopic data and provide new insights into vibronic interactions in the Rydberg-state sandwich molecules.