Molecular and Vibrational Structure of Tetroxo d0 Metal Complexes in their Excited States. A Study Based on Time-Dependent Density Functional Calculations and Franck-Condon Theory

We have applied time dependent density functional theory to study excited state structures of the tetroxo d(0) transition metal complexes MnO4-, TcO4-, RuO4, and OsO4. The excited state geometry optimization was based on a newly implemented scheme [Seth et al. Theor. Chem. Acc. 2011, 129, 331]. The first excited state has a C-3v geometry for all investigated complexes and is due to a "charge transfer" transition from the oxygen based HOMO to the metal based LUMO. The second excited state can uniformly be characterized by "charge transfer" from the oxygen HOMO-1 to the metal LUMO with a D-2d geometry for TcO4-, RuO4, and OsO4 and two C-2v geometries for MnO4-. It is finally found that the third excited state of MnO4- representing the HOMO to metal based LUMO+1 orbital transition has a D-2d geometry. On the basis of the calculated excited state structures and vibrational modes, the Franck-Condon method was used to simulate the vibronic structure of the absorption spectra for the tetroxo d(0) transition metal complexes. The Franck-Condon scheme seems to reproduce the salient features of the experimental spectra as well as the simulated vibronic structure for MnO4- generated from an alternative scheme [Neugebauer J. J. Phys. Chem. A 2005, 109, 1168] that does not apply the Franck-Condon approximation.