Electronic spectroscopy of HRe(CO)5: a CASSCF/CASPT2 and TD-DFT study

The low-lying excited states of HRe(CO)5 have been calculated at the CASSCF/CASPT2 and TD-DFT level of theory using relativistic effective core potentials (ECP) or ab initio model potentials (AIMP). The theoretical absorption spectrum is compared to the experimental one. Despite the similarity between the experimental absorption spectra of HMn(CO)5 and HRe(CO)5 in the UV/visible energy domain it is shown that the assignment differs significantly between the two molecules. The low-lying excited states of HRe(CO)5 correspond to 5d→π*CO excitations whereas the spectrum of HMn(CO)5 consists mainly of 3d→3d and 3d→σ*Mn–H excitations. If the CASPT2 and TD-DFT results are quite comparable for the lowest excited states, the upper part assignment is more problematic with the TD-DFT method.