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 CASS CF/CASPT2 and TD-DFT level of theory using relativistic effective core pote ntials (ECP) or ab initio model potentials (AIMP). The theoretical absorpti on spectrum is compared to the experimental one. Despite the similarity bet ween the experimental absorption spectra of HMn(CO)(5) and HRe(CO)(5) in th e UV/visible energy domain it is shown that the assignment differs signific antly between the two molecules. The low-lying excited states of HRe(CO)(5) correspond to 5d --> pi (CO)* excitations whereas the spectrum of HMn(CO)5 consists mainly of 3d --> 3d and 3d --> sigma*(Mn-H) excitations. If the C ASPT2 and TD-DFT results are quite comparable for the lowest excited states , the upper part assignment is more problematic with the TD-DFT method. (C) 2001 Elsevier Science B.V. All rights reserved.