| Popis: |
Core excitation energies (CEs) are known to depend on the chemical environment mostly through the charge transfered from or to the would-be excited atom in the ground-state molecule. We have made use of this peculiarity to set up a combined method for evaluating the CEs of molecules involving heavy atoms, where cumulated handicaps make direct calculations very difficult. We have evaluated the CEs of np levels in chromium, molybdenum and tungsten hexafluorides and compared the contributions of relaxation and relativity. In a first step, various approximate methods were used to evaluate the amount of charge transfered in the three hexafluorides, using the experimental geometries and testing different definitions of the charge. Results show the following trends: i) the calculated charge transfer increases as CrF 6 ≪ MoF 6 6 ; ii) Mulliken (balanced) charges vary in the order REX ≫ RHF > CISD > DFT, and Weinhold (natural) charges tend to be slightly larger; iii) our best (CISD) calculations give a natural percentage of electron transfer from the metal atom to the bonded fluorines of about 45% for CrF 6 , 56% for MoF 6 , and 59% for WF 6 . In a second step, numerical ab-initio , relativistic, ΔDF calculations of the total and orbital energies were performed on the ground-state and core-excited metal ions involving 1 to 5 valence ionizations. Core excitation energies were deduced and the relative importance of relaxation and relativity effects was discussed. In a last step, the core excitation energies for the molecules were evaluated by interpolating between values previously obtained for the free ions, using the net atomic charges derived for the ground-state molecules in our best previous approximation. The results are particularly striking for WF 6 : 1) for core excitations from the 2p 1/2 , 2p 3/2 and 3p 1/2 , 3p 3/2 levels, experimental energies are reproduced within 0.4–1.2 eV; 2) there is a relaxation alteration of the charge transfer stronger for the 3p than for the 2p levels; 3) relativistic corrections are much larger than and opposite to relaxation corrections. |
