SOLVENT EFFECTS ON MOLECULAR AND IONIC SPECTRA .4. PHOTOCHEMISTRY OF FE2- POSSIBLE MECHANISMS FOR THE PRIMARY ABSORPTION PROCESS LEADING TOELECTRON EJECTION((H2O)(6) IN WATER REVISITED )

Our method (parts I-III1-3) for estimating solvent shifts of species w hich have strong specific interactions (e.g., hydrogen bonding) with t he solvent is applied to inorganic charge transfer spectra; As the sim ulation of the structure of ions in solution is not completely straigh tforward, a number of aspects of the simulation procedure are investig ated, concentrating on solvent shift sensitivity. Specifically, we inv estigate the ultraviolet absorption spectrum of aqueous Fe2+(H2O)(6); for centrosymmetric systems such as this, it is found to be most impor tant to correctly represent the structure of the second coordination s hell. Assumptions such as that of rigidity of the inner shell and the particular choice of boundary conditions are of minor consequence. In a process studied extensively over several decades, ultraviolet light absorption results in electron ejection, leading to the photochemical decomposition of water. Several mechanisms for the primary process hav e been suggested in the past, without consensus being achieved. These include initial metal to ligand charge transfer (MLCT), metallic 3d -- > 4s absorption, direct electron photodetachment producing a partially solvated electron in a preexisting solvent cavity, and charge transfe r to solvent absorption (CTTS). We consider the energetics and solvent shift of the first three of these processes, concluding that the MLCT band is too high in energy, the d --> s band could participate, and t he photodetachment band is at the correct energy and intensity to acco unt for all that is (as yet) observed of the absorption band. In gener al, a rather complicated picture of this process in inorganic complexe s emerges.