Journal of Physical Chemistry, Vol.98, No.43, 11075-11088, 1994
Solvent Effects on Molecular and Ionic Spectra .4. Photochemistry of Fe2+(H2O)(6) in Water Revisited - Possible Mechanisms for the Primary Absorption Process Leading to Electron Ejection
Our method (parts I-III1-3) for estimating solvent shifts of species which have strong specific interactions (e.g., hydrogen bonding) with the solvent is applied to inorganic charge transfer spectra; As the simulation of the structure of ions in solution is not completely straightforward, a number of aspects of the simulation procedure are investigated, concentrating on solvent shift sensitivity. Specifically, we investigate the ultraviolet absorption spectrum of aqueous Fe2+(H2O)(6); for centrosymmetric systems such as this, it is found to be most important to correctly represent the structure of the second coordination shell. 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 have 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 transfer 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 the photodetachment band is at the correct energy and intensity to account for all that is (as yet) observed of the absorption band. In general, a rather complicated picture of this process in inorganic complexes emerges.