SOLVENT EFFECTS ON MOLECULAR AND IONIC SPECTRA .7. MODELING THE ABSORPTION AND ELECTROABSORPTION SPECTRA OF PENTAAMMINE-RUTHENIUM(II) PYRAZINE AND ITS CONJUGATE ACID IN WATER

This work brings to a focus a series of papers concerning the modeling of solvent shifts in systems in which specific solute-solvent interac tions such as hydrogen bonding occur: we consider the interpretation o f the metal-to-ligand charge-transfer (MLCT) absorption and electroabs orption spectra of Ru2+(NH3)(5)-pyrazine and its conjugate acid Ru2+(N H3)(5)-pyrazine-H+ in dilute aqueous solution. The electroabsorption s pectra of these complexes (among the first to be observed for inorgani c complexes) taken in S. G. Boxer's laboratory indicated that very sma ll dipole moment changes occur on excitation from the ground to the ex cited state; it has been found necessary to develop and extensively te st, in earlier parts of this series, a sophisticated model for solvent -solute interactions in order to interpret these experimental results. In our approach, first, nb initio MCSCF and INDO methods are used to estimate the gas-phase electronic excitation energies; second, Monte C arlo simulations are performed to determine the ground-state liquid st ructures; finally, the solvent shifts and excited-state dipole moments are evaluated on the basis of the gas-phase charge distributions and the explicit ground-state solvent structures. A variety of potential s urfaces and boundary conditions are used in the simulations, and some variation in the liquid structures but little variation in the calcula ted solvent shifts and dipole moment changes result. The calculated so lution frequencies agree quite well with those observed, and the anoma lously low values observed for dipole moment change are reproduced; th e Magnuson and Taube model for the electronic structure of Ru2+(NH3)(5 )-pyrazine-H+ is verified.