SOLVENT EFFECTS ON MOLECULAR AND IONIC SPECTRA .5. DEVELOPMENT OF A METHOD FOR SIMULATION OF THE LIQUID STRUCTURE AND SOLVATOCHROMIC SHIFT OF INORGANIC COMPLEXES SUCH AS PENTAAMINOPYRIDYLRUTHENIUM(II) IN WATER

Our method (parts 1-3) for estimating solvent effects on electronic sp ectra in media with strong solute-solvent interactions is extended to interpret the MLCT absorption spectrum of Ru2+(NH3)(5)-pyridine in dil ute aqueous solution: it should be generally applicable to inorganic c harge-transfer spectra. First, both ab initio MCSCF and INDO/S-CI meth ods are used to estimate the gas-phase electronic excitation energies and state charge distributions; second, Monte Carlo simulations are pe rformed to determine the ground-state liquid structure; finally the so lvent shift is evaluated based on the gas-phase charge distributions a nd the explicit ground state solvent structure: no arbitrarily adjusta ble parameters such as ''cavity radii'' are required. Several intermol ecular potential surfaces are used to investigate the relationship bet ween solvent shift and solvent structure, with the most reasonable str ucture determined by comparison with results simulated for dilute Ru2- (NH3)(6) solution: for this complex, the inner solvent shell contain ed 13 water molecules which formed 18 hydrogen bonds to the ammonia li gands. The solvent shift calculated using various methods is -7500 +/- 1500 cm(-1), implying that the gas-phase transition energy should be 32000 +/- 1500 cm(-1), consistent, given the level of approximation us ed, with the INDO/S-CI and ab initio MCSCF predictions which range fro m 34 000 to 39 000 cm(-1). It is proposed that a method which is both practicable and reliable for calculations such as this is to use ab in itio SCF ESP charges combined with Kollman's force field as the interm olecular potential during the simulations and to use INDO/S-CI charges when evaluating solvent shifts.