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 interactions such as hydrogen bonding
occur:  we consider the interpretation of the
metal-to-ligand charge-transfer (MLCT) absorption and electroabsorption
spectra of
Ru2+(NH3)5-pyrazine and
its
conjugate acid
Ru2+(NH3)5-pyrazine-H+
in dilute aqueous solution. The electroabsorption spectra of these
complexes
(among the first to be observed for inorganic complexes) taken in S. G.
Boxer's laboratory indicated that very small
dipole moment changes occur on excitation from the ground to the
excited state; it has been found necessary to
develop and extensively test, in earlier parts of this series, a
sophisticated model for solvent−solute interactions in
order to interpret these experimental results. In our approach,
first, ab initio MCSCF and INDO methods are
used
to estimate the gas-phase electronic excitation energies; second, Monte
Carlo simulations are performed to determine
the ground-state liquid structures; 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
surfaces and boundary conditions are used in the simulations, and some
variation in the liquid structures but little
variation in the calculated solvent shifts and dipole moment changes
result. The calculated solution frequencies
agree quite well with those observed, and the anomalously low values
observed for dipole moment change are
reproduced; the Magnuson and Taube model for the electronic structure
of
Ru2+(NH3)5-pyrazine-H+
is verified.