Hybrid Ab initio/EFP approach for calculating d‐d absorption spectrum of hexaammineruthenium(II) ion in aqueous solutions

Abstract

AbstractAb initio quantum chemical strategies for quantitatively predicting the lowest (1Ag → 1T1g) vertical d‐d excitation energy of hexaammineruthenium(II) ion in aqueous solution are discussed. The scalar‐relativistic ECP/valence basis set on Ru atom developed by the Stuttgart group in a combination with the state‐average CASSCF(d) approach, followed by multiconfigurational quasi‐degenerate second‐order perturbation theory (MCQDPT2) to account for differential correlation effects is proved to be an adequate tool to reproduce the experimental absorption spectrum of the complex for a variety of AO basis sets on ligand atoms. In addition, different ab initio methodologies are examined in order to predict the ground state geometry which is consistent with the follow‐up excitation spectrum calculations. It is observed that the use of the optimized structures of a hypothetical gas‐phase complex lead to substantial underestimation of excitation energies. Solvent effects strongly influence the excitation energy though indirectly, mainly by means of changing the ground state geometry of the solvated complex when compared with the vacuum one. In particular, the ground state structure of the complex surrounded by effective fragments simulating water molecules provides the lowest CASSCF/MCQDPT excitation energy estimate to be within 25,500–26,400 cm−1, in a fair agreement with the experimentally measured value of 25,600 cm−1. At the same time, direct incorporation of solvation effects causes only minor change in the estimated transition energies, within several hundred cm−1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008