The electronic spectrum of Fe2+ ion in aqueous solution: A sequential Monte Carlo/quantum mechanical study

A sequential Monte Carlo/quantum mechanics approach was used to investigate the solvent effects on d → d transition of the Fe2+ ion in aqueous solution. A set of Lennard-Jones parameters were generated by modification of the UFF force field. The structural results obtained for the liquid structure around the Fe2+ ion are in very good agreement with the experimental findings. The water molecules in the second coordination shell interacts strongly with the first shell, with a hydrogen bond energy of −15.6 ± 3.0 kcal/mol, which represents about 20% of the binding energy computed for the [Fe(H2O)]2+ species. The electronic spectrum was calculated within the TD-DFT approach and we show that inclusion of outer solvation shells explicitly in the calculation does not affect appreciably the d → d transition.