Reinterpretation of the Spectra of Hydrated Co ++ : An ab Initio Study

The absorption spectrum of aqueous Co++ is reinterpreted in light of ab initio calculations. CAS-MCSCF and MCQDPT calculations yield the spectral states and their oscillator strengths, as well as the spin−orbit coupling between the quartet and doublet states of four-, five-, and six-coordinate Co++−water complexes. Spectral states and oscillator strengths are also computed for these complexes with one water replaced by a hydroxide ion. The results of the calculations are compared with measured spectra obtained from room temperature and high temperature Co++ solutions. A completely symmetric six-coordinate octahedral metal−water complex will have zero oscillator strength because of its symmetry. We find, moreover, that the calculated oscillator strength remains very small (≤10-6) even when the symmetry of this species is allowed to relax or when we distort the molecule asymmetrically. Therefore, the calculations suggest that the six-coordinate species can contribute little to the observed spectrum, even th...