Theoretical analysis of the oxocarbons: the electronic spectrum of the rhodizonate ion

In the present paper the electronic spectrum of the rhodizonate anion [(C6O6)2−] is calculated in the gas phase and aqueous solution using a sequential Monte Carlo/Quantum Mechanical procedure. The D6h and C2 conformers are considered in addition to their complexed forms ([Li2(C6O6)]) at distinct coordination modes. Semiempirical and ab initio methods were used to calculate the electronic spectra in gas phase with the results being quite sensitive to the level of theory applied. In aqueous solution, only the semiempircal approach was used due to the large size of the water/solute clusters considered to take into account the solvent effect. The counter-ion effect on the electronic spectrum of the rhodizonate ion is more pronounced than those caused by the solvent. In general the theoretical results found for the complex [Li2(C6O6)] are in good accordance with the experimental data.