Ab initio methods were used in conjunction with Monte Carlo simulation to analyze the structure and spectroscopic properties of the croconate ion in the gas phase and in aqueous solution. The infrared and Raman spectra were calculated and band assignments were made showing a good agreement with experiment. The electronic spectrum of the croconate ion was calculated in the gas phase and in aqueous solution, using a
sequential Monte Carlo/quantum mechanical approach, taking into account the solvent and counter ion effects. The electronic spectrum for the free croconate ion in aqueous solution showed two transitions at 479 and 468 nm when the first solvation shell is considered. These transitions were not sensitive to additional solvent molecules beyond the first solvation shell. The experimental electronic spectrum was only reproduced when the combined effects of the solvent and counter ion were taken into account. The calculated spectrum for
the cis-[Li2(C5O5)(H2O)21] complex showed two transitions at 383 and 365 nm, in agreement with the experimental observations of 372 and 351 nm. These results strongly suggest that in order to reproduce the experimental electronic spectrum of the oxocarbons in solution, we must take into account the combined effects of the solvent and the counter ions. A new proposal for the interaction of Li+ with the croconate anion in solution, based on the theoretical electronic spectra, is also discussed.
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