A computational study of the charge-delocalized and charge-localized forms of the croconate and rhodizonate dianions

Ab initio and DFT calculations indicate that the charge-localized forms (C2v symmetry) of croconate (C5O52−) and rhodizonate (C6O62−) dianions cannot exist as a single entity. Instead, this form of the dianions can exist in the presence of counter cation(s) or in hydrogen-bonded networks. For the charge-delocalized (D5h symmetry) form of C5O52−, it is found that the HF/3-21G(d) and CASSCF(n,m)/3-21G(d) levels of theory yield C–C and C–O bond lengths that are within the experimental range. However, for the D6h tautomer of C6O62−, all the theoretical methods that have been tried led to optimized C–C bond lengths that are slightly (about 0.02–0.06 Å) too long.