Re-evaluating the Aromaticity of Oxocarbon Molecules
Mentor:Kimberley Cousins, Professor of Chemistry, California State University San Bernardino
Croconic acid has been recently shown to have strong, room temperature ferroelectric behavior through a proposed “aromatic” transition state. This project consists of re-evaluating the aromaticity, using computational methods, of the four cyclic oxocarbon analogues deltic, squaric, croconic, and rhodizonic acid along with their anion and dianion forms. By using other molecules of similar structure, three sets of isodesmic reactions were evaluated, resulting in a total of forty-two molecules whose properties were calculated. The properties used to evaluate aromaticity are: (1) the optimized geometries of each molecule (specifically bond lengths and bond angles that indicate symmetry), (2) the enthalpies of the isodesmic reactions, and (3) the values from the nucleus-independent chemical shifts scan (NICS scan) of each molecule. Each molecule was evaluated for extent of aromaticity and where available, calculations were compared to reported results. The optimized geometries and the enthalpies were calculated with Spartan 10 using the density functional method, B3LYP/6-311+G**. NICS scan calculations were performed using Gamess, with the optimized geometries from Spartan. As previously reported, oxocarbon dianions showed significant aromatic character. Surprisingly, the neutral deltic acid molecule also exhibited significant aromaticity, and the minimum energy geometry of the rhodizonic dianion was not planar, contrary to expectations. For the croconic series, the most aromatic was determined to be the dianion, as predicted previously.