Bisquaric acid: Unusual solid state NMR, electronic structure, and a predicted order-disorder transition

The molecular structure of the recently synthesized compound bisquaric acid was investigated by high-resolution solid state C-13 NMR spectroscopy and electronic structure calculations. Variable temperature 13C NMR spectra wer e found quite unusual compared with the solution phase spectra and not expl ainable on the basis of the literature data on squaric acid. In particular, the C-13-OH peaks were found to be coincident with the C-13=O peaks. An ex tensive electronic structure analysis using the density functional theory ( B3LYP) was thus performed, including the calculation of the C-13 shifts by the GIAO method. Indeed, the calculations indicate that the linker C-C bond has a substantial double-bond character, thereby leading to a strong deshi elding of the C-OH carbons. In addition, the theoretical study shows signif icant differences between bisquaric acid and squaric acid in several molecu lar properties, such as aromaticity, hydrogen-bonding, dipole moment, and a cid-base behavior. Unlike squaric acid, no phase transition was detected fo r bisquaric acid by C-11 NMR. However, the theoretical results suggest the possibility of such a transition at temperatures lower than about 100 degre esC, but it should be probed by other methods such as specific heat measure ments.