Bisquaric Acid: Unusual Solid State NMR, Electronic Structure, and a Predicted Order−Disorder Transition

Abstract

The molecular structure of the recently synthesized compound bisquaric acid was investigated by high-resolution solid state 13C NMR spectroscopy and electronic structure calculations. Variable temperature 13C NMR spectra were found quite unusual compared with the solution phase spectra and not explainable on the basis of the literature data on squaric acid. In particular, the 13CsOH peaks were found to be coincident with the 13CdO peaks. An extensive electronic structure analysis using the density functional theory (B3LYP) was thus performed, including the calculation of the 13C shifts by the GIAO method. Indeed, the calculations indicate that the linker CsC bond has a substantial double-bond character, thereby leading to a strong deshielding of the CsOH carbons. In addition, the theoretical study shows significant differences between bisquaric acid and squaric acid in several molecular properties, such as aromaticity, hydrogen-bonding, dipole moment, and acid-base behavior. Unlike squaric acid, no phase transition was detected for bisquaric acid by 13C NMR. However, the theoretical results suggest the possibility of such a transition at temperatures lower than about 100 °C, but it should be probed by other methods such as specific heat measurements.