J. Palomar et al., Bisquaric acid: Unusual solid state NMR, electronic structure, and a predicted order-disorder transition, J PHYS CH A, 105(39), 2001, pp. 8926-8930
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.