Is squaric acid square? A combined X-ray and neutron diffraction study of 3,4-dihydroxycyclobut-3-ene-1,2-dione

The single-crystal structure of the title compound has been studied by both X-ray and neutron diffraction. The space group is P21/m with Z= 2 in a unit cell of dimensions (X-ray data): a= 6·119(3), b= 6·133(3), c= 5·297(3), β= 90·05(3)°, λ= 0·7107 Å. The final weighted discrepancy factors, R1, obtained after anisotropic full-matrix least-squares refinement were: X-ray 0·069 (2717 reflections) and neutron, extinction-corrected, 0·038 (714 reflections). The squaric acid molecule is asymmetric with two types of hydrogen bonding to the four oxygen atoms. The two O ⋯ O distances are 2·532(4) and 2·544(4)Å(neutron data). The molecules are planar and located at mirror-plane special positions. Strong intermolecular hydrogen bonding between the molecules in the plane results in the formation of an infinite plane sheet structure. The layers are parallel to the ab plane and the distance between layers is c/2, 2·649 Å.