The molecular and crystal structure of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) at 121 °C: A neutron diffraction study

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Abstract
The molecular and crystal structure of squaric acid (3,4‐dihydroxy‐3‐cyclobutene‐1,2‐dione) has been determined by neutron diffraction techniques at 121°C, a temperature above that of the monoclinic to tetragonal phase transition (∼100°C). The high‐temperature structure is in space group I4/m, a=c=6.137(2) Å, b=5.337(2) Å. Least‐squares refinement has been carried out with 25 variable parameters and 276 reflections, to yield an unweighted R value (on F2) of 0.042. The squaric acid molecule has point symmetry 4/m, and the hydrogen atom is ’’disordered’’ about the inversion center at the midpoint of the O⋅⋅⋅O hydrogen bond. Hydrogen‐bonding parameters are O–H 1.014(4), H⋅⋅⋅O 1.536(5) Å, O⋅⋅⋅O 2.548(2) Å, & O–H⋅⋅⋅O 176.2(6) °. The tetragonal structure corresponds closely to that obtained from the atomic parameters determined in the monoclinic phase at 25°C, by averaging over the pseudofourfold symmetry.