The Molecular and Crystal Structure of the Cyclic Tetramer of Formaldehyde, 1,3,5,7-Tetroxocane

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Abstract
The crystal structure of the cyclic tetramer of formaldehyde, 1,3,5,7-tetroxocane, has been determined by the X-ray diffraction method. The unit cell is monoclinic, with a=11.455, b=4.160, c=12.232 Å, and β=108.4°. The space group is C2/c, and there are four molecules per unit cell. The structure was solved by using a molecular transform method and was refined by the block-diagonal least-squares method to a final R factor of 0.051, using 728 reflections measured with an automated four-circle diffractometer. The molecules in the crystal are of the so-called crown form, with an approximate C4v symmetry but an exact C2 symmetry. The torsion angles about the four independent C–O bonds appear in two distinguishable values: two bonds with 83.7 and −83.6°, and the other two bonds with 93.6 and −94.1°. The average C–O bond length is 1.403 Å, while the average C–O–C and O–C–O bond angles are 116.6 and 113.8° respectively. The deformation from a regular crown form by taking the two distinguishable torsion angles and the increase in bond angles is favorable to compensating for the intramolecular H···H repulsions; the close H···H distances in the deformed crown form are 2.16 and 2.20 Å.