Molecular Structural Studies on Compounds Containing C–O Bond by Electron Diffraction. II. The Molecular Structure of Anhydrous Oxalic Acid

The molecular structure of anhydrous oxalic acid was studied by electron diffraction method. The interatomic distances and valency angles were found as follows: C=O 1.22 Å, C–O 1.37 Å, C–C 1.54 Å, ∠O=C–O 125°, and ∠O=C–C 122° with possible errors within 0.02 Å, 0.02 Å, 0.06 Å, 3° and 5° respectively. The molecule takes planar trans configuration. From these data the following conclusions were derived. The central C–C bond seems to have a small degree of double bond character with no appreciable conjugation between two carbonyl groups. The molecule takes the planar trans configuration in spite of the small degree of double bond character in the C–C bond. This fact in connection with the data of other compounds seems to suggest the possibility of the existence of intermolecular hydrogen bonds in a free oxalic acid molecule. Our data of oxalic acid in the gaseous state were compared with the data of oxalic acid in the crystalline state from x-ray diffraction. Significant differences was found between the results on molecular dimensions, especially in hydroxyl C–O bond lengths. From these fact it was inferred that the structure of a molecule (or a carboxyl group) is affected by the molecular environment such as hydrogen bond formation between molecules.