Ethyl (2E)-2-(hydroxyimino)propanoate

The molecule of the title compound, C5H9NO3, is essentially planar [the maximum deviation for a non-H atom from the mean plane is 0.021 (3) A ˚ ] due to the -conjugation of the hydroxyimino and carbonyl groups, which are trans to each other; ab initio calculations in vacuo at the DFT (B3LYP/6– 311G**++) level of theory confirmed that E conformer is indeed the lowest in energy. The packing in crystal structure is influenced by strong intermolecular O—H N hydrogenbonding interactions between oxime groups and also by - stacking of the molecules due to the carbonyl and oxime group orbital overlap [interplanar distance between adjacent molecules = 3.143 (4) A ˚ ]. Jointly, these factors afford infinite 6.32 A ˚ thick molecular sheets, where the plane of each molecule is perpendicular to the plane of the sheet. Seen from above, the molecules within the sheet are arranged in a herringbone pattern. Such sheets form a stack due to weak van der Waals interactions; the gap between adjacent sheets is 2.07 A ˚ .