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

The mol­ecule of the title compound, C(5)H(9)NO(3), is essentially planar [the maximum deviation for a non-H atom from the mean plane is 0.021 (3) Å] due to the π-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 hydrogen-bonding inter­actions between oxime groups and also by π-stacking of the mol­ecules due to the carbonyl and oxime group orbital overlap [inter­planar distance between adjacent mol­ecules = 3.143 (4) Å]. Jointly, these factors afford infinite 6.32 Å 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 Å.