Crystal structure of 4-oxo-4H-chromene-3-carboxylic acid

In the title compound, C10H6O4, also known as 3-carboxychromone, the non-H atoms of the chromone ring are essentially coplanar (r.m.s. deviation = 0.0057 Å), with the maximum deviation from their least-squares plane [0.011 (2) Å] being for a pyran C atom. The dihedral angle between the fused ring and plane of the carboxy group is 3.06 (2)°. An intramolecular hydrogen bond is formed between the ring carbonyl O atom and the carboxy O—H atom, closing an S(6) loop. In the crystal, molecules are assembled by stacking interactions [centroid–centroid distance between the benzene and pyran rings = 3.844 (3) Å] and C—H⋯O hydrogen bonds, generating a three-dimensional network. Short contacts are also observed between the carboxy O and C atoms [C=O⋯C=O = 3.002 (3) Å].

The mean deviation of the least-square planes for the non-H atoms of the chromone ring is 0.0057 Å, and the largest deviation is 0.011 (2) Å for C2. These mean that these atoms are essentially coplanar (Fig. 1). The dihedral angle between the fused-ring and carboxy plane is 3.06 (2)°. Intramolecular hydrogen bond is formed between the α,β-unsaturated carbonyl O atom and the carboxy O-H atom.
In the crystal packing, the molecules are assembled by stacking interactions [centroid-centroid distance between the benzene and pyran rings of the 4H-chromene units = 3.844 (3) Å] and C-H···O hydrogen bonds, as shown in Fig. 2.

S2. Experimental
The title compound was synthesized from 3-formylchromone according to the literature method (Helguera et al. 2013).
Single crystals suitable for X-ray diffraction were obtained by slow evaporation of an acetone solution of the title compound at room temperature.

S3. Refinement
All hydrogen atoms were placed in geometric positions [C-H 0.95 Å and O-H 0.84 Å], and refined using a riding model with U iso (H) = 1.2U eq of the parent atoms.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level.
Hydrogen atoms are shown as small spheres of arbitrary radius.

Figure 2
A view of the intermolecular interactions of the title compound. Intramolecular O-H···O and intermolecular C-H···O hydrogen bonds are represented as dashed lines.

4-Oxo-4H-chromene-3-carboxylic acid
Special details Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 σ(F 2 ) is used only for calculating R-factor (gt).