Crystal structure of a second monoclinic polymorph of 3-methoxybenzoic acid with Z′ = 1

A second polymorph of 3-methoxybenzoic acid with Z′ = 1 is reported and compared with first polymorph with Z′ = 2.


Chemical context
Methoxybenzoic acid, also called anisic acid, consists of three isomers with molecular formula C 8 H 8 O 3 : the crystal structures of 2-and 4-methoxybenzoic acids with Z 0 = 1 have been reported (Parvez, 1987;Etter et al., 1988;Bryan, 1967;Colapietro & Domenicano, 1978;Fausto et al., 1997;Hathwar et al., 2011) and polymorphism has not been observed for these two isomers in the Cambridge Structural Database (CSD) (Version 5.39, last update August 2018; Groom et al., 2016) to date. In this article, we report a second polymorphic form (I) of 3-methoxybenzoic acid with Z 0 = 1 and compare its properties with those of the previously reported first polymorphic form (I). Polymorph I crystallizes in the monoclinic space group P2 1 /n with a = 13.8034 (17) Å , b = 5.0275 (5) Å , c = 21.446 (3) Å and = 99.320 (13) (Raffo et al., 2014;refcode EFINEO). The asymmetric unit of I consists of two molecules with different conformations (Z 0 = 2), which are connected into a homodimer through strong O-HÁ Á ÁO hydrogen bonds. As described below, these two conformers (A and B) differ in the orientation of the methoxy group and its relative position from the -OH group. DFT calculations suggest that the A conformer of I is more energetically stable than the B conformer (Pereira Silva et al., 2015).

Supramolecular features
In the crystal of I, two inversion-related molecules are joined into a homodimer with an R 2 2 (8) graph-set motif via strong pairwise O-HÁ Á ÁO hydrogen bonds (Fig. 2, Table 1). The homodimers are linked by weak C-HÁ Á ÁO hydrogen bonds between two methoxy groups into zigzag chains with R 2 2 (6) graph-set motifs, which propagate along the b-axis direction. The [010] chains are stacked along the a axis into corrugated sheets parallel to the ab plane via weakinteractions with a centroid-to-centroid distance of 3.8018 (6) Å (symmetry codes: x À 1, y, z and x + 1, y, z) and slippage of 1.676 Å .

Hirshfeld surface analysis
The Hirshfeld surfaces mapped with normalized contact distance d norm and the two-dimensional fingerprint plots for I were generated using CrystalExplorer17.5 (Turner et al., 2017). The large and small red spots on the Hirshfeld surface mapped with d norm (Fig. 3) correspond to the O2-H1O2Á Á ÁO1 and C8-H8AÁ Á ÁO3 hydrogen bonds, respectively. The HÁ Á ÁO distances are 1.09 and 0.16 Å shorter than the sum of van der Waals radii of H and O atoms (2.72 Å ). The HÁ Á ÁH contact is the most populated contact and contributes 42.3% of the total intermolecular contacts, followed by HÁ Á ÁO/OÁ Á ÁH (32.9%), HÁ Á ÁC/CÁ Á ÁH (11.4%) and CÁ Á ÁC (8.1%) contacts (Fig. 4). The tips of pseudo-mirrored sharp spikes at d e + d i ' 1.6 Å represent the shortest HÁ Á ÁO/OÁ Á ÁH contacts, corresponding to the O2-H1O2Á Á ÁO1 hydrogen-bond. The absence of significant C-HÁ Á Á interaction in the crystal structure of I is indicated by the absence of characteristic 'wings' in the fingerprint plot of HÁ Á ÁC/CÁ Á ÁH contacts. The CÁ Á ÁC contacts include the weakinteraction, which appears as a unique 'triangle' focused at d e ' d i ' 1.8 Å . Theinteraction is illustrated as a unique pattern of red and blue 'triangles' on the shape-index surface and a flat region on the curvedness surface of the phenyl ring (see supporting Figures S1 and S2).

Lattice energy calculation
The lattice energies of polymorphs I and I were calculated using PIXEL software (Gavezzotti, 2003) at default settings. The calculated lattice energy of I (107.5 kJ mol À1 ) is larger than that of I (98.5 kJ mol À1 ) and this comparison is in agreement with the report of Pereira Silva et al. (2015), in which I is more stable than I under ambient conditions. Partial crystal packing of I. Dashed lines represent the hydrogen-bonds. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx; Ày þ 2; Àz þ 2; (ii) Àx; Ày þ 1; Àz þ 2.

Figure 3
The Hirshfeld surface mapped over d norm of the central molecule of I hydrogen bonded to two neighbouring molecules.

Figure 1
The molecular structure of I with 50% probability displacement ellipsoids.

Synthesis and crystallization
Single crystals of I were obtained from an unsuccessful attempt of co-crystallization between 3-methoxybenzoic acid and hexamethylenetetramine. Colourless plate-like crystals were obtained from slow evaporation of a methanolic mixture of 3-methoxybenzoic acid and hexamethylenetetramine in equimolar ratio at room temperature.

3-Methoxybenzoic acid
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.