An energetic study of differences in crystallization of N-(furan-3-yl)benzamide and N-(thiophen-3-yl)benzamide.

The crystal structures of N-(furan-3-yl)benzamide, C₁₁H₉NOS, FAP, and N-(thiophen-3-yl)benzamide, C₁₁H₉NO₂, TAP, were determined by single-crystal X-ray diffraction at 173 K. The molecular units in both structures consist of three planar regions: a five-membered aryl ring, an amide linkage, and a phenyl ring. Both compounds crystallize in the space group P1 with no solvent in the unit cell. There are two crystallographically unique, but geometrically similar, molecules in the asymmetric unit of FAP. N-H...O hydrogen bonds in FAP link the molecules into a linear chain lying along the b axis. The asymmetric unit in TAP is a disordered molecule containing contributions from two conformers with different orientations of the thiophenyl ring. N-H...O hydrogen bonds in TAP link the molecules into a linear chain lying along the a axis. Conformations of the gas-phase isolated conformers were predicted with density functional theory (DFT) calculations at the M06-2X/6-31+G(d) level. The conformers in FAP possess similar twist angles with respect to their calculated isolated conformers. However, the DFT calculations revealed a significant difference (>20°) in the twist angles of the thiophenyl rings-amide plane in TAP relative to the predicted gas-phase conformations. The π-stacking ring interactions between hydrogen-bonded molecules in the two crystal structures are not the same and are related to the difference in the magnitude of the dispersion and electrostatic interactions in the FAP and TAP environments..