Crystal structures of N-[4-(trifluoromethyl)-phenyl]benzamide and N-(4-methoxyphenyl)-benzamide at 173 K: a study of the energetics of conformational changes due to crystal packing

As a part of our study of the syntheses of aryl amides, the crystal structures of two benzamides were determined from single-crystal X-ray data at 173 K. Both crystal structures contain molecular units as asymmetric units with no solvent in the unit cells. Crystal structure I, TFMP, is the result of the crystallization of N[4-(trifluoromethyl)phenyl]benzamide, C14H10F3NO. Crystal structure II, MOP, is composed of N-(4-methoxyphenyl)benzamide, C14H13NO2, units. TFMP is triclinic, space group P (1) over bar, consisting of two molecules in the unit cell related by the center of symmetry. MOP is monoclinic, space group P2(1)/c, consisting of four molecules in the unit cell. Both types of molecules contain three planar regions; a phenyl ring, an amide planar region, and a parasubstituted phenyl ring. The orientations of these planar regions within the asymmetric units are compared to their predicted orientations, in isolation, from DFT calculations. The aryl rings are tilted approximately 60 degrees with respect to each other in both experimentally determined structures, as compared to 30 degrees in the DFT results. These conformational changes result in more favorable environments for N-H center dot center dot center dot O hydrogen bonding and aryl ring pi-stacking in the crystal structures. Intermolecular interactions were examined by Hirshfeld surface analysis and quantified by calculating molecular interaction energies. The results of this study demonstrate that both hydrogen bonding and dispersion are essential to the side-by-side stacking of molecular units in these crystal structures. Weaker dispersion interactions along the axial directions of the molecules reveal insight into the melting mechanisms of these crystals.