Crystal structures of -[4-(tri-fluoro-meth-yl)phen-yl]benzamide and -(4-meth-oxy-phen-yl)benz-amide 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 mol-ecular units as asymmetric units with no solvent in the unit cells. Crystal structure I, , is the result of the crystallization of -[4-(tri-fluoro-meth-yl)phen-yl]benzamide, CHFNO. Crystal structure II, , is composed of -(4-meth-oxy-phen-yl)benzamide, CHNO, units. is triclinic, space group , consisting of two mol-ecules in the unit cell related by the center of symmetry. is monoclinic, space group 2/, consisting of four mol-ecules in the unit cell. Both types of mol-ecules contain three planar regions; a phenyl ring, an amide planar region, and a -substituted 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° with respect to each other in both experimentally determined structures, as compared to 30° in the DFT results. These conformational changes result in more favorable environments for N-H⋯O hydrogen bonding and aryl ring π-stacking in the crystal structures. Inter-molecular inter-actions were examined by Hirshfeld surface analysis and qu-anti-fied by calculating mol-ecular inter-action energies. The results of this study demonstrate that both hydrogen bonding and dispersion are essential to the side-by-side stacking of mol-ecular units in these crystal structures. Weaker dispersion inter-actions along the axial directions of the mol-ecules reveal insight into the melting mechanisms of these crystals.