Supramolecular Packing of a Series of N-Phenylamides and the Role of NH···O=C Interactions

A series of seven N-phenylamides [R-C(O)NHPh, in which R: CH₃, C(CH₃)₃, Ph, CF₃, CCl₃, CBr₃, and H] were used as models in this study. Molecular packing and intermolecular interactions were evaluated by theoretical calculations, solution NMR, and quantum theory of atoms in molecules analyses. Crystallization mechanisms were proposed based on the energetic and topological parameters using the supramolecular cluster as demarcation. Concentration-dependent ¹H NMR experiments corroborated the proposed interactions between molecules. For all compounds (except for R: H, which initially formed tetramers), layers (two-dimensional) or chains (one-dimensional) were formed in the first stage of the proposed crystallization mechanisms. The presence of strong intermolecular NH···O=C interactions promoted the first stages. The study in solution provided different values of association constant (K _ass) governed by the hydrogen bond NH···O=C, showing that the stronger interactions are directly influenced by the substituent steric hindrance. A correlation between K _{ass(NH···O=C)} from the solution and the NH···O=C interaction energy in the crystal showed a good trend.