Two conformational polymorphs of 4-methyl­hippuric acid

4-Methylhippuric acid {systematic name: 2-[(4-methylbenzoyl)amino]ethanoic acid}, a p-xylene excreted metabolite with a backbone containing three rotatable bonds (R-bonds), is likely to produce more than one stable molecular structure in the solid state. In this work, we prepared polymorph I by slow solvent evaporation (plates with Z′ = 1) and polymorph II by mechanical grinding (plates with Z′ = 2). Potential energy surface (PES) analysis, rotating the molecule about the C-C-N-C torsion angle, shows four conformational energy basins. The second basin, with torsion angles near-73°, agree with the conformations adopted by polymorph I and molecules A of polymorph II, and the third basin at 57° matched molecules B of polymorph II. The energy barrier between these basins is 27.5 kJ mol-1. Superposition of the molecules of polymorphs I and II rendered a maximum r.m.s. deviation of 0.398 Å. Polymorphs I and II are therefore true conformational polymorphs. The crystal packing of polymorph I consists of C(5) chains linked by N-H⋯O interactions along the a axis and C(7) chains linked by O-H⋯O interactions along the b axis. In polymorph II, two molecules (A with A or B with B) are connected by two acid-Amide O-H⋯O interactions rendering R 2 2(14) centrosymmetric dimers. These dimers alternate to pile up along the b axis linked by N-H⋯O interactions. A Hirshfeld surface analysis localized weaker noncovalent interactions, C-H⋯O and C-H⋯π, with contact distances close to the sum of the van der Waals radii. Electron density at a local level using the Quantum Theory of Atoms in Molecules (QTAIM) and the Electron Localization Function (ELF), or a semi-local level using noncovalent interactions, was used to rank interactions. Strong closed shell interactions in classical O-H⋯O and N-H⋯O hydrogen bonds have electron density highly localized on bond critical points. Weaker delocalized electron density is seen around the p-methyl­phenyl rings associated with dispersive C-H⋯π and H⋯H inter­actions.