Conformational flexibility and pseudosymmetric aggregation in a betainium salt hydrate

Abstract

The cation of 3-(trimethylammonium)-benzoic acid exhibits a considerable conformational flexibility connected to the orientation of carboxyl group, coupled to the proton position, and rotations of the trimethylammonium group. This conformational flexibility considerably affects the aggregation of ions and molecules of 3-(trimethylammonium)-benzoic acid iodide hydrate, which crystallizes in a lowsymmetry structure with four symmetry-independent formula units (monoclinic space group Pc, Z' = 4, model I). However this structure approximates the symmetry of space group P2 1 / c in the same unit cell (Z' = 2, model II), and also even a still more highly symmetric structure of space group P2 1 /c in a half-smaller unit cell (Z' = 1, model III). The independent formula units are differentiated by the conformations of trimethylammonium groups (ordered in model I, but disordered in models II and III), conformations of the carboxyl groups and positions of OH···O bonded water molecules (ordered in molecules I and II, and disordered in model III), while the bulks of the cations and iodide anions are consistent for all models. While the calorimetric study and varied-temperature structural determinations exclude phase transitions of the highly pseudosymmetric model I. This pseudosymmetry is independent of temperature between 100 K and the m.p. at 463 K, and illustrates subtle structural effects leading complex molecular aggregation in the crystal. Theoretical computations confirm the low energy differences between the conformers.