title: Crystal polymorphism of propylammonium chloride and structural properties of its mixture with water.
abstract: The thermal behavior of propylammonium chloride (PAC) has been investigated by parallel beam X-ray powder diffraction in the 303-463 K thermal range. A polymorphic transition has been observed at 403 K, whereas the melting process starts at 438 K. The low-temperature (LT) polymorph is tetragonal, P4/n or P4/nmm, and a = 6.2429(2) Å, c = 7.3830(3) Å, and Z = 2. Thermal expansion is isotropic as the N···Cl interactions have components along the three crystallographic axes. At the transition temperature, a high-temperature (HT) polymorph was observed, with candidate space groups P23, Pm3, P432, P43m, and Pm3m and cell parameter a = 11.715(3) Å, consistent with Z = 12. This phase has features reminiscent of those of plastic phases of molecular crystals. The structural properties of a PAC/water mixture at ambient conditions were also studied by using an integrated approach, which combines X-ray diffraction measurements and molecular dynamics simulations carried out with the SPC/E and TIP5P water models. By using a Cl-water Lennard-Jones parameter previously refined for a similar system, a very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior has been highlighted, in which cations and anions do not possess a completely closed hydration shell of their own, but rather "solvent-shared ion pairs" are formed, where one or more water molecules act as a bridge between the chloride and propylammonium ions.
authors: Dipartimento di Chimica, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy.
authors: Valentina
authors: V
authors: Migliorati
authors: Paolo
authors: P
authors: Ballirano
authors: Lorenzo
authors: L
authors: Gontrani
authors: Olga
authors: O
authors: Russina
authors: Ruggero
authors: R
authors: Caminiti