title: Four Polymorphs of Methyl Paraben: Structural Relationships and Relative Energy Differences.
abstract: Four polymorphic forms of methyl paraben (methyl 4-hydroxybenzoate,1), denoted1-I(melting point 126 °C),1-III(109 °C),1-107(107 °C), and1-112(112 °C), have been investigated by thermomicroscopy, infrared spectroscopy, and X-ray crystallography. The crystal structures of the metastable forms1-III,1-107, and1-112have been determined. All polymorphs contain the same O-H···O=C connected catemer motif, but the geometry of the resulting H-bonded chain is different in each form. TheZ' = 3 structure of1-I(stable form; space groupCc) contains local symmetry elements. The crystal packing of each of the four known crystal structures of1was compared with the crystal structures of 12 chemical analogues. Close two-dimensional relationships exist between1-112and a form of methyl 4-aminobenzoate and between1-107and dimethyl terephthalate. The lattice energies of the four methyl paraben structures have been calculated with a range of methods based on ab initio electronic calculations on either the crystal or single molecule. This shows that the differences in the induction energy of the different hydrogen-bonded chain geometries have a significant effect on relative lattice energies, but that conformational energy, repulsion, dispersion, and electrostatic also contribute.
authors.affiliation: Institute of Pharmacy, University of Innsbruck , Innrain 52, 6020 Innsbruck, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Gelbrich
authors.forename: Doris E
authors.initials: DE
authors.lastname: Braun
authors.forename: Arkady
authors.initials: A
authors.lastname: Ellern
authors.forename: Ulrich J
authors.initials: UJ
authors.lastname: Griesser