title: Influence of N-alkyl substituents and counterions on the structural and mesomorphic properties of guanidinium salts: experiment and quantum chemical calculations.
abstract: A series of N-4-(4'-alkoxybiphenyl)-N',N',N",N"-tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N-alkyl substituents, each with a number of different counterions. X-ray crystal-structure analyses of 1b I, 1b PF(6), 2a I, and 4a I reveal bilayer structures in the solid state and, for the 1b and 1b PF(6) salts, a hydrogen-bond-type connectivity between the guanidinium N-H group and the anion is found. For the N-alkyl homologues 2a I and 4a I the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by (1)H NMR: the N-H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1b I, 2b I, 3b I, and 4b I was studied by temperature-dependent (1)H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three C-N partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N-H and the N-alkylguanidinium salts. A fourth barrier is observed for the rotation about the N-biphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N'-substituted 2-amino-1,3-dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (WAXS/SAXS). All liquid-crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N-H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.
authors.affiliation: Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55D-70569 Stuttgart, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Butschies
authors.forename: Sven
authors.initials: S
authors.lastname: Sauer
authors.forename: Elena
authors.initials: E
authors.lastname: Kessler
authors.forename: Hans-Ullrich
authors.initials: HU
authors.lastname: Siehl
authors.forename: Birgit
authors.initials: B
authors.lastname: Claasen
authors.forename: Peter
authors.initials: P
authors.lastname: Fischer
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Frey
authors.forename: Sabine
authors.initials: S
authors.lastname: Laschat