Four new methoxy-substituted derivatives of E,E-2,5-bis(2-phenylethenyl)pyrazine have been synthesized. The supramolecular structures of the resulting set of five polymorphs have been studied using single-crystal X-ray diffraction to gauge the influence of the position of the methoxy groups on the organization of the molecules in the solid state, as part of an attempt to dispense with the particular polymorphism of the parent compound. The crystal packing patterns were analyzed in terms of the two pyrazine-based synthons found in the parent compound's crystal structure, the π pyrazine 3 3 3 π phenyl stacking synthon, and the pyrazine hydrogen-bonded synthon, as well as in terms of weak intermolecular interactions such as CH 3 3 3 O, CH 3 3 3 N, and CH 3 3 3 π. The analysis shows that the introduction of methoxy groups in positions other than only the para position of the peripheral benzene rings successfully switches off the two synthons seen in the parent compound and that the new compounds adopt other packing strategies, based on methoxy 3 3 3 methoxy and -OCH 3 3 3 3 π contacts. Polymorphism, however, remains.
A very long one-dimensional chain can be obtained through hydrogen bond formation at both ends of the molecule. These compounds configure layered structures in the solid state, as well as most of the polymorphic phases of long-chain compounds, such as n-alkanes, n-alcohols, and n-fatty acids. Dicarboxylic acids exhibit interesting thermal properties and a high enthalpy of melting that make them potential candidates for their use in the field of thermal protection and energy storage. Dimorphism was prevalent in odd carbon members of dicarboxylic acids while one form is only present for even members: α form P2 1 /c with Z=4 and β form C2/c with Z=4. High-quality x-ray powder diffraction analyses of pentadecanedioic acid (C 15 H 28 O 4) have been carried as a function of temperature. A new form, called γ has been observed just before melting: γ form C2/c with Z=4 at 383K. The packing arrangement of the carboxyl groups at both end of the chain is different in the three forms. From the structural point of view for γ form the distance interlayer is intermediate between α and β distances. Crystal structure of the pentadecanedioic acid is presented. The carboxyl's group in α form shows two different angles with middle plain of the chain. The packing arrangement of α and γ forms are radically different even if the molecular conformation are cis in both cases. A reversible transition from β form to γ form was observed before melting. Slurry experiments indicate that β is the most stable form at room temperature.
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