Molecular and crystal structure of 4-hexylbenzoic acid: Design of the mesophase
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Molecular and Crystal Structure of 4Hexylbenzoic Acid: Design of the Mesophase L. G. Kuz’minaa, S. M. Pestovb, A. N. Kochetovb, A. V. Churakova, and E. Kh. Lermontovaa a
Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia email: [email protected] b Moscow State Academy of Fine Chemical Technology, Moscow, 119571 Russia Received March 24, 2010
Abstract—The crystal structure of 4hexylbenzoic acid C6H13–C6H4–COOH, which forms a nematic mesophase upon melting, is determined. The crystal contains three crystallographically independent mole cules. Their molecular skeletons are made up of two almost planar fragments: a benzene ring, πconjugated with the carboxyl group and a planar zigzag aliphatic fragment. One of the independent molecules forms cen trosymmetric dimers via pairs of hydrogen bonds between carboxyl groups, whereas the two others are linked via hydrogen bonds. The dimers in the crystal are packed into pseudostacks with a pronounced nonparallel arrangement of conjugated fragments. There is no good mutual projecting of benzene rings in the stacks, which corresponds to efficient πstacking interaction. The graph describing the mesophase of this compound contains only one structureforming element (a hydrogen bond) and corresponds to the nematic mesophase. DOI: 10.1134/S1063774510050111
INTRODUCTION Alkyloxybenzoic acids (AOBs) are typical liquid crystals (LCs). Higher representatives of the AOB homologous series, which have long alkyl chains (from С7Н15– to С12Н25–), form both smectic and nematic mesophases, whereas the homologues with shorter alkyl groups (from С3Н7– to С6Н13–) form only the nematic mesophase [1]. A structural study and analy sis of crystalline packings have been previously per formed [2] for the entire homologous AOB series. It was established that the mesophase formation is deter mined by the joint action of two factors: (i) the parti tion of the crystal space into closely packed aromatic regions and loosely packed aliphatic regions and (ii) the presence of weak directional interactions in the aromatic regions. The melting of a crystal begins in its loose aliphatic regions and is accompanied by their disordering, whereas the aromatic regions retain their degree of structuring for some time, which conditions the exist ence of the mesophase due to the weak directional interactions in these regions. According to the modern concepts of supramolecular chemistry [3, 4], the for mation of supramolecular architectures both in the solution and in the melt is due to a large extent to a combined set of weak directed interactions of different types: πstacking and/or С–Н···π interactions, con ventional hydrogen bonds, etc. [5–14]). Weak interac tions decay with an increase in temperature, and the system is transformed into an isotropic liquid. The concepts of supramolecular chemistry turned out to be very efficient for studying the phase transi
tions in LC compounds of the AOB group because they made it possible not only
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