Molecular and crystal structure of 4-alkoxybenzoic acids: Design of the mesogenic phase |
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Authors: | L G Kuz’mina N S Kucherepa S M Pestov A N Kochetov N S Rukk S A Syrbu |
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Institution: | 1.Kurnakov Institute of General and Inorganic Chemistry,Russian Academy of Sciences,Moscow,Russia;2.Lomonosov State Academy of Fine Chemical Technology,Moscow,Russia;3.Faculty of Chemistry and Biology,Ivanovo State University,Ivanovo,Russia |
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Abstract: | The crystal and molecular structures of 4-alkoxybenzoic acids C
n
H2n + 1-O-C6H4-COOH, n = 4–12, which are nematic (n = 4–6) or smectic-nematic (n = 7–12) mesogenes, are studied. No direct relationship between the molecular geometry and mesogenic properties is found.
All the crystal structures contain dimers formed by pairs of O…H-O hydrogen bonds between carboxyl groups. Crystals belonging
to the group of smectic-nematic liquid crystals (SNLC) are built of separate regions, namely, loosely packed aliphatic regions
and closely packed aromatic regions with significant π-stacking interactions. Loose aliphatic regions occur in crystals belonging
to the group of nematic liquid crystals (NLC); however, they are surrounded by dense aromatic regions and do not run throughout
the crystal. In NLC, π-stacking interactions are far weaker than in SNLC. At the early stage of melting of SNLC crystals,
structurized associates due to both π-stacking interactions and hydrogen bonds are retained in the melt (smectic phase), whereas,
upon further heating, only hydrogen-bonded associates are left (nematic phase). In the course of melting of NLC, only hydrogen-bonded
dimers are retained in the melt (nematic phase). |
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