Banana-Shaped Molecules Peculiarly Oriented in a Magnetic Field: 2H NMR Spectroscopy and Quantum Mechanical Calculations

• Published in: Chemphyschem 2007-11, Vol.8 (16), p.2321-2330
• Main Authors: Domenici, Valentina, Veracini, Carlo Alberto, Fodor-Csorba, Katalin, Prampolini, Giacomo, Cacelli, Ivo, Lebar, Andrjia, Zalar, Boštjan
• Format: Article
• Language: English
• Subjects: Ab initio calculations; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Density-functional theory; deuterium; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; General molecular conformation and symmetry; stereochemistry; liquid crystals; magnetic properties; Molecular properties and interactions with photons; NMR spectroscopy; Physics; Properties of molecules and molecular ions
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200700377

The orientational properties of the banana‐shaped liquid crystal 4‐chloro‐1,3‐phenylenebis{4‐[4′‐(10‐undecenyloxy)]benzoyloxy} benzoate (ClPbis11BB) are reported in the nematic phase under the effect of an external magnetic field. A new hypothesis, which states that the central ring of the aromatic core is oriented perpendicularly to the external magnetic field, is proposed. In support of this hypothesis, a series of studies based on 2H NMR spectroscopy, both in the bulk and in solution, are discussed. 2H NMR measurements on three selectively deuterium‐labelled isotopomers are presented, together with DFT results from B3LYP/cc‐pvDz calculations performed on the aromatic core. The rather flat shape of the investigated intramolecular energy surface allows for several different conformations to be populated, the computed magnetic susceptibilities of which are consistent with the proposed hypothesis of peculiar orientation of banana‐shaped molecules. Moreover, the orientation of the magnetic susceptibility tensor is shown to be strongly dependent on the internal conformation of the banana‐shaped molecules. Alignment of bent molecules: Banana‐shaped molecules of a benzoate liquid crystal form a nematic phase with a peculiar orientation. Based on 2H NMR experiments and quantum mechanical calculations (see figure), it is proposed that the mesogens aggregate so that the central rings of the aromatic cores are oriented perpendicularly to the external magnetic field.