Banana-shaped molecules peculiarly oriented in a magnetic field: 2H NMR spectroscopy and Quantum Mechanical calculations

The orientational properties of the banana-shaped liquid crystal 4-chloro- 1,3-phenylenebis{4-[4'-(10-undecenyloxy)]benzoyloxy} benzoote (CIPbis11BB) 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 H-2 NMR spectroscopy, both in the bulk and in solution, are discussed. H-2 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.