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          Institute: MPI für medizinische Forschung     Collection: Jahrbuch 2012     Display Documents

ID: 638114.0, MPI für medizinische Forschung / Jahrbuch 2012
The Chirality of a Twist−Bend Nematic Phase Identified by NMR Spectroscopy
Translation of Title:The Chirality of a Twist−Bend Nematic Phase Identified by NMR Spectroscopy
Authors:Beguin, Laetitia; Emsley, James W.; Lelli, Moreno; Lesage, Anne; Luckhurst, Geoffrey R.; Timimi, Bakir A.; Zimmermann, Herbert
Date of Publication (YYYY-MM-DD):2012-07-12
Title of Journal:Journal of Physical Chemistry B
Journal Abbrev.:J. Phys. Chem. B
Issue / Number:27
Start Page:7940
End Page:7951
Review Status:Peer-review
Audience:Experts Only
Intended Educational Use:No
Abstract / Description:One of the defining characteristics of the twist−bend nematic phase, formed by the methylene−linked liquid crystal dimer 1?,7?−bis(4−cyanobiphenyl−4'−yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D∞h symmetry is lost in a chiral phase because its symmetry is reduced to D∞ on removal of the mirror plane. Recording proton−enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C−H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the twist−bend nematic. Strikingly, this equivalence is not lost for the central methylene group where the two protons are homotopic. We also show how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4−octyl−4'−cyanobiphenyl−d2, perdeuteroacenaphthene−d10, and acenaphthene−d4 as examples. For the standard nematic phase deuterium, NMR shows that the deuterons in these methylene groups are equivalent but, as expected, in the twist−bend nematic phase this equivalence is lost. The deuterium NMR spectra of these probe molecules dissolved in CB7CB have been recorded from the isotropic phase, through the nematic and deep into the supercooled twist−bend nematic
External Publication Status:published
Document Type:Article
Communicated by:Wulf Kaiser
Affiliations:MPI für medizinische Forschung/Abteilung Biophysik
MPI für medizinische Forschung/Abteilung Biomolekulare Mechanismen
MPI für medizinische Forschung/Arbeitsgruppe Zimmermann
MPI für medizinische Forschung/Abteilung Molekulare Physik
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