Abstract
7,7'-Disubstituted 2,2'-methylenedioxy-1,1'-binaphthyls are highly efficient chirality inducers in nematic liquid crystals. The absolute configuration of these compounds is, however, hard to determine as they only crystallize as racemic mixtures. In this work a Vibrational Circular Dichroism (VCD) study is reported that provides an unambiguous determination of the absolute configuration of these compounds. An in-depth General Coupled Oscillator (GCO) analysis of the source of the VCD signal reveals that the unusual structure of these binaphthyl compounds inherently leads to strong and robust VCD bands. Combined with linear transit calculations, our VCD studies allow for the determination of key structural parameters.
Highlights
A study was published on 7,70-disubstituted 2, 20-methylenedioxy-1,10-binaphthyls which showed that these compounds have extremely high helical twisting powers.[26]
In order to elucidate why these molecules have such high helical twisting power (HTP) values -and potentially develop in a rational manner novel compounds with even higher HTPs-it is important to compare the twist in these compounds with the helical twisting they induce in Liquid Crystals (LCs)
We show that application of Vibrational Circular Dichroism (VCD) leads quite straightforwardly to the determination of the absolute configuration of these binaphthyls compounds
Summary
A study was published on 7,70-disubstituted 2, 20-methylenedioxy-1,10-binaphthyls which showed that these compounds have extremely high helical twisting powers.[26]. In order to come to a reliable assignment of the absolute configuration and retrieve structural information, a careful comparison between the experimentally recorded VCD spectrum and theoretically predicted spectra is required since it is almost never possible to come to an assignment merely on the basis of the experimental spectrum.[28,29,30,31] Such a theoretical modelling can be challenging as the VCD spectrum is extremely sensitive to the finer details of the molecular conformation.[32,33] This structural sensitivity arises from the fact that intense VCD signals often arise from the coupling between vibrational motions in different parts of the molecule via the General Coupled Oscillator (GCO) mechanism32 – which is the vibrational counterpart of the well-known exciton–exciton interactions in ECD28,34 – and this coupling depends strongly on the relative orientation of the two interacting molecular fragments.
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