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          Institute: MPI für medizinische Forschung     Collection: jahrbuch_2011_archival     Display Documents

ID: 597795.0, MPI für medizinische Forschung / jahrbuch_2011_archival
Conformation and Dynamics of 18−Membered
Hexathiametacyclophanes: A Two Step Racemization as Studied by
Deuterium NMR in Chiral Lyotropic Liquid Crystals
Translation of Title:Conformation and Dynamics of 18−Membered
Hexathiametacyclophanes: A Two Step Racemization as Studied by
Deuterium NMR in Chiral Lyotropic Liquid Crystals
Authors:Lesot, Philippe; Aroulanda, Christie; Berdagué, Philippe; Zimmermann, Herbert; Luz, Zeev
Date of Publication (YYYY-MM-DD):2011-10-20
Title of Journal:Journal of Physical Chemistry B
Journal Abbrev.:J. Phys. Chem. B
Issue / Number:41
Start Page:11793
End Page:11804
Review Status:Peer-review
Audience:Experts Only
Intended Educational Use:No
Abstract / Description:The conformation and interconversion dynamics of two derivatives of the 18−membered hexathia metacyclophane 1 and 2 were studied by 1H NMR spectroscopy in isotropic solvents and by 2H NMR in chiral liquid crystalline (CLC) solutions, as well as by molecular structure computations. For the analysis of the dynamic effects, we made use of the concepts of "average symmetry" and "isodynamic groups", introduced by Altmann (Altmann, Proc. R. Soc.1967, 184, A298). Compound 1, which is unsubstituted in the inner aromatic site, has, according to the NMR and molecular force field calculations, a boat shaped ground conformation with C2 symmetry. It is highly flexible and in the NMR spectrum exhibits two successive dynamic processes. There is a low temperature (170−210 K, Ea = 10.5 kcal/mol) alternate "wing flipping", which corresponds to interchange between pairs of enantiomers and results, in the fast exchange limit, in an average prochiral molecule with C2v symmetry. This process is followed, at higher temperatures (290−320 K, Ea = 28.5 kcal/mol), by an umbrella flipping type inversion with an average structure of D2h symmetry. This second process involves averaging of effective enantiotopic into homotopic sites and can only be studied in chiral solvents. The origin of the chiral discrimination and of their stepwise averaging is discussed. Compound 2, which is substituted with methoxy groups at the inner sites of the benzene rings, is much less flexible and exhibits dynamic effects in the NMR spectrum only at temperatures above 370 K. We were able to study the kinetic parameters of this process in isotropic solvents (Ea = 21.4 kcal/mol). As for 1, the detailed mechanism of this process can in principle be established using dynamic NMR in CLC; however, experimental limitation precluded us from doing so. Possible alternatives and their effect on the 1D and 2D exchange spectra in CLC are discussed in a concluding section
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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