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J. Phys. Chem. B 2009, 113, 10628–10640
Enantiodiscrimination of Flexible Cyclic Solutes Using NMR Spectroscopy in Polypeptide Chiral Mesophases: Investigation of cis-Decalin and THF Christie Aroulanda,*,† Olivier Lafon,†,‡ and Philippe Lesot*,§ Me´thodologie RMN, UMR CNRS-UHP 7036, Institut Jean Barriol, Faculte´ des Sciences, Nancy-UniVersite´, BP 70239, 54506 VandoeuVre-le`s-Nancy cedex, France, Unite´ de catalyse et de chimie du solide (UCCS), UMR CNRS 8181, Ecole Nationale Supe´rieure de Chimie de Lille, UniVersite´ de Lille 1, Baˆt. C7, BP 90108, 59652 VilleneuVe d’Ascq cedex, France, and Laboratoire de Chimie Structurale Organique, RMN en Milieu Oriente´, ICMMO, UMR CNRS 8182, UniVersite´ Paris Sud 11, Baˆt. 410, 91405 Orsay cedex, France ReceiVed: March 12, 2009; ReVised Manuscript ReceiVed: May 7, 2009
The conformational dynamics and orientational behavior of two model cyclic molecules, cis-decalin (cisdec) and tetrahydrofurane (THF), dissolved in weakly ordering, polypeptidic chiral liquid crystals (CLCs) are theoretically discussed and experimentally investigated using deuterium and carbon-13 NMR spectroscopies. The analysis of enantiomeric and enantiotopic discriminations in these compounds is shown to depend on the rate of conformational exchange regime, slow or fast. The slow exchange regime is illustrated through the case of cis-dec at low temperature (243 K). We show that the deuterium NMR spectra in this regime can be qualitatively and quantitatively interpreted by restricting the conformational pathway of cis-dec to two enantiomeric conformers of C2-symmetry. The orientational order parameters of these interconverting enantiomers are calculated by matching the 2H quadrupolar splittings with calculated conformer structures. The fast exchange regime is investigated through the examples of cis-dec at high temperature (356 K) and THF at room temperature (300 K). The 2H NMR spectra above the coalescence temperature are analyzed by introducing the concept of “average molecular structure”. This fictitious structure allows easily identifying NMR equivalences of solutes dissolved in CLC. However, it cannot be applied to determine consistent orientational order parameters. This study emphasizes that enantiotopic discriminations observed for flexible molecules in the fast exchange regime can be quantitatively interpreted only by considering the orientational order of each conformer. I. Introduction The combined analysis of the conformational dynamics and orientational behavior of flexible cyclic molecules using NMR spectroscopy in chiral liquid crystals (CLCs) is a noteworthy challenge. Among possible candidates for such investigations, cis-decalin, denoted 1 or cis-dec, and tetrahydrofurane, denoted 2 or THF, are valuable model molecules. cis-Decalin is an apolar bicycloalkane which interconverts between two enantiomeric conformers (see Figure 1).1,2 The activation enthalpy of this process is around 120 kJ · mol-1, which is much larger than the thermal energy (RT ≈ 2.5 kJ · mol-1 at T ) 300 K),3,4 and this exchange can be almost frozen, on the NMR time scale, at low temperature (
