Analysis of enantiotopic discrimination in the NMR spectra of prochiral solutes in chiral liquid crystals by symmetry factorization of the Saupe ordering matrix

作者：Lesot Philippe^*; Aroulanda Christie; Luz Zeev

来源：Journal of Chemical Physics, 2009, 131(10): 104501.

DOI：10.1063/1.3197853

The enantiotopic discrimination in the NMR spectrum of prochiral molecules dissolved in chiral liquid crystals (CLCs) is governed by the ordering of the solute molecules. The lifting of the spectral degeneracy of enantiotopic sites in such solvents stems from the nonequivalence in their effective ordering. This, in turn, is brought about by a change in the directions (or the equivalence) of the principal axes of their ordering tensor, relative to those in achiral liquid crystals. This discrimination mechanism can only occur for solute molecules belonging to a limited number of ("allowed") improper point groups, viz., D(2d), C(2v), C(s), and S(4) [D. Merlet, J. W. Emsley, P. Lesot, and J. Courtieu, J. Chem. Phys. 111, 6890 (1999)]. In this work it is shown that the ordering tensor of such prochiral solutes in CLC, S(CLC), and likewise, the tensors, T(k), describing the anisotropic magnetic resonance interactions of enantiotopic pairs, can be partitioned into symmetric and antisymmetric (and irrelevant) parts. The NMR results in such solvents can be cast into separate sets of equations depending on either the symmetric or the antisymmetric parts of S(CLC) and T(k). The discrimination observed in such solvents depends only on the latter set of equation, while the former applies to the average splitting of enantiotopic pairs as well as to diasterotopic or homotopic sites in the prochiral molecules. The factorization procedure greatly facilitates the analysis of the ordering properties of prochiral solutes in CLC and provides new insight on the discrimination mechanism. In particular, it allows correlation between independent enantiotopic partners and the identification of NMR signals belonging to common prosterogenic faces in the molecule. Expressions relating NMR observables with the symmetric and antisymmetric parts of S(CLC) are derived for each of the four allowed groups. Model examples are presented and discussed.

• 出版日期2009-9-14