NMR Methods for the Unambiguous Identification of meso-Compounds and the Racemate of Their Corresponding Enantiomers

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NMR spectroscopy is the most important structural characterization method if a molecule of interest cannot be crystallized. A regularly occurring class of molecules with particular difficulties in structure determination is formed by so-called meso compounds, which can hardly be distinguished from the racemate of the corresponding enantiomers in solution. Todays NMR methods allow a distinct identification only in cases when a scalar coupling across the symmetry center can be detected. No applicable experiments exist so far for molecules with overlapping multiplets or with too few NMR-active nuclei. In addtion, existing methods only allow the positive identification of meso-compounds by signal doubling, while the corresponding racemate cannot be distinguished from an accidentally degenerate spectrum. The goal of this project is the development of a set of broadly applicable NMR methods for meso/racemate identification. Central aims are maximum resolution via homonuclear and heteronuclear decoupling as well as the correlation of indirectly coupled spins across the symmetry center via TOCSY-like pulse sequence elements. Also the usage of residual dipolar couplings introduced by partial alingment will be followed, because they allow correlations over large distances and make the positive identification of racemates possible. In compounds with low proton-density fast experiments will be developed for the application of dissolution-DNP and the corresponding boost in senstivity in the detection of low abundance nuclei. Finally maximum distinction of the stereogenic centers is sought by selected residual quadrupolar and dipolar coupling evolution for special cases.