High‐resolution solid‐state NMR study of reversible 1,5‐proton shifts in organic solids

Abstract

AbstractHigh‐resolution CP‐MAS NMR analyses of solid organic samples where fast 1,5‐proton shifts occur between tautomeric structures are described. Attention is focused on the available techniques to establish the existence of such reactions. Dynamic interconversion is unambiguously characterized where complete line shape study involving multiple sites is possible. Reconciliation between this information and that provided by diffraction methods is stressed, the latter often suggesting the presence of unique, static structures in crystals. In cases in which reactions are in the fast limit at all accessible temperatures, the combination of CP‐MAS spectra, x‐ray or neutron diffraction data at variable temperatures and theoretical calculations of molecular properties may be helpful in establishing double minimum potential wells for the proton motion. Among these, porphyrins, arylazonaphthols and 1,3‐diketones in rigid and non‐rigid skeletons are interesting examples of current interest.