Hydrogen/deuterium isotope effects on the 15N NMR chemical shifts and geometries of low-barrier hydrogen bonds in the solid state☆

Abstract In this study, hydrogen/deuterium isotope effects on the geometry and the 15N NMR chemical shifts of strongly hydrogen bonded, solid hydrogen-bisisocyanide salts of the type [MCN⋯L⋯NCM]−X+ 1 (L  H, D) are described both experimentally and theoretically. The theoretical studies include PM 2 6–31+ G(d,p) ab initio calculations of geometries and subsequent calculations of the proton and the deuteron vibrational states in the hydrogen bond of the model compounds [CN⋯L⋯NC]−Li+ 1a (L  H) and 1b (L  D). The Li+, at various fixed CLi distances, represents the external electric field. Furthermore, the 15N NMR chemical shifts were calculated using the Individual Gauge for Localized Orbitals method. The calculated isotope effects depend strongly on the asymmetry of the hydrogen bond caused by the influence of the Li+ cation. These results are compared with those of a solid state 15N cross polarization, magic angle spinning NMR study of the metal-stabilized salts 1c and 1d, where M  Cr(CO)5, X+  AsPh+4 and L  H or D, as well as of those of 1e and 1f, M  Cr(CO)5, and X+  NnPr+4. In the case of 1c and 1d the hydrogen bond is symmetric and isotope effects on the 15N NMR chemical shifts are not observed, as predicted theoretically. By contrast, and in agreement with the calculations, large isotope effects are observed for 1e and 1f, where the hydrogen bond symmetry is lifted by a strong interaction with the counterion. So far, solid state hydrogen/deuterium isotope effects on the NMR chemical shifts of hydrogen bonded nuclei have, to our knowledge, not been observed and may be used as a promising novel tool in hydrogen bond research.