NMR of Strong Hydrogen Bonds: Isotope Effects

Solid state nuclear magnetic resonance (NMR) is used as the main investigative tool to explore isotope effects in strong hydrogen bonds of small molecules. The rare isotopomeric polymorphism phenomenon in 4-methylpyridine pentachlorophenolate was studied by solid state NMR. 1H and 2H solid state NMR, inelastic neutron scattering and theoretic calculations were employed to study isotope effects on the N–H…O hydrogen bond. 1H and 2H MAS NMR confirmed the isotopomers do have different thermodynamically stable crystal structures. Both NMR studies and inelastic neutron scattering results indicate a low barrier double-well potential for the shorter and stronger hydrogen bond in the triclinic form. A model based on differences in zero-point energies can be constructed to account for the relative stability of crystal forms upon isotope substitution. A single crystal NMR study of 17O was performed on potassium hydrogen maleate-d2. Potassium hydrogen maleate has a short intramolecular O–H…O hydrogen bond that is believed to be symmetric. The strong hydrogen bond affects the quadrupole tensors of oxygen in the molecule, especially the neighbor oxygen on the COH sites. The principal values of the 17O quadruple coupling tensors on C=O and COH sites were determined by simulating the rotation plots of quadrupole splitting on satellite transitions. The CQ values are close to experimental values from other methods; the directions of three principal components, determined for the first time, are very close to the local axes of symmetry, which is similar to the results of other carboxylic acids and salts. 2H powder NMR experiments were performed at very low temperature for the Zundel cation (H5O2+) in deuterated H2SO4.4H2O. Zundel cation is a very common form of the hydronium ion in solution; it contains a strong hydrogen bond in the center and forms 4 weak hydrogen bonds with anions in the solid. By simulating the Pake patterns for the powder spectra, quadrupole coupling tensors of deuterons on 2 different hydrogen bond sites were determined. Deuterons were found preferentially occupy weak hydrogen bonding sites at low deuteration levels. This thermodynamic isotope effect can be explained by the different vibrational zero-point energies for different isotopomers. The quantum chemical calculations fully support this point, especially by using an improved, anharmonic vibrational potential model (VSCF) into the theoretical calculations