Proton transfer in H-bond: Possibility of short-range order solvent effect

Abstract

1H NMR spectra of acetonitrile (AN) were studied in the neat substance and in the presence of strongly hydrogen-bonded pyridine- N-oxide (PyO)…HCl as solute. The attribution of observed 1H NMR signals was based on the density functional theory calculation of the proton magnetic screening tensor of suspected solvent clusters and H-bond structures in the 6-311G(d, p) basis set taking into account the solvent effect by the polarized continuum model (IEF-PCM). A perfect agreement between experimental and calculated chemical shift values of AN protons was obtained after short-range molecular ordering and long-range electrostatic interactions were handled by a ‘supermolecule’ model (i.e., 4-molecule cluster of AN molecules) including a solvent reaction field. The optimized geometry of PyO…HCl in vacuo shows no proton transfer between molecules, whereas in the presence of only one properly oriented AN molecule, the proton is observed to be transferred. We report also appearance of an ‘extra’ 1H NMR peak at ca. 3 ppm, which can be considered as the first experimentally observed short-range ordering effect of solvent molecules in the surrounding of the ionic pair PyO H +…Cl −.