NMR study of proton location in strongly hydrogen bonded complexes of pyridine as influenced by solvent polarity

Abstract

1H, 15N and 13C NMR spectra of H-bonded complexes of 15N-enriched pyridine (Y) with various strong proton donors (XH) were obtained in a range of solvents with different dielectric constant at low temperature. Chemical shifts and nuclear magnetic relaxation times, T 1, were measured. A procedure for evaluating the mean NH distance in the complexes using T 1 values for 15N and 13C nuclei treated as determined by dipole-dipole coupling with nearest protons is proposed. The results point to a gradual shift of the average proton position towards the N atom, with increasing the XH acidity. For a given complex, the NH distance falls with the ε dielectric constant of a solvent, linear correlation between rNH and the reactive field factor, F=( ε-1)/(2 ε+1), being observed. The analogous correlation was found for the Δδ values. For interpretation of the effect, an anharmonic two-dimensional model of H-bond in connection with the Onsager-Butcher potential was used. It was found that the reactive field of a solvent causes the NH bond to shorten due to the specific feature of the dipole moment function common for nearly all XH…Y and X −…HY + complexes ( ∂μ/ ∂R Y-H<0). With the required parameters taken from IR spectra, this model gives a reasonable coincidence with the experimental Ar NH values.