NMR-STUDY OF PROTON LOCATION IN STRONGLY HYDROGEN-BONDED COMPLEXES OFPYRIDINE AS INFLUENCED BY SOLVENT POLARITY

H-1,N-15 and C-13 NMR spectra of H-bonded complexes of N-15-enriched p yridine (Y) with various strong proton donors (XH) were obtained in a range of solvents with different dielectric constant at low temperatur e. Chemical shifts and nuclear magnetic relaxation times, T-1, were me asured. A procedure for evaluating the mean N-H distance in the comple xes using T-1 values far N-15 and C-13 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 dis tance falls with the epsilon dielectric constant of a solvent, linear correlation between r(N-H) and the reactive field factor, F=(epsilon-1 /(2 epsilon+1), being observed. The analogous correlation was found fo r the Delta delta values. For interpretation of the effect, an anharmo nic two-dimensional model of H-bond in connection with the Onsager-But cher potential was used. tt was found that the reactive field of a sol vent 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+ co mplexes (partial derivative mu/partial derivative(Y-H)<0). With the re quired parameters taken from IR spectra, this model gives a reasonable coincidence with the experimental Delta r(NH) values.