A Study of Hydrogen Bonding

The proton magnetic resonance technique lends itself to the study of hydrogen bond equilibria. Through PMR spectra, hydrogen bonding can be detected. The specific proton which participates intimately in the hydrogen bond may be identified. The following work has investigated hydrogen bonding systems in which the proton donor is trifluoroacetic acid and the proton acceptor is an N-heterocyclic base, such as quinazoline, quinoline, and isoquinol ine. From measurements of the chemical shift versus concentration, and from measurements of the line width at half peak amplitude versus concentration for the time average exchange signal, the extent of hydrogen bonding may be determined and the participating equilibria may be characterized. Two equilibrium constants have been calculated using the PMR data obtained in this study. Solvent effects upon the hydrogen bonding equilibria have been noted in several cases. These effects were similar to those found in other previous investigations and can be explained. The effect of hydrogen bonding upon the N-heterocyclic ring has been observed. The ring protons are chemically shifted upon hydrogen bond association of the N-heterocycle with the acid. This observation reveals a charge redistribution in the heterocycle upon complex formation. There is insufficient data to calculate the charge densities of these systems, however