Proton NMR studies of the Lewis acid-base reactions between pyridinium chlorides and the acids ZnCl₂ and AlCl₃

Pyridinium chloride (PyHCl), which is the salt formed from the Lowry-Bronsted acid-base reaction between pyridine and HCl, is utilized as the base in a study of chloride transfer equilibria in the Lewis acid-base systems PyHCl + AlCl₃, PyHCl + ZnCl₂, and α-MePyHCl + ZnCl₂. Upfield shifts in the PyHCl proton resonance occur in each case as the Lewis acid is added, providing a probe of the electrical field adjacent to transferred chloride ions, and hence a measure of the effectiveness of the transfer process. In the presence of excess AlCl₃ in the PyHCl + AlCl₃ system, the proton becomes localized on the pyridine nitrogen to the extent that splitting due to interaction with the three spin states of the nitrogen can be observed, as in amines. PyH⁺ may be considered a "free" cation under these conditions. On the basic side of the equivalence point in this system the complex hydrogen-bonded chlorodipyridinium cation (PyH⋯Cl⋯HPy)⁺ is stabilized, with an approximate free energy of formation from the isolated ions of 50 kJ mol^-1. At the maximum Lewis acidity reached in this study the chemical shifts for the nitrogenic proton, and for two of the three types of ring proton, come into agreement with values calculated for the "free" pyridinium cation. An implication is that in molten PyHCl itself the proton exchanges freely between Py and Cl^- entities and probably contributes much of the high electrical conductivity of this liquid.