NMR study of proton transfer interactions in the system pyridine +HCl (0%-95%)

Pyridinium chloride, which has proven useful in a number of molten salt studies where it serves as a strong chloride donor Lewis base, is freely soluble in both excess acid and excess base. To probe the proton transfer interactions involved, proton NMR studies have been performed on pyridine+HCl solutions in the composition range 0%-95% HCl at temperatures ranging from -100 to 200 °C. This system is rich in examples of the way in which competing hydrogen bonding interactions can affect the magnetic environment of the involved protons. The most obvious of these is the increased magnetic shielding that develops in the vicinity of the N-H⁺ proton of the pyridinium cation as excess HCl competes for the Cl^- ions, initially to form HCl ₂^- anion and later to form more extended HCl⋯HCl₂^- aggregates. As excess pyridine is added to PyHCl a shift in the opposite direction occurs, due either to the formation of symmetrical [Py-H-Py]⁺ species, to the repression of PyHCl autodissociation, or to a combination of both effects as expressed by (PyHCl)₂ dimer formation. Less obvious but equally interesting is the deshielding of the proton in the HCl₂^- anion of the PyH⁺-HCl₂^- melt which accompanies addition of pyridine. This results from a competition between Py with Cl^- for the HCl proton and a consequent tendency to produce the [PyH⋯Cl⋯HPy] ⁺ species implied by the stoichiometry of certain crystalline compounds. Finally the ring protons themselves reflect the competition of excess HCl for the Cl^- of PyHCl and the consequent shifts in ring electron density which accompany the inward movement of the N-H⁺ proton. The trends are such as to bring the observed ring proton chemical shifts relative to pyridine into better accord with theoretical predictions for the isolated PyH⁺ cation. The trend is continued when excess HCl is replaced by the stronger Lewis acids ZnCl₂ and AlCl₃ and in the case of excess AlCl₃, agreement with theory is almost quantitive except for the α proton shifts. For the latter, the observed shifts are not even qualitatively correct.