Spectroscopic differences between molecular (O–H ⋯ N) and ionic pair (O–⋯ H–N+) hydrogen complexes

Abstract

The centre of gravity (_H) and chemical shifts (δ_H) of hydrogen-bonded protons in 55 complexes of pyridines with acetic acids have been measured in dichloromethane. If _H and δ_H are plotted against the aqueous pK_a values for protonation of these bases general scatter diagrams are obtained which may be resolved into separate trends for different acids. We noted that this behaviour is controlled by the equilibrium between the molecular complex and the ion pair. Strong overlap of the carbonyl and carboxy stretching bands of the molecular complex and the hydrogen-bonded ion pair leads in most cases to a single band. Characteristic variations of the bandwidth were found. A plot of δ_H against _H can be resolved into a series of straight lines for molecular complexes, hydrogen-bonded ion pairs and their mixtures, respectively. A straight line for molecular complexes consists of data for the monomeric acids. The chemical shifts for N⁺–H proton of the ‘free’ pyridine cation was estimated from the line of protonated pyridines and compared with the calculated value.