Localization of the proton in complexes of trifluoroacetic acid with pyridine N-oxides in dichloromethane by second-derivative infrared spectroscopy

Abstract

A single carbonyl band in the second-derivative FTIR spectra of the equimolar mixtures of six substituted pyridine N-oxides with trifluoroacetic acid in dichloromethane implies that complexation is complete and hydrogen bonds are described by a strongly asymmetrical quasi-single minimum potential. The equimolar complexes interact with a second molecule of the acid, the second hydrogen bond causes only a minor effect on the first hydrogen bond and this effect is independent of proton-acceptor properties of N-oxides. The equimolar complexes interact also with a second molecule of N-oxide to form homoconjugated complex (BHB)⁺A^–. This interaction is controlled by pK_a of the N-oxide.