Hydrogen bonding in complexes of heterocyclic N-oxides with halogenoacetic acids. Part VII. An infrared and nuclear magnetic resonance study of 4-methylpyridine N-oxide complexes

Abstract

I.r. and n.m.r. spectra of complexes of 4-methylpyridine N-oxide with trifluoroacetic, trichloroacetic, dichloroacetic, and monochloroacetic acids have been measured in order to study the association through hydrogen bonding. The complex with trichloroacetic acid was prepared in two crystal modifications. The observed spectral variations were so large that it was necessary to postulate several different structures. The n.m.r. chemical shift of the proton in the hydrogen bond was dependent on both the concentration and the nature of solvent. The i.r. spectra, in contrast, were not noticeably affected by concentration in the range 0·1–0·6M or the nature of the solvent (below 1800 cm^–1). A linear relationship was observed between chemical shift and ΔpK_a. The experimental results can be interpreted in terms of the following equilibria: 2B + 2AH,solv ⇌ 2(B,HA)⇌(B ‥ H ‥ B)⁺(A ‥ H ‥ A)^–⇌(B ‥ H ‥ B)⁺+(A ‥ H ‥ A)^–