Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine N-oxide trifluoroacetates and their deuteriated analogues in dichloromethane

Abstract

The i.r. and ¹H n.m.r. spectra of 11 substituted pyridine N-oxides with CF₃COOH and CF₃COOD were examined in dry dichloromethane. All the investigated complexes show a typical carbonyl band. The variation of ν(CO) frequency with ΔpK_a, its shape and band-width, and changes on deuteraition confirm the assumption that the structure of complexes with strong OHO hydrogen bonding is altered gradually with the rise of the interaction energy, from the molecular A-H … B, to the ionic, A^–… H-B⁺. If the centre of gravity (_H or _D) and the chemical shift of hydrogen-bonded protons (δ) are plotted against ΔpK_a, points fall on two intersecting straight lines. These correlations are used to compare the hydrogen bond strength in complexes of pyridines and their N-oxides. A discrepancy between the i.r. and ¹H n.m.r. data was found and discussed with respect to the contribution of charge to the shielding of strongly hydrogen-bonded protons. A plot of _H/_Dagainst _Hshows a characteristic shape with a minimum, similar to that found by Novak. The variations of _H/_D with proton acceptor properties of N-oxides are consistent with the ‘quasi-symmetrical’ structure A … H … B postulated in the literature.