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

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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 ΔpKa, 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 ΔpKa, 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.

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