A quantitative comparison of spectroscopic and energy data, in solution, of NHO and OHO hydrogen bonds and gas-phase proton affinities: complexes of pyridines and pyridine N-oxides with acetic acids

Abstract

The correlations between the centre of gravity (ν H) of the broad IR vibrations, chemical shifts (δ H) of hydrogen-bonded protons and enthalpies of hydroben bonds (−Δ H) of two series of complexes (pyridines and pyridine N-oxides) with trifluoroacetic, dichloroacetic, chloroacetic and acetic acids, and the difference between the gas-phase proton affinity of bases ( PA(B)) and that of conjugate bases ( PA(A −)) were investigated. Complexes of pyridines with all these acids can be treated as one category and complexes of pyridine N-oxides as another. On the other hand, correlations of ν H and δ H with aqueous p K a values consist of separate lines for each acid. The PA(A −) and PA(B) values provide an excellent basicity scale for hydrogen bonds in solvents of low and medium polarity.