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

The correlations between the centre of gravity (νH) of the broad IR vibrations, chemical shifts (δH) of hydrogen-bonded protons and enthalpies of hydroben bonds (-ΔH) 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 νH and δH with aqueous pKa 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. © 1990.