Aromaticity and tautomerism. Part IV. Free energy: enthalpy correlations for protonation of pyridine bases and azine N-oxides and temperature variation of the H 0 and H A acidity functions

Thermodynamic parameters for the protonation of nine substituted pyridines and nine azine N-oxides are obtained from basicity measurements of the bases at 25, 40, 60, 80, and 90° using the spectrophotometric method. The protonations of both series of bases follow linear ΔH0–pKa correlations. The spectrophotometric measurements are also used to assess the temperature variation of the H0 and HA acidity functions and the relationship HA=mH0+C is found to be approximately temperature independent.