Complexation constants of hydrogen bonded complexes between substituted phenols and substituted pyridines were determined by infrared spectroscopic measurements in carbon tetrachloride at 25OC. It was previously shown that for monosubstitutedJcomplexes the complexation constant Kab obied the relation log KOb = log Ko+pu where u is the Hammett substitution constant. When both the proton donor and the proton acceptor are substituted, it is shown in the present work that the logarithm of the complexation constant can be expressed by the equation:
where KO, refers to the unsubstituted complexe phenol-pyridine: p: is the pa value when ub is zero and p$ the pb value when a,, is zero. The product &,pb is an interaction term where B = -037. This relation shows that an increasing acidity of phenol enhances the substitution effects on the pyridine. This interaction term is discussed in terms of dipolar and polarisation effects. A similar equation was previously found for the phenol-aniline disubstituted complexes. In the present case, the p: and p: values are greater owing to the stronger basicity of pyridine, and to the smaller O . . . N distance. The E constant however is greater with the aniline (-1.70) than with the pyridine complexes. This fact is discussed in terms of the variation of hybridization of the nitrogen atom in the complexes, which in the case of aniline, can be deduced from HNH angle measurements.