A study of cationic heteroconjugation equilibria of substituted pyridine N-oxides in acetonitrile

Cationic heteroconjugation equilibria (resulting in the formation of the [OH…O]+ hydrogen-bonded bridges) of proton-N-oxide systems in acetonitrile (a polar aprotic protophobic solvent) were studied using the potentiometric titration method. For comparison, the heteroconjugation equilibria of the pyridinium cation with N-oxides (the [NH…O]+ bridges) were also studied. For the [O-H…O]+ bridges, the heteroconjugation constants were found to increase with increasing basicity of the proton acceptor and decrease with increasing basicity of the proton donor. Consequently, the highest values of the heteroconjugation constants were obtained for the [OH…O]+ bridges of N-oxides with almost the same basicity. Another factor influencing the heteroconjugation is the hydrogen-bonding propensity of the components defined as the arithmetic average of their homoconjugation constants. A correlation was found that expresses the logarithm of heteroconjugation constant as a linear function of the two foregoing quantities. For the [NH…O]+ bridges, the heteroconjugation constants were found independent of the proton-acceptor basicity.