ANALYTICA CHIMICA ACTA zyxwvutsrqponm ELSEVIER Analytica Chimica Acta 338 (1997) 261-267 A study of cationic heteroconjugation equilibria of substituted pyridine N-oxides in acetonitrile Lech Chmurzyliski*, Ewa Kaczmarczyk, Adam Liwo zyxwvutsrqponmlkjihgfedc Department of Chemistry, University of Gdarisk, Sobieskiego IS, 80 952 Gdarisk, Poland Received 1 July 1996; revised 8 October 1996; accepted 13 October 1996 Abstract 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. Keywords: Cationic heteroconjugation; Nonaqueous solvents; F’yridine N-oxides 1. Introduction Considering the interactions between uncharged [1] or cationic [2] acids and bases [3] in nonaqueous media, one can predict the model of the acid-base equilibria established in such solutions. In order to obtain a complete picture of these equilibria, polar aprotic solvents are used in such studies, as they exhibit very weak, but still measurable amphiprotic properties [2]. Because of their strong differentiating *Corresponding author. Tel.: +48 58 415271; fax: + 48 58 410357; e-mail: lesiu@sunl .chem.univ.gda.pl and very weakly exhibited acidic-basic properties, these solvents are convenient while studying such subtle properties of acid-base systems as homo- and hetero-conjugation equilibria [4], in which hydrogen- bonded complexes are formed between the acid and base [5]. Studying hydrogen-bonding equilibria in apolar aprotic solvents is much more difficult because of experimental limitations, and a far more complex equilibrium model involving higher-order complexes [4]. For systems composed of two kinds of cationic acids (BH+ and BiH+) and conjugated bases (B and Bi), it is reasonable to assume the following 121 equilibrium model: 0003-2670/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(96)00501-6