Temperature dependence of the acid-base equilibrium constants of substituted pyridine N-oxides in acetonitrile

Temperature dependence of acid-base equilibrium constants in systems consis ting of substituted pyridine N-oxides was studied in acetonitrile, a strong ly ionizing polar protophobic aprotic solvent. N-oxides, which have an enha nced tendency towards formation of cationic homo- and heteroconjugates, (OH O+) as a result of their relatively strong basicity, were investigated. For the sake of comparison, also aliphatic trimethylamine N-oxide and pyridine , the parent compound of its N-oxide, were included. Of particular interest were acid dissociation constants of cations of the protonated bases, homoc onjugation equilibria of the cationic acids with conjugate bases, as well a s cationic heteroconjugation equilibria of the protonated bases with non-co njugated bases. The constants were determined at three temperatures, 292.1, 298.1 and 304.1 K. On the basis of the temperature dependence of the const ants, enthalpy changes were calculated for the acid dissociation of the cat ionic acids, as well as for cationic homo- and heteroconjugation. These val ues were derived from the known thermodynamic relationships, ac = -RTln K a nd Delta G = Delta H - T Delta S. It has been found that enthalpy changes f or the dissociation reactions of the cationic acids and for cationic homoco njugation reactions are consistent with the variations of the protonation a nd homoconjugation energies, respectively calculated by both the semi-empir ical and the ab initio methods on the 4-31G level. (C) 1999 Elsevier Scienc e B.V. All rights reserved.