Experimental and theoretical studies of solvent effects on the hydrogen bonds in homoconjugated cations of substituted 4-halo (Cl, Br) pyridine N-oxide derivatives.

Hydrogen bond OHO-type bridges formed between six substituted 4-halo (Cl, Br) pyridine N-oxide systems and their simple cations have been investigated by using the potentiometric titration method. The formation constants of these complexes (expressed as ) have been determined in two non-aqueous aprotic solvents with different polarity, , acetone (AC) and acetonitrile (AN). It has been observed that tri- and tetra-substituted pyridine N-oxides [B] and their cationic acids [BH] form stable homocomplexed cations [BHB] stabilized by O⋯H⋯O bridges in both solvents used. It has been found that the most stable homocomplexed system is formed by 3,5-dimethyl-4-chloropyridine N-oxide (3,5Me4ClPyO). The values for this compound in acetone and acetonitrile are 3.15 and 2.82, respectively. Furthermore, by using methods at the RHF and MP2 levels utilizing the Gaussian 6-31++G basis set, the energies of formation of the homocomplexed cations and Gibbs free energies have been determined . The calculated energy parameters have been compared with the cationic homoconjugation constants determined potentiometrically in acetone and acetonitrile to establish a correlation between these magnitudes. Additionally, the results of potentiometric measurements have been used to determine the acidity constants of the conjugate acids of N-oxides.