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

Abstract

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 lg K BHB þ ) have been determined in two non-aqueous aprotic solvents with different polarity, i.e., 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 OÁ Á ÁHÁ Á ÁO 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 (3,5Me 2 4ClPyO). The lg K BHB þ values for this compound in acetone and acetonitrile are 3.15 and 2.82, respectively. Furthermore, by using ab initio 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 in vacuo. The calculated energy parameters in vacuo 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.

Theoretical Studies of Interactions between O-Phosphorylated and Standard Amino-Acid Side-Chain Models in Water

“…Hydrogen-bond and salt-bridge formation between natural amino-acid side chains was investigated in our previous work. − On the basis of molecular dynamics simulation data, the potential of mean force (PMF) surfaces were calculated and analyzed for like-charged and oppositely charged side chains. We showed that the most significant are the interactions between oppositely charged amino-acid side chains. − The stabilizing electrostatic free energy of the salt bridges arises from the attraction between opposite charges.…”