Ab Initio Study of the Energetics of Protonation and Homocomplexed Cation Formation in Systems with Pyridine and Its Derivatives

Abstract

The energetics of protonation of pyridine and a series of its derivatives, as well as the energetics of formation of hydrogen-bonded N...H...N homocomplexed cations in systems involving substituted pyridines and conjugate cationic acids were investigated by means of restricted Hartree-Fock (RHF) and Møller-Plesset (MP2) ab initio calculations. The Gaussian functional basis set 6-31G* was employed to calculate energy and Gibbs free energy of protonation and cationic homoconjugation in the gas phase. The proton potential of a homocomplexed pyridine cation exhibits a double minimum with a 5.7 kcal/mol energy barrier, which could be reduced to 2.7 kcal/mol by accounting for a thermodynamic correction factor. The calculated protonation energies, ∆E prot , and Gibbs free energies, ∆G prot , have been found to correlate well with the acid dissociation constants (expressed as pK a values) in acetonitrile. On the contrary, the calculated energies, ∆E BHB + , and Gibbs free energies, ∆G BHB + , of formation of the homocomplexes do not correlate with the cationic homoconjugation constant values (expressed as log 10 K BHB + ) determined in acetonitrile.

Theoretical and experimental study of aparisthman: A natural product with anti‐ulcer activity

“… * Δ G was calculated as the energy of the protoned molecule minus the energy of the neutral molecule 19. …”

Potentiometric studies of cationic heteroconjugation equilibria in systems involving free and protonated pyridine derivatives in dimethyl sulfoxide

“…For comparison, pK o a (BH + ) values of the compounds in water (27) are included together with appropriate protonation energies calculated by the ab initio method at the RHF level. (28) A closer inspection of the data in table 1 has led to the following conclusions: both pyridine and its derivatives are weaker bases in DMSO than in water, the pK o a (BH + ) values in the two solvents differing by 1 to 2 factors of 10; the sequence of changes of pK o a (BH + ) values in DMSO and water is the same, and thus pK o a (BH + ) values in both solvents can be correlated. A correlation is expressed by the following equation:…”

The kinetics of substitution reaction of oxydiacetate and thiodiacetate copper(II) complexes with 1,10-phenanthroline and 2,2’-bipyridine

“…The physico-chemical as well as acidbase properties of non-aqueous solvents including DMSO affect coordinating properties of ligands such as pyridine derivatives and consequently it may influence the reaction kinetics in this medium. [7][8][9][10][11] The medium of reaction (the physico-chemical and acidbase properties of solvent) may have an important impact on the rate of the reactions. So far, the kinetic stability of such complexes and the effect of solvent on the rates of formation of complexes have not been studied systematically.…”