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

“…Acid-base equilibria set up in nonaqueous media in systems containing heterocyclic bases such as amines [1][2][3][4][5] (pyridine and substituted pyridines) and amine N-oxides 4,[6][7][8][9][10][11][12][13][14][15][16][17] (pyridine Noxide and its derivatives) have been systematically studied in our laboratory. Our interest was focused not only on acid dissociation [1][2][3][4][5][6][7][8][9][10][11][12][13] of protonated pyridine and pyridine N-oxide, as well as their derivatives, but also on consecutive acid-base reactions 18 occurring in media of limited capability to exchange proton (polar aprotic and medium polar amphiprotic solvents), referred to as cationic homoconjugation: [1][2][3][4][5][6][7][8][9][10][11][12][13] In the preliminary step, the above equilibria were systematically surveyed in the BH + /B systems of substituted pyridine N-oxides in nonaqueous media, as well as ...…”

“…Our interest was focused not only on acid dissociation [1][2][3][4][5][6][7][8][9][10][11][12][13] of protonated pyridine and pyridine N-oxide, as well as their derivatives, but also on consecutive acid-base reactions 18 occurring in media of limited capability to exchange proton (polar aprotic and medium polar amphiprotic solvents), referred to as cationic homoconjugation: [1][2][3][4][5][6][7][8][9][10][11][12][13] In the preliminary step, the above equilibria were systematically surveyed in the BH + /B systems of substituted pyridine N-oxides in nonaqueous media, as well as by ab initio methods 19 in the gas phase. The calculations accomplished at the RHF and MP2 levels revealed that the experimental quantities were well related to the theoretical ones.…”

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

“…(5) Values of lgK f of the cationic standard heteroconjugation constants (table 3) range between 1.5 and 3.20. Bearing in mind that DMSO is regarded as an unfavourable solvent for homoconjugation of the N -bases, the values are astonishingly high.…”

“…The heteroconjugation reactions between protonated base BH + and free base B 1 were studied in o-dichlorobenzene, (1,2) chlorobenzene, (1,2) nitrobenzene, (3,4) nitromethane, (4,5) acetonitrile, (6)(7)(8) acetone, (9)(10)(11) propylene carbonate (12) and methanol (11) on the basis of conductometric, (1)(2)(3) solubility, (4,9) potentiometric, (6-8, 10, 11) and calorimetric (12) methods. The asymmetric hydrogen bond in (NHN) + bridges was also investigated by i.r.…”

“…It can be concluded that the tendency towards cationic heteroconjugation in systems of protonated pyridine derivatives and conjugate free bases in polar aprotic solvents like nitromethane, acetonitrile, and acetone is rather poor. (5,8,11) In most of the systems, the equilibrium is either completely absent or the equilibrium constants are so small that they cannot be measured potentiometrically. It is quite striking that, in the amphiprotic methanol, a solvent regarded as rather unfavourable for conjugation equilibria, the cationic standard heteroconjugation constants could be measured for most of the substituted pyridine systems.…”