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

“…Just the opposite tendency was noticed with the N -oxide systems. (17) That can be explained in terms of the high basicity of DMSO molecules carrying, moreover, oxygen atoms as the basicity centres. Thus, solvent molecules compete successfully with N -oxide molecules in hydrogen bonding formation (the higher the solvent basicity, the stronger the tendency towards formation of heterocomplexes of protonated N -oxide and solvent type) and, consequently, cause the weakening of heterocomplexed N -oxide systems.…”

“…For instance, no relationship could be established between cationic standard heteroconjugation constants and solvent basicities, which is characteristic for substituted pyridine N -oxides. (17) Furthermore, in the case of the (OHO) + bridges, the cationic standard heteroconjugation constant values increase with increasing acceptor basicity and decrease with proton donor basicity, (18)(19)(20)(21) a regularity not observed in the case of the (NHN) + bridges. Another feature influencing the tendency towards cationic heteroconjugation in the systems with the N -oxides is the capacity of the constituents to form hydrogen bonds, as defined by the arithmetic mean of their cationic standard homoconjugation constant values.…”

“…spectroscopy. (13)(14)(15) The results of these investigations have shown that the (NHN) + bridges are less stable than the (NHO) + bridges (16,17) in which one of the N -bases is replaced by an N -oxide of the N -base. Also, the (OHO) + bridges, where different amine N -oxides are bound together with a hydrogen bond, turned out to be less stable.…”

A potentiometric study of cationic heteroconjugation equilibria in nitromethane andN,N- dimethylformamide

“…In this series, donicity numbers of the solvents are: (11) 4.4, 15.1, 14.1, 17.0, and 19.1, respectively; the exception observed in the case of propylene carbonate may be explained in terms of the much stronger polarity of propylene carbonate compared with that of acetonitrile (the dielectric constants (11) are 64.4 and 35.95 for propylene carbonate and acetonitrile, respectively), whereas the proton-donating and proton-accepting properties of propylene carbonate closely resemble those of acetonitrile (donicity numbers (11) of these solvents are 15.1 and 14.1, and the acceptor numbers (11) 18.3 and 18.1, respectively). It has been found (9) that, in the case of comparable basicities of the two solvents, the increase in solvent polarity results in enhancement of the tendency towards cationic heteroconjugation in the N -oxide systems studied.…”

“…Furthermore, cationic homoconjugation shown in equations (3) and (4) is a reaction whereby cationic acid BH + (B 1 H + ) reacts with conjugate base B (B 1 ) to yield a symmetrical complex cation BHB + (B 1 HB 1 ) + , whereas the cationic heteroconjugation phenomenon shown in equation (5) takes place when cationic acid BH + reacts with base B 1 conjugated with another cationic acid to afford an asymmetric hydrogen-bonded complex cation BHB + 1 . Cationic heteroconjugation equilibria in systems comprising amine N -oxides (mainly substituted pyridine N -oxides, as well as aliphatic trimethylamine N -oxide) have been extensively studied in polar protophobic aprotic solvents, such as nitrobenzene, (7) acetonitrile, (8) propylene carbonate, (9) and acetone, (10) as well as in methanol, (10) which is a polar amphiprotic solvent. These studies have shown that the tendency towards heteroconjugation of substituted pyridine N -oxides depends on solvent basicity and decreases with increasing basicity of solvents in the following order: nitrobenzene > propylene carbonate > acetonitrile > acetone > methanol.…”