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Wewnętrzna solwatacja a reakcje przeniesienia protonu w fazie gazowej dla związków azotowych

Raczyńska E.D.

Wiadomości Chemiczne

2000

[Z] 54, 1-2

67-86

Intramolecular hydrogen bonding which can be formed in the neutral or ionic forms of polyfunctional compounds strongly influences the simple proton-transfer reactions and tautomeric equilibria in the gas phase. This effect, called internal solvation increases the proton affinities of nitrogen ligands, such as proton sponges with a rigid structure, and diamines, amidinamines, bipyridines and amidinazines with a flexible conformation by 20-100 kJ/mol in comparison to monofunctional or model compounds. In consequence diamines and amidinamines belong to superbases (PA>1000 kJ/mol). In aqueous solutions, only proton sponges are strong bases. Due to strong steric effects, water molecules cannot open the ionic NźźźH-N+ bridge in the proton sponges. Bidentate nitrogen ligands with a flexible conformation behave in solution similarly as monofunctional derivatives. External solvation by water molecules eliminates the NźźźH-N+ bridge, changes conformation of the monocation (from cyclic to linear), reduces polarizability of the chain, and thus decreases the basicity. For tautomerizing azoles, the internal solvation changes the pK(T) values by more than 2 pK(T) units in comparison to derivatives without this interaction. Application of the Taft and Topsom equation to series of 4(5)-substituted imidazoles indicates that the internal solvation in the gas phase has similar influence on the transmission of substituent effects as the external solvation in water.