Proton Transfer Chemistry in the Gas Phase. Is a Spontaneous Neutralization Reaction a Myth or a RealityReport as inadecuate

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Croatica Chemica Acta, Vol.82 No.1 June 2009. -

Relying on physicochemical knowledge, the proton cannot be spontaneously transferred from a gaseous mineral acid HF, HCl, HBr, HI, HNO3, H2SO4, or HClO4 to a gaseous nitrogen base NH3, alkylamine, aniline, pyridine, amidine, or guanidine. For example, the full proton-transfer from HCl to NH3, followed by the separation of Cl- and NH4+ requires more than 500 kJ mol-1. The same is true for a spontaneous intramolecular proton-transfer for gaseous amino acids, aminophenols and other amphiprotic compounds. From the gas-phase acidity parameter of COOH and the gas-phase basicity parameter of NH2 or other more basic group in the side chain, it appears that proton transfer is endothermic or endergonic. For arginine, an amino acid with a highly basic guanidine function, this difference is still larger than about 300 kJ mol-1. Only solvation of the acid-base system by neutral species e.g., one or more water molecules, complexation by ions e.g., ionic acids or bases, metal cations or even electrons may reduce the

energetic barrier and facilitate the proton-transfer. Recent extension of the gas-phase acidity-basicity scale toward superacids and superbases, and recent development of spectroscopic techniques adapted to the gas phase for less volatile organic compounds give some perspectives for observing the full intermolecular proton-transfer between a molecular Brønsted-Lowry superacid and a molecular Brønsted-Lowry superbase.

acid-base chemistry; gas phase; acidity-basicity scale; proton-transfer; -neutralization- reaction; superacids; superbases; amino acids; zwitterions

Author: Ewa D. Raczyńska - ; Department of Chemistry, Warsaw University of Life Sciences, 02-776 Warsaw, Poland Jean-François Gal - orc



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