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Hydrogen bonding and proton exchange in trifluoroacetic acid-quinoline and methanolorthochlorophenol mixtures by nuclear magnetic resonance spectroscopy. Krakower, Earl

Abstract

High resolution nuclear magnetic resonance is used to investigate proton exchange in two hydrogen bonded systems. The change of the chemical shift of the acid proton with concentration in mixtures of trifluoroacetic acid and quinoline have been used to indicate the formation of a strongly hydrogen bonded ion pair in the pure acid-base system and in acetonitrile as solvent. Spectra at -18°C show resolved peaks for the ≽NH⁺ and -COOH protons confirming the nature of the exchange process. The kinetic processes involved in the exchange are discussed but the experimental difficulty in carrying out a low temperature (0 to -60°C) study of an equimolar mixture of the two components prevents any detailed investigation of the rate processes. The chemical shifts measured for the acid and hetero-ring protons are averages for the various environments so that charge density changes on protonation of a basic species are not proportional to the observed chemical shift changes. Intermolecular hydrogen bonding and proton transfer rates between methanol and orthochlorophenol have been experimentally investigated in the slow exchange approximation for the modified Bloch Equations (0°C to -41.1°C). Interpolations of the results to the fast exchange limit are consistent with experimental line widths in this region. The activation energy obtained for the first order kinetics is 4.58 Kcal mole⁻¹. It is considered that the intramolecular hydrogen bond in orthochlorophenol does not impede the proton exchange process in the two component system. Variations in the line width of the exchange averaged -OH resonance signal are used to indicate the effect of small quantities of water added to the methanol-orthochlorophenol mixtures.

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