Hydrogen Bonding, Tautomerism and Proton Exchange

The work of Allan and Reeves (1962, 1963) illustrates the use of chemical shift measurements to obtain enthalpies of formation for both intra- and inter-molecular hydrogen bonds involving the same molecules. The measurements do not rely on infrared studies of the same systems but the thermodynamic results from the two techniques are in agreement. In orfko-halogenophenols the equilibrinm at finite concentrations in an inert solvent can be represented as two chemical equations 

The intramolecular process (a) amounts to hindered rotation about the C—O bond of the phenol. It may be exclusively studied therefore by obtaining accurate chemical shifts at infinite dilution in an inert solvent at a series of temperatures. The equilibrium constant can be written 

If AS for the cis - trans conversion is reasonably assumed to be zero then the temperature-dependence of JiTi is given by  

If temperature studies are made two useful equations exist between the three unknown Ki, S and Sf The value of the third unknown, 8 is obtained by extrapolating to low temperature, when the equilibrium 

The chemical shift of the —OH proton is linearly dependent on concentration of the phenol in the range 1-4 mole % at all temperatures studied. The variation of this linear slope with temperature can be ascribed to 

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V. Hydrogen Bonding, Tautomerism and Proton Exchange A. Hydrogen Bonding... 

The hydrogenation of the peripheral double bonds strongly influences the tautomerism. The rates of the intramolecular proton exchange of 4 are smaller than of 3 because only the first trans-tautomer is aromatic, whereas the second trans-tautomer has lost its aromaticity and exhibits a higher energy. It is only observable as an intermediate of the exchange of the two inner protons [24]. 

By contrast, this process is slow as compared to hydrogen bond exchange. In other words, in each tautomerization another base molecule always carries the same proton back and forth without intermolecular proton exchange. 27 is stable only at low temperatures, but dissociates at higher temperatures. Thus at low... 

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