Discussion of the proton potential with proton-transfer equilibria: thermodynamic data and infrared continua as a function of temperature

Abstract

The infrared continua caused with intramolecular hydrogen bonds of the type AH⋯B ⇌ A^–⋯H⁺B have been studied as a function of the ΔpK_a of the hydrogen-bond donor and acceptor as well as a function of the temperature. From the behaviour of these continua it is shown that the shape of the proton potential may change with decreasing temperature from a double minimum with the deeper well at the donor to proton potential with the deeper well at the acceptor. The In (K_PT) over 1/T plots result, however, always in negative apparent ‘ΔH° values’. Hence with all such proton-transfer hydrogen bonds the deeper well of the double-minimum proton potentials should always be at the acceptor group.

Using the reaction-field theory, it is shown that for proton-transfer equilibria the slope of these In K_PT vs. 1/T plots remains linear since ∂P/∂T, i.e. the derivative of the Onsager parameter P against the temperature T is constant if it is considered with not too large a temperature range, and since ΔH° changes with temperature by a term which is linear in T. Therefore, from the In (K_PT)vs. 1/T plots apparent ‘ΔH° values’ are obtained from which the true values can be calculated if ∂P/∂T is known.