Biological Sciences: The So-called Thermodynamic Compensation Law and Thermal Death

Abstract

IT has frequently been reported that, for a structurally related series of compounds undergoing a defined chemical reaction, the activation parameters, ΔH^‡ and ΔS^‡, are linearly related: This is sometimes called a thermodynamic compensation law because the form of the rate equation derived from transition state theory (equation (2)) is such that parallel changes in the activation parameters have opposite effects on the value of the rate coefficient, k, As Leffler¹ pointed out, however, the relationship can arise as a computational artefact from experimental errors in the rate coefficients from which the activation parameters are calculated. Exner^2,3 further showed that the form of the equations used in the calculation tended, in any case, to generate the relationship. Hammett⁴, from statistical mechanical considerations, has produced a set of formal conditions which must be satisfied if the relationship is to be observed but these seem to have no simple physical interpretation. Behaviour which appears to be consistent with the compensation law makes a comparison of rate constants of doubtful value since, at a temperature T=1/a, all the rate coefficients are the same. Above this so-called isokinetic temperature the order of rate coefficients is the reverse of that observed below it (see ref. 5).