Enthalpy−Entropy Compensation is Not a General Feature of Weak Association

Relationships between the enthalpy and entropy changes resulting from perturbations of a system have been discussed in the literature for some time. Both positive correlations (compensation) and negative correlations (anti-compensation) between deltaH and DeltaS have been observed in various experimental contexts, including chemical reaction, physical association, solvation, and protein folding. Many examples have been demonstrated to be statistical artifacts, but some are genuine signatures of the perturbations in molecular characteristics. In particular, recent literature claims that compensation is a general feature of bimolecular associations arising from weak intermolecular interactions. We employ a statistical mechanical framework to predict the magnitude and direction of enthalpy-entropy correlation in bimolecular association. The theory links the macroscale thermodynamic correlation to the relationship between the intermolecular potential parameters. Using a harmonic approximation to the Lennard-Jones model and potential parameters taken from the literature, we show examples of both compensation and anti-compensation for gas-phase self-association among five homologous series. Furthermore, an aggregate presentation of data for 48 different chemical species shows no correlation in either direction, for the case of self-association in a dilute gas phase.