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Entropy considerations in kinetic method experiments.

In extended kinetic method experiments, relative binding enthalpies ('affinities') and relative entropies are obtained based on unimolecular dissociation kinetics. A series of ion-bound dimers A-X-B(i) is formed, in which the sample (A) and structurally similar reference molecules (B(i)) are bridged by a central cation or anion (X). The branching ratios of the A-X-B(i) set to A-X and B(i)-X are determined at different internal energies, usually by subjecting A-X-B(i) to collisionally activated dissociation at various collision energies. The dependence of the natural logarithm of the branching ratios on the corresponding B(i)-X bond enthalpies (X affinities of B(i)) is evaluated as a function of internal energy to thereby deduce the A-X bond enthalpy (X affinity of A) as well as an apparent relative entropy of the competitive dissociation channels, Delta(DeltaS(app)). Experiments with proton- and Na(+)-bound dimers show that this approach can yield accurate binding enthalpies. In contrast, the derived Delta(DeltaS(app)) values do not correlate with the corresponding thermodynamic entropy differences between the channels leading to A-X and B(i)-X, even after scaling. The observed trends are reconciled by the transition state switching model. According to this model, the kinetics of barrierless dissociations, such as those encountered in kinetic method studies, are dominated by a family of tight transition states ('entropy bottlenecks') lying lower in energy than the corresponding dissociation thresholds. In general, the relative energies of these tight transition states approximately match those of the dissociation products, but their relative entropies tend to be much smaller, as observed experimentally.