How Is Transition State Looseness Related to the Reaction Barrier?

Using a thermodynamic approach for the reaction class Y + AX → YA + X, a link is drawn between barriers and transition state (TS) looseness. Thermochemical indexes are derived which allow the calculation of the thermochemical looseness of the TS from knowledge of the barriers for the forward and reverse directions (Δe^‡f, ΔE^‡_r). It is shown that high reaction barriers (ΔE^‡_f + ΔE^‡_r) are associated with thermochemically loose TSs. This correlation derives from two fundamental features of reactivity: (a) that high barriers are associated with TSs which are close to their dissociation limits, and (b) that reaction barriers derive from the deformations that are required to carry the ground-state molecules to the TS. The application is extended to solution-phase reactions, and an additional loosening effect emerges which accounts for the stability of the ions, A⁺ + X^-. It is concluded that the thermochemical looseness involves bond distortion effects, which are dominated by the height of the barriers, and electron-density-depletion effects which are dominated by the stability of the ions. Computational and experimental data are discussed. Thermochemical looseness and geometric looseness correlate whenever bond stretchings are the main distortions which establish the TS. Thermochemical and geometric looseness will not correlate when bond stretchings are not the principal distortions which establish the TS. The lack of correlation between the two types of looseness may provide therefore, some information about the activation process. Potential application to other reactions is discussed.