The activation strain model and molecular orbital theory: understanding and designing chemical reactions

Abstract

In this Tutorial Review, we make the point that a true understanding of trends in reactivity (as opposed to measuring or simply computing them) requires a causal reactivity model. To this end, we present and discuss the Activation Strain Model (ASM). The ASM establishes the desired causal relationship between reaction barriers, on one hand, and the properties of reactants and characteristics of reaction mechanisms, on the other hand. In the ASM, the potential energy surface ΔE(ζ) along the reaction coordinate ζ is decomposed into the strain ΔEstrain(ζ) of the reactants that become increasingly deformed as the reaction proceeds, plus the interaction ΔEint(ζ) between these deformed reactants, i.e., ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). The ASM can be used in conjunction with any quantum chemical program. An analysis of the method and its application to problems in organic and organometallic chemistry illustrate the power of the ASM as a unifying concept and a tool for rational design of reactants and catalysts.

Highly accelerated inverse electron-demand cycloaddition of electron-deficient azides with aliphatic cyclooctynes

“…5 is a schematic representation of the frontier molecular orbital interactions between the BCN and phenyl azide reactants in the transition state that emerge from our quantitative Kohn-Sham electronic-structure analyses. Activation-strain analyses 32 confirm that the overall orbital interactions indeed contribute ca À 35 kcal mol À 1 of stabilization, but that net interactions between the reactants in the TS are small, at most À 0.8 kcal mol À 1 , due to substantial Pauli repulsion between closed-shell orbitals on each of the two fragments. The dominant donor-acceptor orbital interactions are between the p electrons of BCN and the relatively low-energy LUMO of the azide.…”

Eight-coordinate fluoride in a silicate double-four-ring

“…2, differs from the instantaneous interaction ΔE int as defined above, by ΔE strain . This is the energy needed to deform the box to adjust to the halide bonding (30). These terms are plotted in Fig.…”

Asymmetric identity SN2 transition states: Nucleophilic substitution at α-substituted carbon and silicon centers

“…The computed PESs along the reaction coordinate ζ were further analyzed using the activation strain model (ASM) [11,[40][41][42] in order to obtain insight into how the activation barriers arise.…”