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WIREs Comput Mol Sci
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The activation strain model and molecular orbital theory

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The activation strain model is a powerful tool for understanding reactivity, or inertness, of molecular species. This is done by relating the relative energy of a molecular complex along the reaction energy profile to the structural rigidity of the reactants and the strength of their mutual interactions: ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). We provide a detailed discussion of the model, and elaborate on its strong connection with molecular orbital theory. Using these approaches, a causal relationship is revealed between the properties of the reactants and their reactivity, e.g., reaction barriers and plausible reaction mechanisms. This methodology may reveal intriguing parallels between completely different types of chemical transformations. Thus, the activation strain model constitutes a unifying framework that furthers the development of cross‐disciplinary concepts throughout various fields of chemistry. We illustrate the activation strain model in action with selected examples from literature. These examples demonstrate how the methodology is applied to different research questions, how results are interpreted, and how insights into one chemical phenomenon can lead to an improved understanding of another, seemingly completely different chemical process. WIREs Comput Mol Sci 2015, 5:324–343. doi: 10.1002/wcms.1221

This article is categorized under:

  • Structure and Mechanism > Molecular Structures
Schematic representation of (a) oxidative addition and (b) SN2 reaction mechanisms.
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Schematic substrate LUMO composition for (a) CH3CH2OH and (b) CH3CH2OH2+.
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Activation strain analyses of the model substitution and elimination reaction profiles under (a) basic (OH + CH3CH2OH) and (b) acidic (H2O + CH3CH2OH2+) conditions. A dot designates a TS. Energies and bond stretch are relative to reactants.
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Activation strain analyses of the SN2 reaction profiles for variation of (a) the leaving group and (b) the nucleophile. A dot designates a TS. Energies and bond stretch are relative to reactants.
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Different overlap situations for the metal d orbital with (a) a carbon–hydrogen bond and (b) a carbon–halogen bond.
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Activation strain analyses for the oxidative addition of methane and halomethanes to Pd. A dot designates a TS. Energies and bond stretch are relative to reactants.
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Activation strain analyses for the oxidative addition of methane to Ni(PH3)2, Pd(PH3)2, and Pt(PH3)2. A dot designates a TS. Energies and bond stretch are relative to reactants.
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Activation strain analyses for the oxidative addition of methane to Pd, Pd(PH3)2, and Pd[PH2(CH2)2PH2]. A dot designates a TS. Energies and bond stretch are relative to reactants.
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Simplified Walsh diagrams for bending ML2 complexes (a) without and (b) with π backbonding, as they emerge from Kohn–Sham MO analyses (+/− indicate bonding/antibonding). A more detailed scheme of the intermixing occurring for the a1 orbitals is available in the supporting information of Ref .
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Activation strain analyses for the oxidative addition of methane to Pd (black), PdPH3 (blue), and Pd(PH3)2 (red). A dot designates a TS. Energies and bond stretch are relative to reactants.
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Orbital interaction diagrams for the most commonly appearing interactions.
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Comparison of generic activation strain analyses of two different reactions. The dashed lines indicate the results from a single‐point analysis at the transition state geometry.
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