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The activation strain model and molecular orbital theory.

Wolters LP, Bickelhaupt FM - Wiley Interdiscip Rev Comput Mol Sci (2015)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University AmsterdamAmsterdam, The Netherlands; Dipartimento di Scienze Chimiche, Università degli Studi di PadovaPadova, Italy.

ABSTRACT

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) = ΔE strain(ζ) + ΔE int(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.

No MeSH data available.


Related in: MedlinePlus

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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fig07: 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.

Mentions: Other studies have investigated the effect of using catalysts with metal centers other than Pd,42,47 such as the d10-ML2 catalysts Ni(PH3)2 and Pt(PH3)2 (see Figure 7). Although all three catalyst complexes are rather similar, activation strain analyses revealed intriguing differences in both the strain and interaction terms. First, the weaker interaction for the Pd catalyst results mainly from the electron-donating capabilities: Ni(PH3)2 is a better electron donor than Pd(PH3)2 due to its higher-energy d-derived orbitals, whereas Pt(PH3)2 has larger d-derived orbitals that provide better overlap with the substrate σ*C–H orbital. Second, the relatively low barrier for Ni(PH3)2 is also partly the result of a softer strain term, already in an early stage of the reaction, originating from the catalyst's contribution. Surprisingly, it appears that bending Ni(PH3)2 comes with a smaller energy penalty than bending the isoelectronic Pd(PH3)2 or Pt(PH3)2. Thus, there is a ‘bite-angle effect’, even though all three M(PH3)2 catalyst complexes have linear equilibrium geometries and their L–M–L angles are decreased to similar values in the course of the oxidative addition. This result indicates that the bite angle itself is not necessarily sufficient to predict the activity of the catalyst. Considering the bite-angle flexibility, that is the ease of decreasing the bite angle of a catalyst, gives better insights into catalyst activity.47


The activation strain model and molecular orbital theory.

Wolters LP, Bickelhaupt FM - Wiley Interdiscip Rev Comput Mol Sci (2015)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4696410&req=5

fig07: 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.
Mentions: Other studies have investigated the effect of using catalysts with metal centers other than Pd,42,47 such as the d10-ML2 catalysts Ni(PH3)2 and Pt(PH3)2 (see Figure 7). Although all three catalyst complexes are rather similar, activation strain analyses revealed intriguing differences in both the strain and interaction terms. First, the weaker interaction for the Pd catalyst results mainly from the electron-donating capabilities: Ni(PH3)2 is a better electron donor than Pd(PH3)2 due to its higher-energy d-derived orbitals, whereas Pt(PH3)2 has larger d-derived orbitals that provide better overlap with the substrate σ*C–H orbital. Second, the relatively low barrier for Ni(PH3)2 is also partly the result of a softer strain term, already in an early stage of the reaction, originating from the catalyst's contribution. Surprisingly, it appears that bending Ni(PH3)2 comes with a smaller energy penalty than bending the isoelectronic Pd(PH3)2 or Pt(PH3)2. Thus, there is a ‘bite-angle effect’, even though all three M(PH3)2 catalyst complexes have linear equilibrium geometries and their L–M–L angles are decreased to similar values in the course of the oxidative addition. This result indicates that the bite angle itself is not necessarily sufficient to predict the activity of the catalyst. Considering the bite-angle flexibility, that is the ease of decreasing the bite angle of a catalyst, gives better insights into catalyst activity.47

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), VU University AmsterdamAmsterdam, The Netherlands; Dipartimento di Scienze Chimiche, Università degli Studi di PadovaPadova, Italy.

ABSTRACT

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) = ΔE strain(ζ) + ΔE int(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.

No MeSH data available.


Related in: MedlinePlus