Intrinsic binding energies

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Intrinsic binding energies ( iBE ) describe the strength of a covalent chemical bond in the undisturbed state of the bond. This quantity is not an observable and must therefore be derived through theoretical approaches. The iBEs are linked to the bond dissociation energies (BDE) in such a way that the BDEs are made up of the iBE and the reorganization energy (RE), which is obtained through electronic and geometric relaxation of the molecule fragments when the bond is broken (BDE = iBE + RE).

Elegant approaches to derive the intrinsic binding energies from an analysis of the electron density (an observable) were presented by Bader and Grimme. Paul von Ragué Schleyer and Kai Exner have critically examined and expanded the approach of Grimme, the concept has now found its way into textbook literature. Intrinsic binding energies allow important insights into the nature of chemical bonds.

Applications

The concept of intrinsic binding energies allows many long-standing questions about the stability of chemical bonds to be re-examined. Compared to the CH bonds in methane (BDE = 444 kJ / mol, iBE = 435 kJ / mol, RE = 4.187 kJ / mol), the dissociation of the tertiary CH bond in isobutane (BDE = 833 kJ / mol, iBE = 435 kJ / mol, RE = −33.5 kJ / mol) or methyl CH bonds in propene (BDE = 373 kJ / mol, iBE = 431 kJ / mol, RE = −62.8 kJ / mol) almost exclusively attributed to the corresponding reorganization energies and not to the intrinsic CH bond energies.

The tension energies of cyclopropane and cyclobutane, which are almost identical at 115.1 and 111 kJ / mol, respectively, are based on this approach to stabilization through significantly stronger CH bonds in cyclopropane (compared to the reference cyclohexane, cyclopropane is around 49 kJ / mol, cyclobutane around 15, 5 kJ / mol stabilized by stronger CH bonds), which almost compensates for the increasing ring tension (reflected in the model by reduced CC bond strengths). Furthermore, from a further analysis of the intrinsic CH and CC bond energies, a new value for the stabilization of cyclopropane by sigma aromaticity of 47.3 kJ / mol can be derived.

Individual evidence

  1. ^ Richard F. W. Bader, Ting Hua Tang, Yoram Tal, Friedrich W. Biegler-König: Properties of atoms and bonds in hydrocarbon molecules . In: Journal of the American Chemical Society . tape 104 , no. 4 , February 1982, p. 946-952 , doi : 10.1021 / ja00368a004 .
  2. ↑ S. Grimme: Theoretical bond and strain energies of molecules derived from properties of the charge density at bond critical points . In: Journal of the American Chemical Society , 118, (1996), pp. 1529-1534, doi: 10.1021 / ja9532751 .
  3. ^ A b Kai Exner, Paul von Ragué Schleyer: Theoretical Bond Energies: A Critical Evaluation . In: Journal of Physical Chemistry A . tape 105 , no. 13 , April 2001, p. 3407-3416 , doi : 10.1021 / jp004193o .
  4. ^ FA Carey, RJ Sundberg: Advanced Organic Chemistry. Part A: Structure and Mechanism . Springer Science & Business Media, 2007, Chapter 11.1. Relationship between Bond and Radical Stabilization Energies , pp. 1052-1053.