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Department of Chemistry and Biochemistry
University of Colorado at Boulder
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Bond Energies, Radical Heats of Formation, and Organic Thermochemistry

We [1] have reviewed the means by which bond dissociation energies of organic molecules are measured and calculated. This is a revision of an earlier, more detailed account [2] . There are two thermochemical tables from the Accounts [1] paper:

Table of Molecular Bond Dissociation Energies

Table 1: Molecular Bond Dissociation Energies: RH. → R + H

Experimental Bond Enthalpies and Radical Heats of Formation at 298 K (kcal mol-1)

Table of Experimental Bond Enthalpies

Table 2: Experimental Bond Enthalpies, DH298/kcal mol-1

Acidity/Electron Affinity Thermochemistry

Gas phase acidity studies use heterolytic bond cleavages to extract molecular thermochemistry. In the case of an R-H bond at 298 K the heterolytic bond enthalpy is known as the enthalpy of deprotonation, ΔacidH298 (reaction 1).

RH → R- + H+ ΔacidH298(R-H) (1)
R- + hν → R + e- E0 (R); (2)
H+ + e- → H + h ν -IE0(H) (3)

Summing reactions 1-3 gives [3] the homolytic bond enthalpy in eq. (4).

H289(RH) = ΔacidH298(RH) + EA0(R) - IE0(H) + [thermal correction]         (4)

The [thermal correction] is a set of heat capacity integrals, ∫[Cp(R) - Cp(R-) + Cp(H) - Cp(H+)]dT, which may be evaluated as previously described, but their value is generally less than 0.3 kcal mol-1. By rearranging eq. (4) one can estimate typical values for ΔacidH298(RH); since DH298(RH) for many organics is approximately 100 kcal mol-1, the IE0(H) is 313.6 kcal mol-1, and the electron affinities [4] of most radicals are about 23 kcal mol-1, ΔacidH298(RH) is [100 + 313 - 23] or roughly 390 kcal mol-1. Notice that heterolytic dissociation of a molecule into isolated positive and negative ions (reaction 1) requires a great deal more energy than does homolytic cleavage, ΔacidH298(RH) » DH298(RH). The additional energy required for heterolytic bond cleavage arises from the need to overcome the Coulombic attraction between the positive and negative fragments formed in this process.

Experimental atomic and molecular Electron affinities tables from the Chemical Reviews [4] paper are available in PDF format. If necessary, download the latest version of Acrobat Reader to view the PDF documents.

Table 10: Experimental Atomic and Molecular Affinities determined by photoelectron spectroscopy. Each species is ordered by the mass to charge ratio, m/z, and the corresponding electron affinity (EA) is listed in units of eV. (PDF, 440KB)

Table 11: Experimental Atomic and Molecular Affinities determined by photoelectron spectroscopy. All atoms and molecules are listed in order of increasing electron affinity (eV). (PDF, 334KB)

References

  1. Stephen J. Blanksby and G. Barney Ellison, "Bond Dissociation Energies of Organic Molecules", Acct. Chem. Res., 36, 255-263, 2003.
  2. J. Berkowitz, G. B. Ellison, D. Gutman, "Three Methods to Measure RH Bond Energies", 1994, 98, 2744.
  3. K. M. Ervin, "Experimental techniques in gas-phase ion thermochemistry", Chem. Rev., 101, 391-444, 2001.
  4. (4) J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer III , S. Nandi, G. B. Ellison, "Atomic and Molecular Electron Affinities: Photoelectron Experiments and Theoretical Computations", Chem. Revs., 102, 231-282, 2002.