Homolytic Bond Dissociation Energies

Homolytic bond dissociation energies or just bond dissociation energies (BDE) is a measure of a particular bond strength. BDE are also referred to as bond enthalpies.

For example, the BDE for the C-C bond in

H₃C-CH₃ --> H₃C• + •CH₃

can be determined using the calculated enthalpies of formation (ΔH_f) using the following method:

Using WebMO/Gaussian to calculate the ΔH_f(H₃C-CH₃), returns the following:

The RHF (Restricted HF) energy, ie. ΔH_f(H₃C-CH₃) is reported in units of Hartree, -79.2287548119 Hartree

Using WebMO/Gaussian to calculate the ΔH_f(CH₃•), returns the following:

The RHF (Restricted HF) energy, ie ΔH_f(CH₃•) is reported in units of Hartree, -39.5589916118 Hartree

ΔH_rxn = ΔH_f(Products) - ΔH_f(Reactants)

ΔH_rxn = 2*ΔH_f(CH₃•) - H_f(H₃C-CH₃)

ΔH_rxn = 2*-39.5589916118 - (-79.2287548119)

ΔH_rxn = 0.110771588 Hartree = 290.830826448 kJ/mol

According to Engel, 2nd, Chapter 15, table 15.2, the actual value is 406 kJ/mol)

As can bee seen in this table (shown in all Gen Chem textbooks), the "average bond enthalpy" for a C-C bond is 348 kJ/mol.

Using a more accurate computational approach, B3LYP/6-311+G(2d,p):

- B3LYP Energy -39.8561207613 Hartree

- B3LYP Energy -79.8583705307 Hartree

ΔH_rxn = 0.14613 Hartree = 384 kJ/mol