It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species.
[3] As a typical example, the bond-dissociation energy for one of the C−H bonds in ethane (C2H6) is defined as the standard enthalpy change of the process To convert a molar BDE to the energy needed to dissociate the bond per molecule, the conversion factor 23.060 kcal/mol (96.485 kJ/mol) for each eV can be used.
A variety of experimental techniques, including spectrometric determination of energy levels, generation of radicals by pyrolysis or photolysis, measurements of chemical kinetics and equilibrium, and various calorimetric and electrochemical methods have been used to measure bond dissociation energy values.
This vast difference is accounted for by the thermodynamic stability of carbon monoxide (CO), formed upon the C=C bond cleavage of ketene.
[7] The difference in availability of spin states upon fragmentation further complicates the use of BDE as a measure of bond strength for head-to-head comparisons, and force constants have been suggested as an alternative.
The BDFE of a bond A–B can be defined in the same way as the BDE as the standard free energy change (ΔG°) accompanying homolytic dissociation of AB into A and B.
[13] In contrast to the BDE, which is usually defined and measured in the gas phase, the BDFE is often determined in the solution phase with respect to a solvent like DMSO, since the free-energy changes for the aforementioned thermochemical steps can be determined from parameters like acid dissociation constants (pKa) and standard redox potentials (ε°) that are measured in solution.
One consequence to these data are that many reactions generate silicon fluorides, such as glass etching, deprotection in organic synthesis, and volcanic emissions.
[19] For the same reason, B–F bonds are also very strong, possibly stronger than Si−F, with the BDE for F2B−F computed to be 172 kcal/mol at the CCSD(T)/CBS level of theory.
[21] Held together entirely by the van der Waals force, helium dimer, He2, has the lowest measured bond dissociation energy of only 0.022 kcal/mol.
For molecular hydrogen, the alternatives are: In the gas phase, the enthalpy of heterolysis is larger than that of homolysis, due to the need to separate unlike charges.