Solution and gas-phase bond dissociation enthalpies

A "true" BDE should be defined with all the species in the gas phase. In other words, the enthalpy of the reaction where the O—H bond in ArOH is cleaved, will be identified with the O—H BDE only if ArOH and the radical fragments are in the gas phase.

ArOH (g) → ArO (g) + H (g)

The PAC results, however, are obtained in solution. BDEs in solution are different because now the molecules are not isolated; they are interacting with the solvent molecules. The process by which a molecule is taken from the gas-phase and placed in the middle of the solvent is called solvation. Schematically, we can consider that the same reaction in solution involves, besides breaking the O—H bond (which corresponds to the true or gas-phase BDE), “breaking” the interactions between ArOH and the solvent and “forming” the interactions between the product radicals and the solvent. These interactions also contribute to the measured enthalpy in solution, accounting for the difference between solution and gas-phase BDEs.


Although BDEs defined in solution are also very useful, because most important chemical and biochemical processes occur in solution, the reliability of PAC results should be assessed by comparing them with values obtained from other techniques, most of which yield gas-phase data. To bridge solution and gas-phase BDEs, we need information about the solvation energetics of each one of the species involved. Therefore, the effect of the solvent, particularly the solvation of free radicals, is presently a subject of much debate and active research. By providing reliable solution values, PAC results can thus be used to further our understanding of solvation by comparison with gas-phase results and with those obtained from quantum chemistry or other theoretical methods.