Barriers of hydrogen abstraction vs halogen exchange: an experimental manifestation of charge-shift bonding

This paper shows that the differences between the barriers of the halogen exchange reactions, in the H + XH systems, and the hydrogen abstraction reactions, in the X + HX systems (X = F, Cl, Br), measure the covalent-ionic resonance energies of the corresponding X-H bonds. These processes are investigated using CCSD(T) calculations as well as the breathing-orbital valence bond (BOVB) method. Thus, the VB analysis shows that (i) at the level of covalent structures the barriers are the same for the two series and (ii) the higher barriers for halogen exchange processes originate solely from the less efficient mixing of the ionic structures into the respective covalent structures. The barrier differences, in the HXH vs XHX series, which decrease as X is varied from F to I, can be estimated as one-quarter of the covalent-ionic resonance energy of the H-X bond. The largest difference (22 kcal/mol) is calculated for X = F in accord with the finding that the H-F bond possesses the largest covalent-ionic resonance energy, 87 kcal/mol, which constitutes the major part of the bonding energy. The H-F bond belongs to the class of "charge-shift" bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D. L.; Hiberty, P. C. Chem. Eur. J. 2005, 21, 6358), which are all typified by dominant covalent-ionic resonance energies. Since the barrier difference between the two series is an experimental measure of the resonance energy quantity, in the particular case of X = F, the unusually high barrier for the fluorine exchange reaction emerges as an experimental manifestation of charge-shift bonding.