Identity hydrogen abstraction reactions, X-center dot+H-X '-> X-H+X '(center dot) (X = X ' = CH3, SiH3, GeH3, SnH3, PbH3): A valence bond modeling

Breathing orbital valence bond (BOVB) computations (Hiberty, P. C.; Humbel, S.; Archirel, P. J. Phys. Chem. 1994, 98, 11697) are used to obtain identi ty barriers for hydrogen transfer reactions between X groups, X = H, CH3, S iH3, GeH3, SnH3, and PbH3. Modeling of these barriers by means of VB state correlation diagrams (Shaik, S.; Shurki, A. Angew. Chem. 1999, 38, 586) lea d to simple expressions for the barriers (eqs 21 and 22). These expressions show that the organizing quantity of the barriers is the singlet-triplet e xcitation energy (DeltaE(ST)) or bond energy (D) of the X-H bond that under goes activation. The larger the DeltaE(ST) or D, the higher the identity ba rrier. These equations are successfully applied to deduce barriers for hydr ogen transfers between electronegative groups, X = X' = F, Cl, Br, and I. T he "polar effect" (Russell, G. A. In Free Radicals; Kochi, J. K., Ed.; Wile y: New York, 1973; Vol 1, p 293-298) is shown to be significant but virtual ly constant in the series. Thus, identity processes mask the polar effect w hich is more clearly expressed in nonidentity hydrogen transfer reactions. Generalization of the model to other atom transfer reactions is discussed.