VB curve-crossing diagram

In the next few sections, we will describe examples where we used VB curve crossing diagrams to predict trends in oxygen atom transfer reactions. 

Fig. 11. VB curve-crossing diagram for substrate epoxidation by Compound I of P450.

To understand the correlation between sulfoxidation barrier height and ionization potential of the substrate, also here a VB curve-crossing diagram was set up (106), which is shown in Fig. 13. In contrast to aliphatic hydroxylation and epoxidation discussed above, the sulfoxidation reaction is concerted without a reaction intermediate. Therefore, the reactants connect with products directly and the promotion gap as a result reflects the excitation energy from the reactant wave function to the product wave fimc-tion in the reactant geometry. The excited wave fimction Fp essentially refers to the one-electron transfer from substrate to oxidant hence it is proportional to the IE of the substrate and the electron affinity of the oxidant. Consequently, the correlation... 

The stabilization energy, of the transition state relative to the localized structure at the crossing point in the VB state correlation diagram, due to resonance mixing of the VB curves. VB curve crossing diagrams... 

This article discusses key elements of the valence bond (VB) curve crossing diagram which has been formulated to meet the requirements of an interface model, and has been applied in the last decade and a half to a variety of reactivity problems in organic as well as organometallic chemistry. "... 

To complete the curve crossing diagram we need to mix the remaining two configurations 17 and 18 into each of the curves in Fig. 4. Following the VB mixing... 

Other delocalization properties which lead to a high / factor are delocalization of the radical anion over a few identical C-X linkages as in CH4, CH2CI2, and so on [4b, 32b, 46]. Delocalization of the odd electron in X , as for example in PhCH2 leads to a large / factor. The relationship between the size of / and the extent of radical anion delocalization has been retrieved recently with the ab initio VB computations df the curve crossing diagram [12 a] for H + CH4, and the results are discussed later in this chapter. 

Fig. 17. A curve crossing diagram for H + CH4 [CH5 ] CH4 -f showing separately the Lewis and intermediate VB structures 45-48. Reprinted with permission from J. Am. Chem. Soc. (1990), 112, 1407. American Chemical Society...

The VBSSA rule can be used to select relevant VB structures and construct the semiquantitative and quantitative curve-crossing VB diagram. Combining the MO method and VB calculations, we investigated atom exchange reactions H + HLi —> H2 + Li and H + LiH—>HLi + H, and discussed the effect of energy differences between the VB structures on the activation energy and properties of the transition state [70],... 

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