Valence bond diagram

FIGURE 2.7. (a) Three active pz orbitals that are used in the quantum treatment of the X + CH3-Y— X-CH3 + Y Sw2 reaction, (b) Valence-bond diagrams for the six possible valence-bond states for four electrons in three active orbitals, (c) Relative approximate energy levels of the valence-bond states in the gas phase (see Table 2.4 for the estimation of these energies). 

Valence bond diagrams, for SN2 reactions, 60 Valence bond (VB) model for diatomic molecules, 15-22 empirical (EVB), 58-59 EVB mapping potential, 87, 88... 

Connectivity indexes in particular may be used easily because each index can be calculated from valence bond diagrams. These indexes can also be correlated... 

Excitation of the protonated Schiff s base causes a movement of positive charge from the N toward the ring. The C-ll-C-12 bond loses much of its double-bond character. The valence bond diagrams shown here should not be taken too literally, but they give a good qualitative picture of the redistribution of electrons that occurs when the molecule is excited. 

Isomerization of the retinal Schiff s base can occur when the molecule is excited with light, because the C-l 1-C-12 bond loses much of its double-bond character in the excited state. The valence bond diagrams of figure S2.7 illustrate this point. In the ground state of rhodopsin, the potential energy barrier to rotation about the C-l 1-C-l2 bond is on the order of 30 kcal/mol. This barrier essentially vanishes in the excited state. In fact, the energy of the excited molecule probably is minimal when the C-11 -C-l2 bond is twisted by about 90° (fig. S2.8). The excited molecule oscillates briefly about this intermediate conformation, and when it decays back to a ground state it usually settles into the ail-trans isomer, bathorhodopsin. 

S. Shaik, A. Shurki, Angew. Chem. Int. Ed. Engl. 38, 586 (1999). Valence Bond Diagrams and Chemical Reactivity. 

S. Shaik, P. C. Hiberty, The Valence Bond Diagram Approach A Paradigm for Chemical Reactivity, in Theory and Applications of Computational Chemistry The First Fourty Years, C. Dykstra, Ed., Elsevier, New York, 2005, pp. 635-668. 

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