Valence Bond State Correlation Diagram Application to Photochemical Reactivity

11 VALENCE BOND STATE CORRELATION DIAGRAM APPLICATION TO PHOTOCHEMICAL REACTIVITY 

FIGURE 6.21 (a) The VBSCD showing the twin-states formed by avoided crossing along the reaction coordinate of the thermal process leading from reactant R to product Pi. (b) The crossover of the twin-states generates a conical intersection (Cl) along a coordinate that stabilizes T and leads to product P2. (c) The conical intersection will be anchored in three minima (or more) R, Pi, and P2. 

It is clear from Equation 6.29 that 1P involves a bonding interaction between Ha and Hc and will be lowered by the bending mode that brings Ha and Hc together. Furthermore, based on the semiempirical VB approach 

Since Ha and Hb are far away from each other in the linear structure, we can neglect (3ac. With the remaining two terms, it is clear that B is negative, so that the ground state (A) is the positive combination of R and P, while the 

Rearranging Equation 6.34 to Equation 6.35 reveals a stabilizing 3e bonding interaction between Ha and Hc, of the type (Ha Hc Ha HC)  

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