State correlation diagrams avoided crossing

The photochemical disrotatory closure of butadiene to cyclobutene has been described with a state-correlation diagram, like that shown in Figure 21.4. It is based on the familiar orbital-correlation diagram of Woodward and Hoffmann," from which the intended correlations indicated by the dashed lines can readily be deduced. The solid lines indicate that there is an avoided crossing, which is put in as a result of the quantum mechanical noncrossing rule. It says that two states of the same total symmetry cannot cross. Instead, as they approach each other in energy, they will mix and separate, as the solid lines indicate. 

Figure 21.4. A state-correlation diagram depicting the avoided-crossing representation of butadiene photochemistry.

Fig. 25 State correlation diagram for an SN2 reaction. The lower states are ground states of reactants and products, while the upper states are the corresponding charge transfer states. P is the degree of avoided crossing. AE is the reaction barrier...

Figure 14.9. (a) Orbital correlation diagram for the direct insertion of carbene into an olefin to form cyclopropane. Symmetry classification is with respect to the vertical bisecting mirror plane. (b) State correlation diagram showing the intended correlations and the avoided crossing of states So and S2. 

Figure 62. Partial electronic state correlation diagram for N+-H2 system. At left, N+ is assumed to approach H2 along perpendicular bisector of bond. At right, collinear approach is assumed. Crossings that are avoided in the more general conformations of Cs symmetry indicated by dotted lines.480...

VBSCD Valence bond state correlation diagram. A VB diagram that views the barrier formation as a result of avoided crossing between two state curves that are anchored in the ground and two excited states of reactants and products. The VBSCD is a paradigm for the barrier in chemical reactions (see Chapter 6). 

In Section 4.2.2 it has been shown that due to intended or natural orbital correlations, crossings may occur in the configuration correlation diagram that are avoided in the state correlation diagram. If no avoided crossing is... 

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