Symmetry correlation reactant-product

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

If the symmetries of reactant and product orbitals match up, or correlate, the reaction is said to be symmetry-allowed. If the symmetries of reactant and product orbitals don t correlate, the reaction is symmetry-disallowed. 

Reactant states will only correlate with product states of the same spatial symmetry and spin multiplicity. 

The difference in reactivity ofC( D) from C( S) is explained by Donovan and Husain (310) on the basis of symmetry correlations between reactants (for example, carbon atom and hydrogen molecule) and products (methylidyne and atomic hydrogen). Figure IV—2 shows the correlation of C + H2 with CH + H. The reaction C + H2 is assumed to form CH2 of Cs symmetry (or C2. symmetry), which dissociates subsequently into CH + H. Correla-... 

Correlation rules relate the symmetry of reactants to the symmetry of products. More precisely, they give the symmetry of the fragments which can result when a molecule or transition state is distorted in the direction of reactants or products32,33. A familiar example is the correlation of the states of a diatomic molecule with those of its constituent atoms. Within the Bom-Oppenheimer separation we can deal with strictly electronic correlation rules, valid when there is negligible coupling between electronic and vibrational wave functions. When such coupling is important, correlations forbidden on a strictly electronic basis may be allowed, so the validity of purely electronic correlation rules is hard to assess for polyatomic molecules with strongly excited vibration. 

A chemical reaction always involves bond-breaking/making processes or valence electron rearrangements, which can be characterized by the variation of VB structures. According to the resonance theory [1, 50], the evolution of a system in the elementary reaction process can be interpreted through the resonance among the correlated VB structures corresponding to reactant, product and some intermediate states. Because only symmetry-adapted VB structures can effectively resonate, all VB structures involved in the description of a reaction will thus retain the symmetry shared by both reactant and product states in the elementary process. Therefore, we postulate that the VB structures of the reactant and the product states for concerted reactions should preserve symmetry-adaptation, called the VB structure symmetry-adaptation (VBSSA) rule. 

However, if we promote an electron to 71 3 in cyclohexadiene (obviously by irradiation), then the orbitals of the reactant with C2 symmetry correlate with the first excited state of the product (Eqn 2.8). 

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