State correlation diagram dimerization

Fig. 19 State correlation diagram for the dimerization reaction of two ethylene molecules...

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

The rules for the state correlation diagrams are the same as for the orbital correlation diagrams only states that possess the same symmetry can be connected. In order to determine the symmetries of the states, first the symmetries of the MOs must be determined. These are given for the face-to-face dimerization of ethylene in Table 7-1. The D2h character table (Table 7-2) shows that the two crucial symmetry elements are the symmetry planes a(xy) and v"(yz). The MOs are all symmetric with respect to the third plane, vide supra). The corresponding three symmetry operations will unambiguously determine the symmetry of the MOs. Another possibility is to take the simplest subgroup of D2t, which already contains the two crucial symmetry operations, that is, the C2v point group (cf.,... 

Figure 7-13. State correlation diagram for the ethylene dimerization,...

The application of correlation diagrams to photochemical processes goes back to the early work of Laidler and Shuler (1951). More recently, Longuet-Higgins and Abrahamson (1965) showed how orbital-symmetry correlations may be converted into state-symmetry correlations. Here the fundamental consideration is that states of the same symmetry do not cross, i.e. the result is an avoided crossing, while states of different symmetry do cross. The principles involved are illustrated in Fig. 19 for the dimerization of... 

Figure 7.10 An orbital correlation diagram for ethylene dimerization. Left two widely separated ethylene molecules. Center two ethylene molecules close enough for significant interactions to occur. Right cyclobutane electron configurations correspond to the ground state for each stage.

The tetramer ground state is 000> when is zero, since both dimers are in their ground states. A correlation diagram of vs E, for... 

Fig. 14. Orbital and state symmetry correlations for the dimerization of ethylene. The orbital symmetry designations in the upper diagram are with respect to the planes of symmetry tjyt and < ...

Lcl us consider A2II6 as a dimerization product of two AH3 units. 10.11 shows the least-motion approach of two CH3 radicals to form ethane C2H6. The MO correlation-interaction diagram in 10.12 shows that, given conservation of symmetry, the occupied MOs of two -CH3 radicals lead only to those of the ground state of ethane. Hence 10.11 is a symmetry-allowed reaction. If dimerization of... 

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