Symmetry correlations

Figure 4. Orbital symmetry correlation diagram for R2PS2N3 and 1,3-...

Figure 5. Orbital symmetry correlation diagram for the planar (left) and folded (right) models of E(NSN)2E, where tt = as-(Reproduced from Inorg. Chem. 1982, 21, 982.)...

A symmetry correlation scheme that is capable of addressing SIKIE must combine the geometry-independent treatment of all the molecular degrees of freedom characteristic of Quack s approach with the connection of reactant and transition-state syrrrmetries characteristic of a Woodward-Hoffrnann-type analysis. To make the problem somewhat less abstract, consider the cartoon in Figure 1... 

We have recently developed a new symmetry correlation scheme that connects symmetry-distinct rovibronic states of the isolated reactants with distinct electronic symmetries of the system in the interaction region independent of the detailed geometry of the collision complex. Briefly, the approach can be logically divided into four steps. First, the appropriate PI group for describing the isolated reactants is chosen. Second, P rvet reactant supermolecule system and the electronic... 

Application of the symmetry correlation scheme to reaction (12) is summarized in Table 4 where N is the Himd s coupling case (b) rotational quantum number for O2 and - 5 is the difference of the Hund s coupling case (a) quantum numbers of total angular momentum and electron spin (5 = 1/2) angular momentum, respectively. To consider the high-symmetry isotopomer system first, the results in Table 4 indicate that only odd / collisions - 5 = odd) with 2 can lead... 

In the context of the symmetry correlation schemes in Table 5, the experimental determination of enhancement factors for the individual elements, EF( C) and EF( 0), are more relevant than the directly measured EF(89) and EF(90). Because the experiment is done under natural abundance conditions, EF(90) can be taken... 

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... 

R. B. Woodward, Symposium on Orbital Symmetry Correlation in Organic... 

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-... 

Fig. [V-2. Symmetry correlations between C + H2 and CH + H. The formation of CH2 in C, symmetry (or C2. ) is assumed. The reaction C( D) + H2 proceeds through A and A" to form CHfX1 ) + H with high efficiency. On the other hand, C( S) + H2 correlates with highly excited CH + H and the reaction requires a high activation energy. The C(. S) atoms are probably deactivated to C(1 Z>). C(3P) + Il2 forms CH2(.V- ). Reprinted with permission from R. J. Donovan and D. Husain, Chern. Rev., 70,489(1970). Copyright by the American Chemical Society.

Example 7.4-1 Find all the states that arise from the configuration d2 in a strong field of 0 l symmetry. Correlate these states with those of the free ion, and of the ion in an intermediate field. 

Silver8 adopted a similar approach, and proposed a further category of unfeasible reactions in which neither individual orbital correlation rules nor total symmetry correlation rules (Wigner-Witmer rules) were obeyed. Silver and Karplus87 have shown how to derive W-H symmetry rules using a simple valence-bond formalism. 

The state-symmetry correlation also indicates that electrocyclic radical interconversion favors a conrotatory path from the first excited state and a disrotatory path from the second excited state. Because of the proximity of the energy levels and the violations of the noncrossing rule, it is probable that the excited state process will not be highly stereoselective. The same detailed considerations must be applied to the five-atom five-electron system and yield the results given in Table 1. Differences between the stereochemical predictions of Table 1 and those of others (Woodward and Hoffmann, 1965a Fukui and Fujimoto, 1966b Zimmerman, 1966) tend to be limited to the excited-state reactions of odd-atom radicals. 

The stereochemical consequences of XnC reactions can be examined by the usual orbital symmetry correlations. The predictions for trans-frems-butadiene are indicated in (51) closures from 02 (thermal) or ifi9... 

---