Orbital symmetry effects

C3.2.2.4 BRIDGE ORBITAL SYMMETRY EFFECTS IN CHEMICAL SYSTEMS... 

Figure 9. Dyads used for investigating orbital symmetry effects on ET rates. Centre Orbital symmetries of the active MOs involved in the ET processes for 2(8) and 4(8).

The second example (Figure 9) of orbital symmetry effects concerns photoinduced charge separation in the norbornylogous dyads, 5(7) and 6(7), which is symmetry forbidden in the former system but symmetry allowed in the latter. In these systems, orbital symmetry effects are more pronounced than in 2(8) and 4(8), amounting to a 28-fold modulation in the magnitude of Vj201... 

A. M. Oliver, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, Orbital Symmetry Effects on Intramolecular Chaige Recombination , Chem. Phys. Lett. 1992, 191, 371-377. 

An interesting illustration of the importance of orbital symmetry effects is the contrasting stability of azo compounds 16 and 17. Compound 16 decomposes to norbomene and nitrogen only above 100°C. In contrast 17 eliminates nitrogen immediately on preparation, even at —78°C.221... 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

Comparison of the time resolved data in several solvents for 28 with that for a similar molecule in which one of the quinones was replaced by a dimethoxyphenyl group showed an increase in the lifetime of the fluorescence decay, as would be expected from the removal of a second acceptor, as discussed above. However, the data are complicated by the fact that the decays of the dimethoxyphenyl analog were decidedly biexponential. This result was interpreted in terms of an orbital symmetry effect on the electron transfer reaction. Interesting solvent and temperature dependencies were also observed with 28. 

Just as there are secondary interactions in cycloadditions, so too are there ancillary orbital-symmetry effects in sigmatropic reactions. In process (79), Jones and Jones (1967) find no products of [1,3] hydrogen or methyl shifts, e.g. 1,4,7-trimethylheptatriene, which ostensibly (Table 5) are photochemically allowed. They point out that in the first... 

The rigid, symmetric norbornylogous systems are well-suited for investigating orbital symmetry effects, two examples of which are shown in Figure 50. Charge... 

In summary, orientation effects on ET rates are important, amounting to modulation of rates by factors as large as 20. They are certainly as important as bridge configuration effects (i.e., the all-trans rule) and orbital symmetry effects in influ-... 

Orbital symmetry effects are observed in concerted reactions, that is, in reactions where several bonds are being made or broken simultaneously. Woodward and Hoffmann formulated rules, and described certain reaction paths as symmetry-allowed and others as symmetry-forbidden. All of this applies only to concerted reactions, and refers to the relative ease with which they take place. A symmetry-forbidden reaction is simply one for which the concerted mechanism is very difficult, so difficult that, if reaction is to occur at all, it will probably do so in a different way by a different concerted path that is symmetry-allowed or, if there is none, by a stepwise, non-concerted mechanism. In the following brief discussion, and in the problems based on it, we have not the space to give the evidence indicating that each reaction is indeed concerted but there must be such evidence, and gathering it is often the hardest job the investigator has to do. 

Another bridge in the VBSCD is to orbital symmetry effects [6] and to frontier orbital theory [36]. The effect of orbital symmetry and frontier orbitals is implicit in the expression for B. It is the factor B that makes distinction between allowed and forbidden reactions, and determines the preferred trajectory of the reaction, thereby forming bridges between the VB diagram and MO-derived concepts of reactivity. This has been elaborated amply in previous reviews of the field [7,11,13,14,28], and a brief discussion, by way of an example, is given later. 

In earlier research by one of us pronounced orbital symmetry effect on intramolecular charge-transfer absorption and emission spectra were reported from studies on cyclophanes containing aromatic D/A pairs in a close-contact sandwich type arrangement. The pairs 12(5,5)/12(4,8) and 13(6,6)/13(3,8) epitomize the plethora of D/A cyclophanes investigated in these studies (see Fig. 9). 

Figure 11. Structure of rigidly bridged D/A systems used to investigate orbital symmetry effects. The electrochemical reduction potentials of the acceptor groups incorporated are given inVolts vs. SCE in acetonitrile an asterisk indicates that the electrochemical reduction is irreversible.

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