Jahn-Teller effect principles

As noted previously, the outstanding success of orbital symmetry rules in organic chemistry (75, 242) has led to many attempts to extend these rules to organometallic chemistry and metal-catalyzed processes. These qualitative analyses based on the principle of conservation of orbital symmetry or second-order Jahn-Teller effects have been reviewed extensively (91, 142, 143, 173, 175, 179, 221, 225) and will not be considered in any detail here. 

Let us assume that the polymerization of quasisymmetric dienes proceeds in accordance with the principle of local symmetry, Le., via the stage of formation of a locally symmetric intermediate. The structure of the locally symmetric diene intermediate (Scheme 11) is tentative. However, this is not very important since the train of thoughts is based only on the hypothesis of the existence of the Jahn-Teller effect in this case. 

Let us assume that we are dealing with an alternating copolymerization of butadiene (or isoprene) with a monomer drastically differing from butadiene. In this case the symmetric diene will be attached all the time to an alien radical active center and, vice versa, an alien monomer will be bonded to a diene radical center. The principle of local symmetry is no longer valid, and in the absence of the Jahn-Teller effect the formation of only the 1,4 (4,l)-structuie of diene units in the copolymer should be expected. 

Elliott et al. [136) were unable to make a clear cut decision between a static and a pseudorotational d3mamic Jahn-Teller effect for the hexani-trito complexes, as is the case also for NiF, but this distinction would not in principle affect the spectroscopic consequences, since such measurements reflect the instantaneous rather than the time averaged situation. 

Octahedral Vanadium(III). It has been pointed out by Jorgensen (19) that the vanadium(III) ion may not be the most suitable example on which to study the reduction of interelectronic repulsion parameters. In principle, both the vanadium(III) and the cobalt(II) ions to be discussed below are configurationally instable owing to the influence of the Jahn-Teller effect (76) on the orbitally d enerate ground states Ti, and respectively. However, in contrast to Ej states, it may be expected that the distortion should be relatively small in electronic Ti, and T g states. Since any indication of a dissymmetry is lacking in the electronic spectra studied here, the influence of the Jahn-Teller effect will not be considered farther. 

The general principles for the construction of the vibronic Hamiltonian and the symmetry selection rules are the same as indicated above and discussed in more detail in Chapter 7. The previous distinction between trigonal and tetragonal point groups does not play a role, and the first-order coupling is always accomplished by doubly degenerate (e) vibrational modes. In an analogous notation as in Eq. (3), the Hamiltonian for the linear (E + A) (g) e pseudo-Jahn-Teller effect is found to... 

PRINCIPLE CR2 (of second order Jahn-Teller effect) An energetic narrow LUMO - HOMO band predisposes in general to an increase in chemical reactivity. 

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