Delocalization, isomerizing systems

B. Adiabatic Versus Nonadiabatic Delocalization in Isomerizing Systems... 

Solid state spectra of 70 exhibited a transition at 650 nm with an g of 1.21 eV, while 71 gave a transition at 760 nm and an gOf 1.12eV [95,106]. The g values determined from the CV onsets of oxidation and reduction showed a greater difference between the polymers with values of 1.26 eV for 70 and 0.61 eV for 71 [95,107]. As with polymer 69 above, the doping processes of polymers 70 and 71 have also been characterized by both optical and total reflection FTIR spectroelectrochemistry [95-98,108]. The results of these studies for 70 were fairly similar to those for 69 [95-98]. In the case of 71, however, it was found that the delocalization for the charge carriers upon n- or p-doping was much more equivalent than in the other two isomeric systems [96-98,108]. 

Most radicals located on double bonds (e.g. 4, 5) or aromatic systems (e.g. 6) are a-radicals. The free spin is located in an orbital orthogonal to the it-bond system and it is not delocalized. The orbital of the vinyl radical (4) containing the free spin can be cis- or trans- with respect to substituents on the double bond. The barrier for isomerization of vinyl radicals can be significant with respect to the rate of reaction. 

Aryl derivatives of 101 (R=Ar) are planar, and the bond lengths indicate a fully delocalized 10 re-electron aromatic system. By contrast, the (dimethyl-amino) derivative 102 adopts a folded structure. Dithiatetrazocines with exo-cyclic N(Me)Bu groups exhibit both cisltrans and ring inversion isomerism.257... 

One-electron reduction of a norcaradiene derivative produces the corresponding anion-radical. The conditions of the odd-electron delocalization in this anion-radical are less favorable than in its skeletal isomer. According to calculations, the incorporation of the unpaired electron in the nonatetraenyl n system lowers the energy content by 0.62p. However, the anion-radical initially formed is less stable than the benzotropylidene anion-radical. The latter is the end product of the isomerization (Gerson et al. 1978 Scheme 6.33). 

This conversion is directed so as to create the most favorable conditions for the delocalization of the nnpaired electron within the aromatic nucleus. It is worth noting here that thermal treatment (150—190°C) also initiates isomerization of the initial neutral molecule of norcara-diene into the benzotropylidene system. At the same time, the reductive transformation of Scheme 6.33 proceeds smoothly even at negative temperatures. Under comparable reaction conditions (25°C), the rate of conversion of the neutral molecule is 15 orders lower than that of the anion-radical. 

The dibenzotetraazapentalene ring system was first discovered by chemists at DuPont and is a planar system with six electrons delocalized over four nitrogen atoms. There are two isomeric arrangements of these four nitrogens which lead to the l,3a,4,6a- (220) and l,3a,6,6a-(91) ring systems. Nitro derivatives of both isomeric dibenzotetraazapentalenes have been explored as thermally stable explosives. 

Again it has to be noted that the frontier orbitals participating in such a valence isomerization are delocalized over the whole molecule [22]. This has consequences for the orbital symmetry and, thereby, a prior analogy with comparable processes involving 6 t-electrons only is not given. However, compared with smaller Jt-systems the selection rules for orbital symmetry controlled processes in fullerenes seem to be less restrictive, since a large number of tt-orbitals with small energy separation are available. Calculations at the AM 1 and PM3 level show that the photocycKzation... 

Homo-l//-azepines (29), unlike their carbocyclic counterparts, show little tendency to isomerize to the thermodynamically less favoured bicyclic aziridine tautomers (30). Presumably, the homoazepine gains its stability from delocalization of the nitrogen lone pair through the dienamine system <71AG(E)ll). However, the aziridine tautomers (30 R1 = C02Me, R2 = Ac, C02Me or p-Ts) have been trapped as their all-cTs bis-l,3-dipolar cycloadducts with diazomethane (76CB3505). 

The chemistry of unsaturated azepines is dominated by their polyene character. The absence of ir-delocalization confers instability on the ring system as witnessed by the many and various ring transformations undergone in acid and base solution, and under thermal and photolytic conditions. Most of the major reactions of azepines involve the neutral molecule e.g. cycloadditions (Section 5.16.3.8.1), metal carbonyl complexation (Section 5.16.3.8.2), dimerizations (Section 5.16.3.2.3) and photo- and thermo-induced valence isomerizations (Section 5.16.3.2.1). 

These compounds are isomeric with the 1,2-azoles, e.g. isoxazole, pyrazole and iso thiazole. The aromatic characters of the oxazole, imidazole and thiazole systems arise from delocalization of a lone pair of electrons from the second hetero-atom. 

Thus by NMR it is possible to illustrate the variation of the charge distribution under various conditions of the apparently delocalized systems. It is also possible to illustrate the various rates of rotation and isomerization about the various bonds, and the structure of the ions. It is these factors coupled with the rates of addition of monomer which determine the structure of the polymer formed. 

A second difficulty arises from consideration of allylic systems. Because the resulting cationic center is stabilized by interaction with the -it electrons, allylic halides ionize readily to produce the delocalized allylic ion, 2. The free ipn theoiy predicts that isomeric allyli halides that give the samp intermediate,-upon ionization should yield a product distribution independent of the isomeric origin of thp ion Scheme 2 illustrates the argument. The prediction is sometimes, but... 

---