Semibullvalene, radical cations

Optical spectroscopy has merits in identifying radical cations, particularly when their spectra are known independently. For example, the radiolysis of quadricyclane led to the observation of the known spectrum of norbornadiene radical cation. In another study, irradiation of cyclooctatetraene radical cation caused the color of the sample to change from bright red to royal blue, suggesting the conversion to a different species, the previously identified semibullvalene radical cation. Further irradiation of the latter led to a characteristic banded (vibrationally resolved) spectrum the nature of this spectrum suggested that the rearranged species may be a linear conjugated radical cation and helped in its identification as 1,4-dihydropen-talene radical cation. ... 

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

Dai et al. (1990) have reported direct spectroscopic measurements on a similar bishomobenzene radical cation. The system they studied was the bicyclo[3.3.0]octa-2,6-diene-4,8-diyl radical cation [159] generated by radiolytic oxidation of semibullvalene [83] in Freon matrices. The hydrogen... 

Radical cations (21 +) derived from semibullvalene + or barbaralane belong to a different structure type. The ESR spectrum of 21 + (a = 3.62 mT, 2H a = 0.77 mT, and strong CIDNP effects support a structure in which... 

Another structural possibility for a hexadiene radical cation arises, when the two allylic moieties are linked in pairwise fashion to two- or three-carbon spacers. This structure type can be approached by oxidation of molecules such as semibullvalene [391-393] or barbaralane [394]. In the resulting radical cations, the two allylic moieties are held in close proximity model considerations suggest a non-bonding C—C distance of 2.2-2.3 A, considerably closer than for the previously discussed structure type. At this distance, a moderately strong interaction of the twin moieties cannot be excluded. Accordingly, we assigned cyclic conjugated structures to radical cations derived from semibullvalene (-> 138 cf. 

In contrast, CIDNP results indicate that the radical cations of barbaralane (157, X = C = 0) and semibullvalene (157, X = —) correspond to the elusive structure type with a single minimum [391, 424]. The spin density resides primarily on the termini (C-2,4,6,8) of the twin allyl moieties, whereas the remaining (internal) carbons of the 5 jr-electron perimeter have negative spin density. This spin density distribution reflects the coefficients of orbital 158, the HOMO of a bis-homoaromatic structure (Fig. 32) [424], More recently, ESR results have confirmed this assignment [392, 393],... 

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