Dewar benzene radical cation

One might argue that the results least subject to ambiguity are those with the shortest delay between the generation of the radical cation and its observation. In this respect, the time-resolved ODMR results of Trifunac and Qin (Fig. 24) [368] and time resolved CIDNP results observed in the author s laboratory (Fig. 25) [380], may provide the least distorted view of the species in question. Of course, neither of these experiments qualifies as the coveted direct observation. Thus, the direct observation of the elusive hexamethyl-Dewar benzene radical cation must await further scrutiny. 

In summary, the radical cations of the benzene valence isomers show several interesting structures. Although most of their structural features can be rationalized by considering the HOMOs of the precursor molecules, some of the species show substantial changes in individual bond lengths. Accordingly, species such as the 2B2 and 2A1 Dewar benzene radical cations, the 2Bj and 2At benzvalene radical cations, or the 2Bt prismane radical cation cannot be expected to qualify as Koopmans radical cations. To date, most of the information available in this series is based on CIDNP results and ab initio calculations. It is safe to predict increasing involvement of ESR spectroscopy in this area. 

The lowest state of prismane ( Si) radical cation lies 16 kcal mol above the Bj state of the Dewar benzene radical cation (at the MP2/6-31G level) the energy difference is considerably less than that between the ground states of the corresponding neutral systems (37 kcal mol ) [234]. The ground electronic states of prismane and Dewar benzene ions do not correlate their interconversion is forbidden from both state-symmetry and orbital-symmetry considerations. Nevertheless, the CIDNP experiments indicate that the actual barrier is quite small [233]. 

S. Matzinger and T. Bally, work in progress, 1999. The Dewar Benzene Radical Cation Electronic Structure and the Kinetics of Its Conversion to the Benzene Radical Cation. 

Finally, pulse radiolysis has the advantage of being able to measure the kinetics of reactions, also in liquid solvents, where bimolecular reactions can be studied, with a time resolution down to tens of nanoseconds. It is with this technique, that the G bicki/Marcinek group in Lodz, who had previously applied it to follow the kinetics of the isomerizations of the Dewar benzene radical cation [8], was able to study the tunneling kinetics of the enolization of radical cations. 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

Among the radical cations derived from strained ring hydrocarbons those of composition (CH)6, the valence isomers of benzene (128) (Scheme 11), have attracted considerable interest. In addition to the one-electron oxidation products of Dewar benzene (126) [360-368), bicylopropenyl (129) [369], benzvalene (130)... 

The first valence isomer to be investigated was the bicyclic (hexamethyl-, HM-) Dewar benzene. However, y irradiation of this substrate produced the electronic spectrum of HM-128 [360]. Similarly, nsec time-resolved laser spectroscopy failed to reveal evidence for the bicyclic radical cation [362]. The first indication for the existence of such a species as a discrete entity was provided by a CIDNP study [361]. These results are best discussed in connection with ab initio calculations carried out for the parent C6H6 system. At the 6-31 G level these calculations support the existence of two cationic states with the unpaired spin density either... 

Roth, Schilling and coworkers [47-54] have investigated the nuclear spin polarization behavior of cation-radicals of numerous strained hydrocarbon systems produced by PET to strong electron acceptors such as chloranil, anthraquinone and cyanoaromaties. These systems include cyclopentadiene dimers, methyl-enebicyclo[2.2.0]hexenes [48], bicyclo[1.1.0]butanes [49], hexamethyl (Dewar benzene) [50], norbomadiene [53], quadricyclene [53], and 1,2-diphenylcyclopropanes [54]. 

The fate cS the contact ion pair [RH A 1 is critical to electrm-transfer oxidation. Oxidative efficimey is the highest with those organic donors diat yield unstable radical cations, such as hexamethyl(Dewar benzene), which und goes spontaneous rearrangement (equation 7). > ... 

For example, the quantum yield for a CdS-, TiOj-, or ZnO-mediated valence isomerization of hexamethyl-dewar benzene to hexamethylbenzene is greater than xmity [106]. A cation radical chain reaction mechanism accounts for this observation (Fig. 2). 

Recent efforts to elucidate the structure of the radical cation of hexamethylprismane (Table 8 entry 20b) have shown that a short-lived species (ns range) is generated in the PET reaction when anthraquinone is used as electron transfer sensitizer Its structure is different from the radical cation of hexamethyl Dewar benzene and it rearranges to the latter and hexamethylbenzene. Both products are also generated in the pyrolysis and direct photolysisof hexamethylprismane (Table 8 entries 20a, c). 

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