Radicals and Radical Ions of Alternant Hydrocarbons

Radicals with an odd number of electrons can be either uncharged odd hydrocarbons or radical ions of even hydrocarbons or systems derived from such hydrocarbons. Of special interest are the relationships that exist for radicals and radical ions of alternant hydrocarbons. 

In contrast to closed-shell systems, there are no higher multiplicity transitions that correspond to these lowest-energy excitations. The HOMO LUMO transition is the first one for which a quartet configuration is possible, in addition to two doublet configurations. One of the doublet configurations, is obtained from the ground configuration only by 

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If the transition considered is the HOMO LUMO transition of an alternant hydrocarbon, then first-order theory predicts that inductive perturbation will have no effect at all, because for = fo as a consequence of the pairing theorem. Small red shifts are in fact observed that can be attributed to hyper conjugation with the pseudo-7t MO of the saturated alkyl chain.290 On the other hand, alkyl substitution gives rise to large shifts in the absorption spectra of radical ions of alternant hydrocarbons whose charge distribution is equal to the square of the coefficients of the MO from which an electron was removed (radical cations) or to which an electron was added (radical anions), and these shifts are accurately predicted by HMO theory.291... 

Figure 2.24. Orbital energy levels of alternant hydrocarbon ions a) anions and cations of odd-alternant systems, and b) radical anions and radical cations of even-alternant systems in the HMO approximation and c) in the PPP approximation.

Unlike the corresponding ions, benzylic radicals are true alternant hydrocarbon species. It was deemed of interest to see whether rates of formation of sudi species could be related to changes in delocalization and also whether a similar didiotomy of results between HMO and SCF correlations would be observed. 

Recently, a nonempirical rr-electron SCF approach was reported and applied to interpretations of spectra of various conjugated hydrocarbon radicals (147). The greatest attention, however, has been paid to radical ions derived from even alternant hydrocarbons (10, 58-60, 63, 125, 135, 148-153). Here, numerous experimental material suitable for systematic testing of the MO methods has been accumulated. In particular, the following sources of experimental data should be mentioned Hamill and collaborators (24) prepared... 

Certain aromatic hydrocarbons, such as 9,10-diphenylanthracene, give relatively stable radicals and cation radicals upon electrochemical reduction and oxidation, respectively. If one arranges to have the radical ions from both processes mixed, either by normal DC electrolysis in a suitably designed cell or by using an alternating current for the electrolysis, the phenomenon of electrochemiluminescence appears (Hercules, 1971 McCapra, 1973). 

To date the mercurated arene radical cation is known for biphenylene [87], ace-naphthene, pyracene, hexahydropyrene, triptycene, p-terphenyl, tetramethylnaph-thopyran, anthracene, dibenzodioxin [89], and 4-tert-butylanisole [88]. In certain cases multiple mercuration is observed, for example in case of diphenylene [87] and dibenzodioxin [89]. Mercuration causes a decrease in -value and always occurs at the site where the local coefficient of the Huckel HOMO of the hydrocarbon is greatest, and there is a constant ratio of about 20.6 between the hyperfine couplings by the Hg [l, abundance 16.84 %) which has been introduced, and by the proton which has been displaced [89]. EPR spectroscopic evidence is reported for 8, 9, 10, 11, 12, 13, 14, 15 and 16 as new examples of recently recognized alternative mechanism of arene mercuration in which collapse of ArH +Hg(TFA)2 radical ion pair leads to arylmercury trifluoroacetate ArHg(TFA) + [90]. 

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

An alternative interpretation of this polymerization was advocated by Schlenk and Bergmann8. In his early work reported in 19149 Schlenk described the addition of alkali metals to aromatic hydrocarbons leading to intensely colored solutions. The concepts of free radicals or radical-ions were unknown at that time, hence Schlenk referred to the adduct as a complex. He also showed that a similar reaction of 1,1-diphenyl ethylene resulted in its colored dimer which yielded 2,2,5,5-tetraphenyl adipic add on carboxyla-tion. When the concept of free radicals was established, Schlenk and Bergmann argued that the initially formed adduct is a free radical, e.g. 

Dodd 81) has found that Eq. (46) combined with the model accounting for the hyperconjugation effect reproduces reasonably well the magnitude and direction of the shifts of characteristic bands of the naphthalene and anthracene radical ions upon methyl substitution. In general, with alternant hydrocarbons the shifts of the first bands upon substitution by groups with the inductive effect (e.g. CHs or NH3) can be expected to be more considerable for radical ions than for the parent hydrocarbons, inasmuch as the first transition energy in closed-... 

The dominance of the thermochemically more advantageous decomposition paths is illustrated in Fig. 45 which shows the mass spectrum of paraffin oil (a mixture of liquid paraffins, i.e. of h Mrocarbons of the structure CnH +a) The peculiar alternation of intensities is well-defined. The ions with an odd number of hydrogen atoms possess an even number of electrons and thus are not radical ions. The heat of their formation is lower than that of the ions with neighbouring masses and odd number of electrons, i.e. of radical ions that exhibit the less intense lines. This intensity alternation in hydrocarbon mass spectra, particularly evident for long-chain hydrocarbons, was first reported in [93]. 

As a general treatment of the problem of substitution, Wheland s method has the disadvantage of not being very easy to apply. For one class of aromatic substances, namely the alternant hydrocarbons and heterocyclic compounds isoaromatic with them, a comprehensive and rapid treatment has been developed. Alternant hydrocarbons have no odd-numbered rings. They have the property that the constituent carbon atoms fall into two sets distinguished as starred and unstarred, such that no atom of one set is adjacent to another of the same set. Odd alternant hydrocarbons are radicals or ions. As an... 

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