Hydrocarbon radical anions

For aromatic hydrocarbon radical anions, this approach works pretty well. Figure 2.7 shows a correlation plot of observed hyperfine splitting versus the spin density calculated from Hiickel MO theory. It also correctly predicts the negative sign of aH for protons attached to n systems. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

Decay of arene hydrocarbon radical-anions formed during preparative scale electrochemical reduction in the presence of general acids involves protonation as... 

The major oxidation product isolated was anthracene, perhaps formed in part from the hydroperoxide (I). However, significant amounts of potassium superoxide accompanied the anthracene. This result suggests that the major source of anthracene involved the oxidation of the dianion. In pure DMSO in the presence of excess potassium tert-butoxide, a trace of oxygen converts 9,10-dihydroanthracene, 9,10-dihy-drophenanthrene, or acenaphthene to the hydrocarbon radical anions. These products are apparently formed in the oxidation of the hydrocarbon dianions. 

The study of the photochemistry of aryl carbanions has been restricted to aryllithiums with only a limited number of studies available. Hence, a general picture of their photochemistry is not available at this time. Photolysis of phenyllithium in the presence of aromatic hydrocarbons such as naphthalene, biphenyl, phenylene, etc. in diethyl ether results in electron transfer from the phenyllithium to the aromatic hydrocarbon, with production of the corresponding hydrocarbon radical anion, as observed by ESR spectroscopy [6-8] (Eq. 1). Photolysis of phenyllithium or 2-naphthyllithium alone gave the corresponding biaryl products and metallic lithium [9-10]. For this reaction, it is possible to write a mechanism which does not require electron transfer from the anion [9,10],... 

Given the degeneracy of the LUMO in polycyclic hydrocarbons, addition of an electron into these orbitals is more or less facile and leads to thermally stable radical anions redox potentials for a number of polycyclic hydrocarbon-radical anions are listed in Table 8. 

Table 8. Reduction potentials of polycyclic hydrocarbon-radical anions relative to FeCp2 in DMF .

Winkler et al. (1966) have obtained the spectra of the lithium salts of the radical-anions of aromatic hydrocarbons such as naphthalene and biphenyl by irradiating the corresponding hydrocarbon in the presence of phenyl-lithium this method has several advantages over others for generating hydrocarbon radical-anions, one being that studies may be made in a wide range of solvents. 

Section 15.2 contains hydrocarbon and substituted hydrocarbon anion radicals. It is divided into two main parts Section 15.2.1 deals with hydrocarbon radical anions whilst Sect. 15.2.2 deals with substituted compounds and is subdivided into 3 subsections. Section 15.2.2.1 is concerned with the substituted radical anions, Sect. 15.2.2.2 deals with perfluoro and perchloro substituted radical anions, and Sect. 15.2.2.3 contains substituted fluorenones. These have been included for completeness and because we feel that they may not be included elsewhere. Mono-, di- and trianions are also included. 

The period covered is from 1985 to flie end of 1999. This period overlaps slightly with that covered in volume n/17f, any duplication of data has been avoided where possible. Extensive use was made of the specialist periodiocal reports of the Royal Society of Chemistry and Chemical Abstracts in order to obtain the data. Ever effort was made to ensure completeness however unless the paper title or abstract contained the key words such as for Sect. 15.2 hydrocarbon radical anion or substituted hydrocarbon radical anion or for Sect. 15.4 nitro radical anion there is the possibility that the paper has not been cited. It should be noted that in 1996 the Chemical Abstracts switched fiom the paper version to an electronic version held centrally on a computer network, but every attempt has been made to avoid inconsistencies in the searching. 

The entries are in increasing order of carbon and hydrogen atoms, where relevant di- and trianions immediately follow the mono-anion. The deuterated hydrocarbon radical anion immediately follows its parent... 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

Figure 8-18 ESR splitting constants in gauss versus HMO unpaired spin densities. The systems are fused ring alternant hydrocarbon radical anions (naphthalene, anthracene, tetracene, pyrene). The underlined point is thought to result from negative spin density. (Data from Streitwieser [7].)...

Figure 8-19 ESR hyperfine splitting constants in gauss versus for the highest occupied MO of the radical. All data are from hydrocarbon radical anions or cations of alternant or nonaltemant type. ( ) Anion radical (x) cation radical. Uncertain assignments (o) anion ((g)) cation. The two correlation lines are merely sight fitted to the anion and cation points separately and suggest that Q for cations should be larger than for anions. Points thought to result from negative spin density are underlined. [See Tables I, III, VIII, XII, XIII, XIV, XV, XVI, XVII, XVIII of Gerson and Hammons [9], and Table 6.2 of Streitwieser [7] for data plotted here.]...

Hydrocarbon Radical Anion and Dianion Alkali-Metal Compounds.— The orystal structure of (Ph—Ph), K+(tetraglyme)2 is essentially the same as that of the Rb analogue the cations are surrounded by ten oxygens of the two tetraglyme molecules. The effect of pressure on the ion-pair equilibria, solvent-separated ion-pair (s.s.i.p.)v contact ion-pair (c.i.p.), for Naph, Na+ in THF was studied as was the volume change for the conversion c.i.p. -> s.s.i.p. Disproportionation equilibria [equation (1)] were variously studied, e.g. for ArH = perylene (Pe), ... 

Periasamy et at. obtained aldehydes in good yield from poly-cyclic aromatic hydrocarbon radical anions prepared by the addition of sodium to the aromatic hydrocarbon in THF, followed by formylation with carboxylic acid esters or N,N-dialkyformamides. Reactions of sodium naphthalenide, -anthracenide and -phenan-threnide with ethyl formate yielded the corresponding aldehydes. Substituted naphthalenes e.g. acenaphthene and 2-methylnaphthalene are also formylated using A/,A/-dialkylformamides, but in low yields (20% and 26% respectively). 

A large number of aromatic hydrocarbon radical anions has been reacted with Wurster s Blue-type radical cations, e.g. the radical anions of 1-phenylnaph-thalene, 8,8 -dimethyl-naphthalene, 1,1 -binaphthyl, p-terphenyl, chrysene, 1,2-dimethylchrysene [29]. They all represent energy-deficient systems, so that triplet-triplet annihilation had to be regarded as the mechanism for the production of the emitting singlet state. The quantum yields were in the range 10" to 10" Einstein/mol [29]. 

Table 11. Radical ion recombination chemiluminescence of aromatic hydrocarbon radical anions with subst. triphenylamin radical cations (after Zachariasse [29])...

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