O Radical Anions

Table 4.5 E Electron spin densities in cobalt(o) radical anions...

Cgo- Instead, the spectral signatures of the one-electron oxidized exTTF + radical cation at 660 nm and of the one-electron reduced C o radical anion at 1010 nm were detected (Fig. 9.6). The singlet excited-state lifetimes, as determined from an average of first-order fits from time-absorption profiles at various wavelengths are listed in Table 9.1. 

The charge-separation dynamics as deduced from the decays of the Fc radical cation and the C o radical anion characteristics as a function of donor-acceptor distance (Fig. 9.65) yielded a linear relationship. From the slope [1 was determined... 

Quenching of Cgo by electron donors occurs efficiently, and the mechanism is primarily electron transfer, as shown by the formation of the well-known transient absorptions (350 to 800 nm) of aromatic amine radical cations [64]. Because of the broad visible absorption of these radical cations, it is difficult to confidently assign visible absorptions to the C o radical anion. However, with an infrared-sensitive germanium detector, a prominent transient with maxima at 950 and 1075 nm appears assigned to the Cgo radical anion, [64] in good agreement with simultaneous and later measurements of others [17, 56,65-67]. 

Bivalent radicals of the form O—Y—O are named by adding -dioxy to the name of the bivalent radicals except when forming part of a ring system. Examples are —O—CHj—O— (methylene-dioxy), —O—CO—O— (carbonyldioxy), and —O—SOj—O— (sulfonyldioxy). Anions derived from alcohols or phenols are named by changing the final -ol to -olate. 

The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). 

In the case of the (CD3)2SOv radical anion it is expected that a larger part of the spin will be on the O-group (see Figure 1). 

The ESR spectrum of the thioxanthene S, S-dioxide radical anion itself shows that the two possible conformers coexist, since the two methylene protons are not equivalent. In the case of the 9-monoalkyl derivatives, the large coupling constant observed for the 9-proton leads to the conclusion that the 9-substituent is in the boat equatorial position as in II1 F Thus the radical anions and the neutral molecule display different conformations. The protons in the 9-position of the radical anions of cis-9-methylthioxanthene S-oxides (2, n — 1, R1 = H, R2 = CH3) have an appreciable coupling constant10 which suggests that these radical anions have the substituent in the pseudo-axial position. Furthermore, in the radical anions the S—O bond is pseudo-axial. These situations are exactly the opposite of that observed for the neutral compound. 

At room temperature under photostimulation a-nitrosulfones react with a variety of nucleophiles via radical anion chain reactions interestingly, in none of the cases where the PhSOj group is involved in SrnI type of substitution does the O end of the ambident anion " play a role. This strong regioselectivity is reminiscent of the one reported for other ambident anions involved in these radical chain substitutions. ... 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

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