Benzoquinone radical anion

Figure 2.107 (a) UV-visible absorption spectra of 5 x 10 3 M 1,4-benzoquinone (BQ), its radical anion (BQ ) and dianion BQ2 ) in dimethylsulphoxide solution containing 0,5 M tetra-ethylammonium perchlorate, (b) FTIR absorbance difference spectra of 0.02 M 1,4-benzoquinone in dimethylsulphoxide solution containing 0.5 M tetraethylammonium perchlorate. Positive absorbances are due to the 1,4-benzoquinone radical anion (BQ ) and dianion (BQ2 ) recorded at -1.00 V and -1.80 V respectively. Negative absorbances are due to 1,4-benzoquinone (BQ) present at the reference potential +0.1 V. From Ranjith et ai (1990). 

The first reported low-spin iron(III) semiquinonate complex contained the 3,5-di-t-butyl-l, 2-benzoquinonate radical anion and a tetraazamacrocycle (tazm). It reacts reversibly with acetonitrile to give [Fe (tazm)(MeCN)2] plus 3,5-di-t-butyl-l,2-benzoquinone. " The related 1,2-iminobenzosemiquinonate (ibsq) complexes [Fe(ibsq)2X] have 5 = 5/2 for X = C1 and 5 =3/2 for X = I the complex with X = Br is a mixed spin (5=5/2, 3/2) species. " ... 

Figure 9 shows internal reflection spectra taken during the electrolysis of p-benzoquinone on a Ge prism electrode at potentials < - 0.35 V vs. NHE showing the formation of the p-benzoquinone radical anion in DMSO, as reported by Tallant and Evans in 1969 [25 and references cited therein]. They also detected the anion radical of Benzil, as well as unassigned reduction products of acetophenone and benzophenone. Because of the relatively low sensitivity of the technique, rather high concentrations of reactants had to be used in order to be able to detect any products (> lOmM). An additional problem was the relatively long settling times before steady-state concentra-... 

That the fate of the Ph3CO radical was further oxidation and not dimerization was confirmed by in-situ electrochemical ESR experiments. Day performed oxidising-reducing potential sequences as carried out with TPA. When the potential was held at + 2.20 V before stepping to - 1.80 V, the resultant ESR spectrum was identical to that shown in Fig. 29, i.e. a mixture of benzophenone and benzoquinone radical anions produced by the mechanism as detailed earlier for the fate of Ph3CO+ species from TPA oxidation. 

The p-benzoquinone radical anion has been taken as prototype for the electron-transfer reaction products involving quinone-based acceptors. Using time-resolved photoelectron spectroscopy and ab initio calculations, it has been shown that excitation at 400 and 480 nm yields excited states that decay in less than 40 fs through conical intersections with lower-lying excited states, eventually leading to the ground anionic state. ... 

Now only s-orbitals have a finite probability density at the nucleus p-, d-, and /-orbitals all have nodes at this point. Consequently, the contact interaction can only occur when the electron occupies a molecular orbital with some s-character. It is therefore not immediately obvious how hyperfine splittings arise in the cases of, for example, either the p-benzoquinone radical anion (Figure 9), where the electron occupies a 7r-orbital which has a nodal plane coincident with the plane of the aromatic ring, or the ethyl radical (CH3CH2 ), which shows splittings of 26.87 G from the methyl (or j8-) protons—a larger value than is seen from the a-protons (22.38 G) ... 

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