Benzosemiquinone radical

As stated above, FBC/FBC represent the neutral/reduced forms of free base chlorin and Q/Q the p-benzoquinone molecule and / -benzosemiquinone radical anion, respectively. 

The present experiment involves an investigation of the ESR spectra of the unsubstituted ortho and para-benzosemiquinone radical anions, along with one or more of the methyl and t-butyl derivatives. Aspects of interest include the elucidation of splitting constants from complex spectra, the examination of substituent effects in ESR spectra, and the interpretation of spectra using molecular-orbital (MO) calculations. 

The parent para-benzosemiquinone radical anion may gradually increase in concentration and will last about 2 h the methyl- or t-butyl-substituted radicals form more quickly and have lifetimes of about 15 min. The color serves as some guide to the optimal concentration a reddish color suggests that too much base has been added, while a brown-black color indicates that radical-radical combination has occurred. 

For the benzosemiquinone radical anion, one has nine tt electrons, with two each in the fonr lowest levels and the nnpaired electron residing in 5. Using the HMO resnlts in the tables, calcnlate p for the carbons attached to the protons and compare with the valnes obtained from your experimental results and the McConnell relation [Eq. (9)] with Q = -22.5 ganss. Note that the calculations provide a basis for assignment of the hyper-fine splitting constants to specific ring protons in orfAo-benzosemiqninone. Draw out the possible valence-bond resonance structures for both ortho and para compounds and dis-cnss the relative importance of these. 

The reaction affords two products, an oxolane Pi and an oxetane P2, which exhibit a mirror-image relationship of their CIDNP patterns. The three most strongly polarized signals, of Hi, H7, and H7, with intensity ratios of about —2 to + 3 to +3.5, have been shown in the figure all the other protons are also polarized, but more weakly. The observed pattern is found to be in excellent agreement with the relative proton hyperfine coupling constants of the neutral benzosemiquinone radical and of the tert-butoxybicyclo[2.2.1]heptenyl radical, which were tested as model compounds for the two radical moieties.The biradical BRi is thus the source of the polarizations. It is formed in a triplet state, its singlet exit channel produces the oxolane Pi, and its triplet exit channel the oxetane P2. 

One-electron reduction of p-benzoquinone to give the p-benzosemiquinone radical anion gives bond length changes whose direction may be predicted from the LUMO of p-benzoquinone, shown in Figure 6. Since the... 

Figure 7. p-Benzoquinone and resonance structures of the p-benzosemiquinone radical anion demonstrating the more benzenoid character of the radical anion. 

Approximate mode descriptions and experimentally measured(65-55, 69-71) and scaled, calculated(54) vibrational frequencies(cm ) of p-benzoquinone (PBQ) and p-benzosemiquinone radical anion (PBSQ). Calculations were done using the B3LYP/6-31G(d) method. Frequencies greater than 1000 cm were scaled by 0.9613 and frequencies... 

For p-benzosemiquinone radical anion, the B3LYP-derived vibrational modes are shifted to lower frequencies than in the neutral p-benzoquinone (see... 

After initial tests for tyrosyl radical, -benzoquinone, and p-benzosemiquinone radical anion showed the power of DF-based methods for calculating accurate structures, vibrations, and spin properties, their use to predict the properties of other important radicals exploded. Published HF/DF calculations to characterize the photosynthetic electron transfer cofactors plastoquinone, menaquinone, and ubiquinone are described next. 

Resorcinol and substituted resorcinols give m-benzosemiquinone radicals when oxidized in either alkaline solution (ferricyanide) or acidic solution (ceric ion) in the latter conditions the radical-anion is pro-tonated (Stone and Waters, 1964b), e.g. 

The m-benzosemiquinone radical anion exhibits a Raman spectrum that has a CO stretching frequency similar to that of phenoxyl radical and another band at a much lower frequency that is ascribed to a second CO stretching. This suggested that the two CO groups are not equivalent and that the m-benzosemiquinone anion is more similar to a 3-hydroxyphenoxyl. ESR spectra of the m-semiquinone anion, however, indicate complete symmetry, probably due to rapid spin interchange. 

Fig. 8-12, Concentration of p-benzosemiquinone radical (arbitrary units) and dissolved O2 in solution as a function of time (Kung and McBride, 1988).

The products of the (thermal) reaction of decafluorobenzophenone with meth-oxide ion have been identified by conversion in situ in an e.p.r. spectrometer into the corresponding radical anions [e.g. (4-MeOC6F4)aC—O ] (cf. pp. 395 and 422), and the method may be of more general applicability, The transient p-tetrafiuoro-benzosemiquinone radical produced by photolysis of fluoranil in dioxan shows aw values of 3.8 (3-position with respect to OH) and 14.1 G (2-position) and decays by disproportionation with an activation energy of ca. 10.0 1.7 kJ mol"M The e.p.r. spectra of the photoexcited triplet states of tetraphenylporphine and tetrakis-... 

The dependence on for the phenylthiyl radicals is about one half that of the phenoxyl radicals. As in the case of the BDEs, discussed above, the reason is that in radicals of the former kind the interaction with the n aromatic system is much less, and the unpaired electron is more localized on the sulfur than it is on the oxygen of phenoxyl radicals [37, 57]. The smaller interaction in the phenylthiyl radicals also affects the pA gS of the benzosemiquinone radicals, P-HX-C6H4-Y , where X and Y can be O or S atoms [37]. Note that the tendency for less interaction with n aromatic systems is also observed for Se and Te [85]. In fact, in general the extent of interaction decreases in the series O > S > Se > Te. 

Computed and experimental excitation energies (eV) and oscillator strengths of the p-benzosemiquinone radical anion... 

Pou-Am rigo, R., Serrano-Andres, L., Merchan, M., Orti, E., Forsberg, N. (2000). A theoretical determination of the low-lying electronic states of the p-benzosemiquinone radical anion. Journal of the American Chemical Society, 122, 6067-6077. 

The monoprotonated tetrafluoro-p-benzosemiquinone radical has been prepared from the parent quinone by photochemically-induced hydrogen-atom abstraction in a variety of solvents. The radical dissocates readily in hydroxylic solvents and in some cases rapid proton-transfer equilibria have been observed which give time-averaged e.s.r. spectra. ... 

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