Triplet biradical

ESR Spectra of Biradicals, Triplet States, and other S>lf2 Systems... 

ESR Spectra of Biradicals, Triplet States, and other S>l/2 Systems At constant frequency, v0 = AE/h, the resonant fields are ... 

Fig. 4 Esr spectra of high-spin trianthrylene [21a] at 120 K in MTHF/K A, biradical triplet state [21a]2- (labelled O) B, triradical quartet state [21a]3- (labelled ) C, biradical triplet state [21a]4- (labelled ).

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

However, the more sterically constrained bisphenol 8 is oxidized to a product that apparently is an equilibrium mixture of a fully bonded singlet 9 and a biradical triplet species 10.88... 

Benzophenone (Amax = 340 nm, log e = 2.5, n-ir electronic transition) can be used as a photochemical reagent and eq. 4.25 shows a radical Michael-addition reaction with benzophenone. The formed benzophenone biradical (triplet state, Tx) abstracts an electron-rich a-hydrogen atom from methyl 3-hydroxypropanoate (62) to generate an electron-rich a-hydroxy carbon-centered radical [III], then its radical adds to the electron-deficient (3-carbon of a, (3-unsaturated cyclic ketone (63) through the radical Michael addition. The electrophilic oxygen-centered radical in the benzophenone biradical abstracts an electron-rich hydrogen atom from methyl 3-hydroxypropanoate (62) [70]. So, an a-hydroxy carbon-centered radical [III] is formed, and an electron-deficient a-methoxycarbonyl carbon-centered radical [III7] is not formed. 

Electron paramagnetic resonance (EPR) is a spectroscopic technique detecting chemical species that have unpaired electrons. A great number of materials contain such paramagnetic entities, which may occur either as electrons in unfilled conduction bands, electrons trapped in radiation damaged sites, or as free radicals, various transition ions, biradicals, triplet states, impurities in semiconductors, as well as other types. Solids, liquids and gases are all accessible to EPR. 

Thermal decomposition or direct irradiation yields the singlet biradicals whereas use of sensitizer normally produces a triplet biradical. Triplet biradicals have a longer lifetime, and therefore more chance of bond rotation and loss of stereochemistry, because they must undergo spin inversion before the ring can close (Scheme 4.67). 

A third primary process was also observed in ketone photoly b (61, 69) this results in the formation of a cyclobutanol derivative. This reaction, however, was recognized in only a few of the ketones studied. The last primary reaction to be considered is the excitation of the carbonyl group to the biradical triplet state (5) and subsequent intermoelcular hydrogen abstraction from the substrate RH with formation of free radicals... 

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