Biradicals polarization

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

Closs G L and Forbes M D E 1991 EPR spectroscopy of electron spin polarized biradicals in liquid solutions. Technique, spectral simulation, scope and limitations J. Phys. Chem. 95 1924-33... 

Forbes M D E, Avdievich N I, Schulz G R and Ball J D 1996 Chain dynamics cause the disappearance of spin-correlated radical pair polarization in flexible biradicals J. Phys. Chem. 100 13 887-91... 

We have also examined the behavior of copolymers of o-tolyl vinyl ketone and methyl vinyl ketone (CoMT). In this case the light is absorbed exclusively at the aromatic carbonyl chromophore and the reaction proceeds from this site, while the methyl vinyl ketone moieties provide a relatively constant environment but prevent energy migration along the chain. The values of Tg and Tip in benzene have been included in Table II. These copolymers axe also soluble in some polar solvents for example, we have used a mixture of acetonitrile acetone methanol (30 30 Uo, referred to as AAM). This mixture is also a good solvent for the electron acceptor paraquat (PQ++) which has been shown to be good biradical trap in a number of other systems (9.). 

Complicating aspects of the reactions include the solvent dependence of the dimerization of cyclopentenone, the head-to-head ratio increasing with polarity of solvent ".ioo)( and the plethora of products from the reaction cyclohexenone-isobutylene, where several olefinic products are not shown in Eq. 30. 97> The complicated features of these reactions are so well-described in a recent review article 94> that no furhter outline will be provided here. Biradical mechanisms can account for a great... 

The cyclic enediynyl sulfide 93 is also prone to undergo prototropic rearrangement (Scheme 20.21) [57]. When the l,8-diazabicydo[5.4.0]undec-7-ene (DBU)-induced isomerization was conducted in carbon tetrachloride, three cycloaromatized products, 96 to 98, were isolated, indicating the formation of the biradical 95a as a transient intermediate. In a polar solvent, such as methanol or ethanol, the formation of 99 can best be accounted for by regarding the biradical 95a as the zwitterion ion 95b. A related process involving the oxidation of 93 with selenium dioxide has also been reported [58],... 

Scheme 60). Griesbeck et al. assume that in a non-polar solvent such as benzene the intramolecular electron transfer from the methionic sulfur group is much faster than the abstraction of hydrogen from the hydroxyl group of the unprotected amino acid. C-Hydrogen abstraction leads to 313, whereas previous lactonization of the zwitterionic biradical 311 yields 314. Since the cis-hydroxy acid is not detected it is conceivable that it cyclizes immediately to the lactone 314. Photolysis of the corresponding methyl ester under the same conditions attains improved yields (84% combined) of two diastereomeric tricyclic products in a ratio of 48 52. 

Thermal rearrangement of 0-acyl Af-hydroxycarbamates carrying a cyclopropane substituent was reported (equation 255). When subject to flash vacuum thermolysis at 500 °C the carbamate 573 generates the Af-acyl imine 574 that rearranges to pyrroline 577 in 21-37% yield. The formation of a biradical intermediate 575 or a polar zwitterionic structure 576 was proposed. 

In solution, the triplet biradical 14b dimerizes, and the dimeric products are formed with strong chemically induced nuclear polarization. The absolute rate of the dimerization at 146 K, as monitored in viscous solution by ESR spectroscopy, is just about that predicted by the spin-corrected encounter frequency under those conditions. The cycloaddition of the triplet with a typical alkene, acrylonitrile, also can be followed in this way. 

In view of the present calculated results, the SET mechanism would be described as follows. Basically, the polar four-center reaction in Scheme 14 leads to C—C bond formation. However, when the alkyl group is bulky, only the two-center (Mg—O) reaction takes place. The aUcyl-Mg bond is cleaved homolytically owing to the persistent Mg tetravalency and the stability of the resultant radical species. Hence, biradical intermediates are formed not by a single electron transfer but by the C—Mg homolytic scission. 

When polymerizable vinyl compounds are added to this system, radical polymerization is induced by intermediate radicals instead of producing the 1 1 adduct. This mechanism indicates that the CT interaction does not always produce a polarized or ionic intermediate but also facilitates the formation of a biradical or triplet state. 

Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-1,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish II intermediates in a zeolite site with at least one metal ion is expected to be closer to 100 ns than to 1 ns. 

The lifetime of BR generated from 97 (n = 19) was found to be 64 5 and 70+ 5 ns in the isotropic and hexatic B phases of BS [319], The lack of influence on the lifetime of the biradical by the hexatic phase when the E/C ratios are clearly affected is at first puzzling. However, it can be cited as evidence that the T - S rate is independent of the conformation in which a BR is held [263]. Note that the BR from 97 (n = 21) as shown in Figure 60 has its hydroxyl group far removed from the cross-sectional segment of the BS-provided reaction cavity cylinder which is quite polar. In any case, the long lifetime of the BR found in hexatic BS and its near equivalence to that in the isotropic phase indicate that the various biradical conformers have equilibrated in the cylindrical reaction cavity prior to collapsing to products. 

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