Lifetime biradical

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

A study of the lifetimes of the triplet biradicals D, E, and F, which were generated from the corresponding azo compounds, found that lifetime decreased in the order... 

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

Thus the triplet states of the two diastereomers react to yield different product distributions although this effect is far less marked for the triplet than for the singlet reaction, which is essentially stereospecific. The singlet reaction could be either concerted or due to an extremely shortlived biradical. Since the product distributions of the triple reaction of these two diastereomers are different, it is clear that cleavage must occur before complete equilibration. Thus the lifetime of the aliphatic ketone derived biradical must be considerably shorter than the corresponding biradical derived from an aryl ketone. 

The measurement of Tg is straightforward when Trp<triplet lifetime can be shortened by addition of triplet quenchers, and the values of Tg for PPVK and CoPT(l) have been obtained using this approach (6). Figure 2 shows a typical trace corresponding to the decay of the biradical from PTVK, as monitored at l 15nm. The triplet state is in this case too short lived to be detectable the residual absorbance observed after decay of the biradical is due to the enol. 

Two types of biradicals can be produced in the copolymers, depending on whether the reaction occurs at a PVK or a TVK site. The change is reflected in the quantum yields, as well as in the lifetime for both, the triplet and the biradical, see Table II. 

If the alkene can exist as cis and trans isomers then we need to be aware of the stereospecificity of the reaction. If the reaction involves an excited triplet state then the biradical formed will be able to undergo bond rotation in the lifetime of the excited state. The reaction is, therefore, nonstereospecific, forming a mixture of oxetane isomers from either alkene isomer ... 

This photoreaction has been investigated by laser flash photolysis58 and quantum yield measurements that identify the triplet state (r = 6 nanoseconds) as the reactive species, and show intermediate 82 is sensitive to hydroxylic molecules, but the logical precursor biradical intermediate 81 could not be detected owing to a short lifetime (equation 48). 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

Both cis- and frana-butene-2 are formed from each of the dimethyl-cyclobutanes. They are not however formed in equilibrium amounts. Further, more a -butene-2 than the equilibrium amoimt is formed in the decomposition of cis-l,2-dimethylcyclobutane. The fact that the cis- and cyclo-butanes, this does imply that either the lifetime of the free biradical is of the same magnitude as the time for one rotation of the groups in the biradical, or that the biradical is never strictly a free biradical . In either case the configmation of the reactant will, to some extent, determine the stereochemistry of the products. 

As shown by the scheme on p. 118, the triplet diazoalkene 44 can lose N2 to give the triplet carbene 45, which can now add to the olefin (b) in a non-stereo-specific manner. Since the triplet state of the diazoalkane 44 has a longer lifetime, it can also add to the olefin directly, producing the biradical 47 which after ex-... 

The photoreactivity of o-methyl acetophenone 11 has been studied exten-sively it is somewhat different from 1 because the singlet excited ketone (Sik) in 11 intersystem crosses to its triplet state in less than quantitative yields, as observed for 1 (Scheme 8). Thus, Sik in 11 decays by both intramolecular H-atom abstraction to form exclusively photoenol Z-13 and intersystem crossing to Tik of 11. Haag et al. estimated that Tik of 11 has a lifetime of 10 ns in benzene and decays by intramolecular H-atom abstraction to form biradical 12. The maximum... 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

Rather phase-insensitive Norrish II photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, a-cycloheptyl, a-cyclooctyl, and a-norbonyl groups [282], In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits -/-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. 

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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