Radical pair recombination

The study of the decomposition of optically active 1-methyl-2,2-diphenylcyclopropanoyl peroxide proved the retention (37%) of the product of the geminate radical pair recombination [90]. The radical center in the formed cyclopropyl radical is so strained that the racemization rate is unusually slow. 

The higher the viscosity of the solvent, the higher the amount of the parent molecules formed due to the geminate recombination of radicals. The observed rate constant of decomposition of the initiator decreases with an increase in viscosity [3,90], This was observed in the decomposition of peresters and diacetyl peroxide in various solutions. Subsequently, the fraction fT of the radical pairs recombining to the parent molecule increases with an increase in the viscosity ... 

The oriented elongation of the polymer increases the packing of macromolecules and decreases the molecular mobility in the polymer. This was observed by the EPR spectra of the nitroxyl radical in these films. Therefore, one can expect an increase in radical pair recombination in the cage with an increase in y. However, experiment showed an opposite pattern the more the y, the higher the e value. These results found explanation within the scope of the... 

This is the result of cage effect. The cage of a solid polymer matrix is rigid (see earlier) and the most part of the forming radical pairs recombine in the cage. Hence, the probability of... 

All these quantities are shown in figure 3.2, where Ais the the activation enthalpy for the cage radical pair recombination and Ais the activation enthalpy for diffusive cage escape. 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

Let us assume that fci is equal to k9, the rate constant for the gas phase decomposition (15), where no cage effect is expected. This assumption does not always hold (15, 18). For example, it is known (18) that di-f erf-butyl peroxide (DPB) decomposes about 30% slower in the gas phase than in solution. We can calculate from our value of k8 and the known value of kg, from the work of Szwarc (7, 21), a value for the fraction of acetoxy radical pairs recombining, fR, where... 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

There are several criticisms of the diffusion equation approach to radical pair recombination [5]. In particular, the treatment of the radical... 

The effect of a magnetic field on radical pair recombination... 

In Chap. 6, Sect. 4, the effect of a magnetic field on the probability and the rate of (neutral) radical radical-pair recombination was discussed. Much of this can be extended to the case of ion-pair recombination. There are three means by which spin correlation may be lost [403]. 

Supramolecular organic and inorganic photochemistry radical pair recombination in micelles, electron transfer on starburst dendrimers, and the use of DNA as a molecular wire [N. J. Turro, Pure Appl. Chem. 1995, 67(1), 199-208]. 

Careful study of (S)- (+) -2-phenylpropiophenone reveals that approximately half of the radical pairs recombine before diffusing out of the initial solvent cage 50>. This conclusion follows from the 44% quantum weld of scavengable benzoyl radicals and the 33% quantum yield for racemization. Alkyl thiols are excellent radical scavengers in carbonyl photochemistry because they quench triplet ketones fairly slowly 51>. Lewis has shown that concentrations of thiol above 0.03 M generally trap all free benzoyl radicals as benzaldehyde 50>. Of course, the minimum concentration for complete scavenging depends on conversion. 

Scheme 25 Absolute asymmetric photocyclization involving phenyl migration and radical pair recombination.

The photolyses of dlbenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by mlcelllzatlon and hyperflne induced intersystem crossing In the geminate radical pairs. This latter effect is the basis of Chemically Induced Dynamic Nuclear Polarization (CIDNP), a phenomenon which is well known in homogeneous solution. 

It is of interest to see how the information on the magnetic properties of the individual radicals relates to the spin-chemicaUy observed spin relaxation in the Ru-chromo-phore linked radical pairs. As detailed in Ref. 23, the relaxation rate constant defined in Scheme 10.1 can be extracted from the magnetic field dependence of the radical pair recombination kinetics. The pertinent data points for DCA-POZ and DCA-PSZ are shown in Fig. 10.5 together with the theoretical field dependence of and its various contributions. For DCA-PTZ, no significant deviation from the DCA-POZ case is observed. 

Fig. 3-3 shows how two radicals in radical pairs recombine with each other or separate from each other. Let ns consider the probability for recombination in the interval (t,... 

If the Stevens rearrangement is a concerted reaction, it is a symmetry-forbidden process based on the Woodward-Hoffmann rules. Indeed, it was shown to occur via an intramolecular hemolytic cleavage-radical pair recombination process, which explains the lack of crossover products and the observed retention of configuration at the migrating... 

The above discussions may be summarized as follows when the acceptor side of the reaction center is oxidized, i.e., it is in the [Pd] QA-state, light activation produces the P -state, i.e., the [P" -r]-Q -state, where I is the transient intermediary electron acceptor, namely a BO molecule. When the reaction center is pre-reduced to the [Pdj QA -state, light activation produces the P -state, i.e., the [ P-l]-Q -state with a lifetime of 10 ns. Most of the radical pairs recombine to reform the original [P-I] state, but some form the triplet state of P. In the initial excited singlet state [P 4 ], the spins on and r are antiparallel. During the 10-n lifetime of the excited singlet state, the spins of the unpaired electrons on the radical pair interact with nuclear spins on the two molecules, or with the electron spins on or the nonheme iron atom, but in any case there is a rephasing of these two unpaired spins. Recombination of these radical pairs, now with a predominantly triplet character, leads to the formation of the triplet state of P, i.e., theP -state [P -r] QA" [P -H-Qa" -> Pdl-QA. ... 

The difference spectrum for [Aq -Ao] shows a major bleaching at 690 nm plus a shoulder at 675 nm. Except for the 675-nm shoulder, the net spectrum obtained here also agrees with that obtained by Nuijs et using a 532-nm excitation pulse [cf. Fig. 6 (C)]. It also agrees with tbe in vitro spectrum for the Chl-a anion radical of Fujita et alf, except for a -25 nm red shift. Shuvalov et al thus concluded that the primary electron acceptor Ao in photosystem 1 is a Chi a-type molecule with a major absorption near 690 nm and has a forward electron-transfer time of 32 5 ps. In the pre-reduced state, the induced radical pair recombines in -55 ns, with partial conversion into triplet P700. 

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