Radical pair dynamics

The excitation of these complexes generates intimate ion pairs which are well suited for the quantitative study of ion-pair and radical-pair dynamics. The behaviour of the ion pairs generated by this method parallels the behaviour of ion pairs generated by usual solvolytic reactions215. 

Combining spin dynamics and radical pair dynamics, 84... 

The radical pair mechanism thus links spin dynamics (the evolution of the spin state of A B , hence of spin correlation of the pair, under the influence of magnetic interactions) and radical pair dynamics (the diffusion, relative to one another, of the two radicals, and the coupling to chemical processes). Spin dynamics will be treated in Sections II.B.3 and II.B.4, and then combined with radical pair dynamics in Section II.B.5. Although most of the discussion is centered around radical pairs, and the described mechanism of intersystem crossing is generally called radical pair mechanism, much of the following applies also to biradicals, which can be regarded as a special case of radical pairs, namely, pairs with restricted interdiffusion. 

Combining Spin Dynamics and Radical Pair Dynamics. In a very general form, spin dynamics and radical pair dynamics can be treated simultaneously with the so-called stochastic Liouville equation [10b],... 

The spin-dependent decay of P+H" is complex. It comprises two decay pathways as shown in the kinetics scheme of Fig.3. We will briefly explain how spin-dependent recombination dynamics are responsible for inherent deviations from monoexponentiality in Fig.2 and can fully account for them, as shown by rigorous simulations. On the basis of a brief summary of radical pair dynamics, the effect will be outlined in the following section. 

RADICAL PAIR DYNAMICS IN THE BACTERIAL PHOTOSYNTHETIC REACTION CENTER... 

In general, one has to treat the interactions between the vibronic states of P+BH", and the manifold of multimode vibrational states corresponding to each of the electronic singlet (ip ) and triplet ( P ) states. At low temperatures only the low vibrational states of P+BH are populated and the radical pair dynamics correspond only to these states. Accordingly, we shall focus on the exchange integral for the ground vibrational state of P+BH . 

Avdievich N I and Forbes M D E 1995 Dynamic effects in spin-correlated radical pair theory J modulation and a new look at the phenomenon of alternating line widths in the EPR spectra of flexible biradicals J. Phys. Chem. 99 9660-7... 

Forbes M D E, Schulz G R and Avdievich N I 1996 Unusual dynamics of micellized radical pairs generated from photochemically active amphiphiles J. Am. Chem. Soc. 118 10 652-3... 

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... 

Thurnauer M C and Norris J R 1980 An electron spin echo phase shift observed in photosynthetic algae. Possible evidence for dynamic radical pair interactions Chem. Phys. Lett. 76 557-61... 

C. The Dynamic Behaviour of Badical Pairs A simple model (Fig. 4) will assist in visualizing the time-dependent variation of the separation and hence the interaction of the components of a radical pair. 

The effect has been most commonly encountered in the decomposition of symmetrical diacyl peroxides where it is easily recognized since the symmetrical radical dimer, for which Ag must be zero, is formed and shows net polarization. Clearly, studies of such systems are capable of providing valuable information on the dynamics of radicals and radical pairs in solution, the polarization process providing a time base for events (see Section V,B). 

Magnetic Field Effects on the Dynamics of the Radical Pair in a Cgo Clusters-Phenothiazine System... 

MFEs on the dynamics of the radical pair in CtoN" clusters (C oN " ) -MePH system were examined in TH F-H2O (2 1) mixed solvent. M FEs on the decay profiles of the transient absorption at 5 20 nm due to the phenothiazine cation radical (P H " ) are shown in Eigure 15.9b. The decay was retarded in the presence of the magnetic field. In addition, the absorbance at 10 (is after laser excitation increased with increasing magnetic field. The result indicated that the yield of the escaped PH increased with the increase in magnetic field. Therefore, the MFEs on the decay profile were clearly observed. 

Yonemura, H., Kuroda, N., Moribe, S. and Yamada, S. (2006) Photoindiced electron-transfer and magnetic field effects on the dynamics of the radical pair in a Cgo clusters-phenofhiazine system. C. R, Chim., 9, 254-260. 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

The critical role of the ion-radical pair in the cycloaddition reactions in equation (75) is demonstrated by a careful measurement of the quantum yields as a function of the dienophile concentration and by a study of the effect of solvent and salt on the dynamics of the ion pair ANT+ , MA-. 212 However, in the reported cases, back electron transfer effectively competes with the coupling within the ion-radical pair and thus limits the quantum yields for the formation of the Diels-Alder adduct.212... 

The explanation for the dichotomy between aromatic nitration versus de-alkylative oxidation in equation (84) lies in the dynamics of ion-radical triad (which is predictably modulated by solvent polarity and added inert salt). For example, the nonpolar dichloromethane favors aromatic nitration via a radical-pair collapse of ArH+, NO, 239 i.e.,... 

Dynamics of the ion-radical pair D+, A- are directly observable by time-resolved (fs, ps) spectroscopy during charge-transfer photoactivation. 

The chemically induced dynamic nuclear polarization (DNP) opened perspective to study products formed from free radicals [102], The basis of this study is the difference in NMR spectra of normal molecules and those formed from free radicals and radical pairs. The molecules formed from radicals have an abnormal NMR spectrum with lines of emission and abnormal absorption [102]. DNP spectra help to obtain the following mechanistic information ... 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

The total quantum yield [4>cs(total)] for CS is decreased to 0.17 in dimethyl-formamide (DMF) due to the competition of the CSH from Fc-ZnP-H2F+-C6o (1.63 eV) to Fc-ZnP- -HzP-Cso (1.34 eV) versus the decay of Fc-ZnP-Fl2P -C6o to the triplet states of the freebase porphyrin (1.40 eV) and the Ceo (1.50 eV) [47]. In contrast to the case of most donor-acceptor-linked systems, the decay dynamics of the charge-separated radical pair (Fc -ZnP-H2P-C6o ) does not obey first-order kinetics, but, instead, obeys second-order kinetics [47]. This indicates that the mframolecular electron transfer in Fc -ZnP-H2P-C6o" is too slow to compete with the diffusion-limited inter-molecular electron transfer in solution. 

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