Radical diffusion

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

Evidence in support of radical intermediates with MMO from both M. capsulatus (Bath) and M. trichosporium 0B3b was reported from experiments in which substrate radicals were trapped during turnover (89, 90). The amount of trapped radical, however, was not quantified in these experiments. In other reports, no diffusable radical species were detected in reactions with MMO from M. trichosporium 0B3b (61). 

In dichloromethane, the acidic ESE cation radical undergoes a rapid proton transfer (k = 1.9 x 109 s ) to the CA anion radical within the contact ion pair (CIP) to generate the uncharged radical pair (siloxycyclohexenyl radical and hydrochloranil radical) in Scheme 6. Based on the quantum yields of hydro-chloranil radical (HCA ), we conclude that the oxidative elimination occurs by geminate combination of the radical pair within the cage as well as by diffusive separation and combination of the freely diffusing radicals to yield enone and hydrochloranil trimethylsilyl ether, as summarized in Scheme 6. 

Walter, R. I. Absence of Detectable Freely Diffusing Radicals during the Formation of an Aromatic Grignard Reagent. J. Org. Chem. 2000, 65, 5014-5015. 

It is helpful to choose conditions under which the ionic reactions will predominate In the solid phase at low temperature secondary processes of decomposition and freely diffusing radical interactions are expected to be greatly inhibited. The solid lattice will tend to prevent fragmentation and to promote rapid energy dissipation so that the formation of radicals with the possible exception of hydrogen atoms will be largely prevented. Furthermore, alkyl radical reactions are restricted to radical pairs that are formed adjacent to one another... 

Assume that the lifetime of H20 is longer than the lifetime of the radicals which disappear in intraspur reactions. Then the rate-determining step for inhibition of intraspur H20 formation would be reaction of solute with H20. Let us assume that the rate constant r,r for intraspur combination of freely diffusing radicals is about 1010 sec. 1, and that the lifetime of H20 is about 4 X 10 10 sec. Then the concentration of radicals during intraspur formation of H2 must be greater than 0.25Af which is not unreasonable. 

It is noteworthy that some product yields do not change very much whether the solutions were saturated with air or with N2O/O2, despite the fact that the OH yield is halved in air-saturated solutions (cases in point are for ssDNA and in the units of Table 12.5 Cg (37.9), Tg (43.4), 8-oxo-G (62.0) for further values see Fuciarelli et al. 1990). Whether this means that (V- that is an abundant and freely diffusing radical under such conditions plays an important role in the... 

J. F. Garst, Grignard Reagent Formation and Freely Diffusing Radical Intermediates, Acc. Chem. Res. 1991, 24, 95-97. 

The diffusion coefficients associated with translational motions when the radii of the diffusing radicals are not much larger than that of the solvent are expressed more accurately by D = kTI6nrr T (where r is the radius of the diffusing radical assuming a spherical shape and r (=yxr ) is the microviscosity. The value of /, the microfriction factor, can be calculated or taken equal to DsE/f exptb the ratio between the Stokes-Einstein diffusion coefficient (that considers van der Waals volumes, but not interstitial volumes) and the experimentally measured diffusion coefficient, Dexpti- As will be discussed later, these relationships appear to hold even in some polymer matrices. 

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

Some peroxyl radicals release HO /O spontaneously and/or base-induced [OH and/or buffers cf. reactions (41), (45) and (47)], and nearly all peroxyl radicals give rise in one of their bimole-cular routes (the oxyl radical pathway) to radicals that are capable of undergoing such reactions. For DNA free-radical chemistry, the formation of HO /O/ is an interesting reaction insofar as 0 that dominates at neutral pH [pJreact with the much less mobile DNA radicals. 0 is a rather long-lived radical, as in a cellular environment there are only few sinks... 

The nature of the equilibrium between the dormant system 41 and the pair of radicals 37 and 40 has been probed and exploited by a number of groups. The exact nature of the radical pair, caged pair of radicals, or freely diffusing radicals was probed by a series of crossover experiments.94 This is an important synthetic issue the nitroxide counter-radicals are associated with the same polymeric chain end during the course of the polymerization, or do they diffuse freely to the reaction medium, which affects the ability to insert functional groups at the chain ends. In these experiments, the potential diffusion of the mediating radical from the propagating chain end... 

Figure 4. Photooxidation of unstabilized PE effect of bi(macro)-molecular diffusion (radical termination) on time to failure, e.g.,...

Leemans L, Jerome R, Teyssie Ph. Diffusive radical entry as the rate-determining step in amphiphilic block polyelectrolyte mediated emulsion polymerization. Macromolecules. 1998 31 5565—5571. 

Concerning the other interactions listed in Section II.B.3, we finally note that spin-orbit coupling plays no role for intersystem crossing of freely diffusing radical pairs, because its efficiency is negligible for most of the life of the pair, that is, unless the two radicals are very near one another [26], In the case of biradicals, however, this mechanism is important and competes with intersystem crossing by the radical pair mechanism [23,27]. 

Fig. 7.5. CIDNP in CH protons of CH3CH Mgl from the reaction of ethyl iodide with Mg in di- -buiyl ether [46]. Top normal spectrum. Bottom CIDNP spectrum (E/4 multiplet effect). This is the expected spectrum for CIDNP in RMgl formed in step r from radicals R that have escaped from pairs R R formed when independently diffusing radicals meet [47]. There were no indications of net effects, which w ouid have been expected if diffusing radical pairs 1R Mg ] were significant generators of CIDNP [47]. Reprinted from Bodewit/. ct aL 46j. Copyright 1972. Page No. 283. with permission from Elsevier Science.

Transient absorption spectra of some "satellite ions" closely resemble the spectra of olefin radical cations. In cyclohexane, a band centered at 270 nm (at 2 ns [22]) is observed from 250 ps [25] after the ionization event (this band overlaps with the strong 240 nm band of cyclohexyl radicals [22]). The scavenging behavior and the decay kinetics of the UV-absorbing species suggest that they are normally-diffusing radical cations [25]. In the first few nanoseconds after the ionization event, the VIS absorbance is dominated by solvent excited states [22,57]. When the thermalized electrons are rapidly scavenged using a suitable electron acceptor (halocarbons or N2O), this... 

Ingold, K. U. MacFaul, P. A. Distinguishing Biomimetic Oxidations from Oxidations Mediated by Freely Diffusing Radicals. In Biomimetic Oxidations Catalyzed by Transition Metal Complexes, Meunier, B., Ed., Imperial College Press London, 2000 pp 45-89. 

It is difficult for even a freely diffusing radical to escape reaction at a sufficiently reactive surface when it is generated near that surface. 

The additional source of free radicals from irradiation environment participates to the increase in the insoluble fraction that is formed by the diffusion of reactive molecules [94A1, 05Z2, 09Y1]. The reaction of diffused radicals through irradiated polymer interferes with the own crosslinking leading to the increase in gel amount (Table 3). 

Garst JF (1991) Grignard reagent formation and freely diffusing radical intermediates. Acc Chem Res 24 95-97... 

Photoionization. - 2.1.1 Sulfite anion. The photoionization of the sulfite anion SOf was studied by Fessenden et a few years ago. The steady-state EPR spectra of the hydrated electron and the sulfite radical-anion SOs show no CIDEP effects and indicate the lifetime of the hydrated electron to be about 100 ps. The sulfite radical-anion is often used as g factor standard in photolysis EPR experiments [ [(SOs") = 2.00316]. Bussandri et al. studied the laser photolysis of sulfite ions in basic solution by FT EPR with very high time resolution. They observed the FT EPR spectra of hydrated electrons and sulfite radical-anions with absorption/emission (A/E) pattern caused by the radical pair mechanism (RPM CIDEP) with the electron line in emission [fif(eaq ) = 2.00044 at room temperature] and the sulfite radical-anion in absorption. In the time profiles of both lines, oscillations of the EPR intensities were observed in the first 300 ns. This coherent oscillation in both radicals is the first direct EPR observation of zero quantum coherence in freely diffusing radicals. Previously zero... 

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