Radical pair in homogeneous solution

MAGNETIC FIELD EFFECTS ON RADICAL PAIRS IN HOMOGENEOUS SOLUTION... 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

The initial rise of the rate was confirmed soon (e.g. Refs. [21, 69], whereas the left branch of the parabola could not be detected until recently (AG > —0.5 eV) In a series of communications [70 — 74], Gould, Farid, and co-workers account on their investigations of back electron transfer in laser flash generated radical ion pairs in homogeneous solution. 

Equations (2) and (3) represent two coupled, first order, linear, non-homogenous differential equations of the form dxidt + a(> x =. For radical pairs in liquid solution x and f(x) are... 

If an electron acceptor is available in homogeneous solution, photochemical reaction can be observed. For example, when 2 is excited (X > 350 nm) in anhydrous dimethylsulfoxide (DMSO), methylation occurs, ultimately giving rise to 9,9-dimethyl-fluorene in >80% yield. By analogy with Tolbert s mechanism for photomethylation in DMSO (4), such a process may be initiated by electron transfer to DMSO to form a caged radical-radical anion pair from which subsequent C-S cleavage occurs (eqn 4). 

Photolyses of 31-34 in homogeneous solution results in the formation of diphylethanes 39 (5-15%), phenols 38 (5-15%), ortho-hydroxyphenone 36 (40-60%), and para-hydroxyphenones 37 (20-25%). Small amounts of phenyl benzyl ether 35 (3-8%) were also detected. However, photolyses of all of the four esters on NaY zeolite and Nafion only produce ortho rearrangement products 36. Molecular models suggest that esters 31-34 can enter into NaY zeolite internal surface and the inverse micelle of Nafion. We believe that the preference for formation of ort/zo-hydroxyphenones 36 in the products is a consequence of the restriction on diffusional and rotational motion of the geminate radical pair. 

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. 

Unlike constrained RPs, those in homogeneous solution present a unique situation in that radicals that escape from their geminate cage to bulk solution have the possibility of encountering other radicals that have shared the same fate. Such an encounter leads to the formation of a new RP. Such an RP is formed through free diffusion and is referred to as a freely diffusing RP or f-pair. Until recently, only a single publication... 

Homolytic oc-cleavages (Type I reactions to produce a radical pair) involving the n, re states of ketones, have been studied extensively in homogeneous solutions 9c>, but only recently have ketones been employed to investigate this important class of photoreactions in micellar environments. For ease of discussion, we can classify reactions of this type in terms of the resulting products ... 

On the other hand, the Russian group carried out the following procedure (3) They added ACl to a frozen solution of n-butyl lithium the latter concentration is unclear, but it may be 2 or more mol dm. If the MFEs reported by the Russian group were true, they may be due to heterogeneous interfaces with frozen solution and/or deposited salt, where the radical pair lifetime becomes much longer than that in homogeneous solution. 

T and its saturated value is about -20 % even in a noteworthy that confinement of radical pairs in micells appearance of MFEs due to the d-type TM. fluid homogeneous solution. It is and biradicals is not necessary for the... 

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