Geminate radical pair

CIDNP involves the observation of diamagnetic products fonned from chemical reactions which have radical intemiediates. We first define the geminate radical pair (RP) as the two molecules which are bom in a radical reaction with a well defined phase relation (singlet or triplet) between their spins. Because the spin physics of the radical pair are a fiindamental part of any description of the origins of CIDNP, it is instmctive to begin with a discussion of the radical-pair spin Hamiltonian. The Hamiltonian can be used in conjunction with an appropriate basis set to obtain the energetics and populations of the RP spin states. A suitable Hamiltonian for a radical pair consisting of radicals 1 and 2 is shown in equation (B1.16.1) below [12]. 

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... 

There is an additional problem that must be considered for radicals that escape an original geminate radical pair. Such radicals are polarized. If they form new (encounter) radical pairs, they could yield combination products after undergoing T-S mixing again. However, because of the rapidity of nuclear relaxation in free radicals, the F-type polarization generally predominates. 

Consider a geminate radical pair, its component radicals in close proximity and therefore having correlated electron spins reflecting the electronic spin state of the precursor. Three different situations can be envisaged. 

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. 

Primary geminate Secondary geminate radical pair radical pair... 

The term geminate radical pair is derived from the word gemini = twins, the two geminate radicals being formed together at the same time. 

Selective combination of the secondary geminate radical pairs occurs in the micelle, compared to nonselective free-radical combination reactions in solution. This results from the micelle host effectively constraining the separation of the geminate radical pair. 

Photochemical excitation results in a-cleavage to produce a primary, geminate radical pair, which may undergo radical combination reactions (1) in competition with decarbonylation or (2) to produce a secondary geminate radical pair. The latter may undergo radical combination (3) or produce a free-radical pair (4). The free radicals undergo radical combination reactions (5). 

Regioselectivity is the preferential formation of one product over all other possibilities. The zeolite host effectively constrains separation of the secondary geminate radical pair A and B . 

The photochemical dissociation of a molecule AB often leads to the formation of a pair of radicals A 4- B, e.g. as in Figure 4.33. If the reaction takes place from the lowest triplet excited state of AB, the radicals will have parallel spins and cannot recombine unless a spin flip takes place to bring them to the singlet state of the geminate radical pair. 

The triplet state of the geminate radical pair has three distinct levels (aa, /3/3, a/3 4- /3a) which are isoenergetic in the absence of any magnetic field. They interconvert very rapidly with the isoenergetic singlet state, so that the spin flip is fast and recombination is efficient. 

We suggest that the cyclodextrin cavity provides an environment wherein recombination of the geminate radical pair (from Type I) is favored and this results in lower yield of products from Type I process relative to Type II. Experiments are underway to test this cage effect with other examples. 

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 key observation in the case of 152 is that photolysis in benzene conforms to the expected a-cleavage and decarbonylation reactions to form diphenylmethyl (A") and benzyl radicals (B"), which are free to diffuse apart. The statistical combination of free radicals A" and B gives a 1 2 1 mixture of products 154,155, and 156. In contrast, photochemical excitation in the crystalline phase led to the exclusive formation of 153 by combination of the geminate radical pair A B with a 100% cage effect. 

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

Thermal decomposition of cis-27 leads then to the geminate radical pair 28 giving the products a-methylstyrene 29, benzene and 30. The main products after isolation are biphenyl (20%) and dicumyl (48%). This study shows that the decomposition of azo compounds proceeds via a diazenyl radical in the singlet reaction (see also 40>). No such studies are available at the present time for the triplet. 

Radical pair Two radicals in close proximity, usually within a solvent cage or at least sufficiently close to allow spin correlation. The radicals may be formed simultaneously by some unimolecular process, e.g., photochemical bond breaking, or they may have come together by diffusion. A radical pair is called geminate radical pair provided that each radical partner is a descendant of the same parental 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. 

With the diffusion model it became apparent that polarization is influenced by the probability of the two radicals being able to reencounter after diffusing apart. In an independent approach to calculate this probability, Adrian (4-7) arrived at the same formulation as developed by Noyes (103) many years ago. The final estimated expectation value of the electron polarization in a geminate radical pair is... 

Of particular relevance to the present discussion is the observation that the CSS, which is a biradical cation, is formed with essentially pure triplet spin correlation. For energetic reasons, this triplet radical pair cannot recombine to form the MLCT state and can only form the singlet ground state. Therefore, direct recombination is spin forbidden. Moreover, because the radical pair which constitute the CSS product can separate only to a limited distance, essentially every CSS recombination event is between the same geminate radical pair—in other words, every reduced acceptor is ultimately oxidized by the donor radical cation that was formed from the same initial photochemical event. The spin behavior of the DC A triad CSS can be effectively explained by application of the relaxation mechanism of Hayashi and Nagakura. ... 

A detailed description of bulk polymers as hosts for geminal radical pairs and their precursors is also beyond the scope of this chapter. For general sources of information about photochemical and photophysical processes in bulk polymers, we recommend the classic book by Guillet as well as the book edited by Winnik, the journal Polymer Degradation and Stability (incorporating the defunct journal. 

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