Spin-selective radical reactions

Spin-selective Radical Reactions. The CIDNP effect arises from magnetic interactions in pairs of free radicals. 

CIDNP in RMgX further demonstrates that it is formed from an R- intermediate (I igure 7.5) 112.46-51], CIDNP results from nuclear-spin selection in reactions of radical pairs 159.60]. 

Net F-polarization (A at 56-4 MHz) has been observed during the reaction of p-fluorobenzyl chloride with sodium naphthalene in tetra-hydrofuran solution to give p, p -difluorobibenzyl (Rakshys, 1971). The spin selection is believed to take place within a radical pair of the -------------------------------F... 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

It is now well established that both CIDEP and CIDNP have their origins in the formation and removal reactions of free radicals. As a result of this, it is now possible to gain information, not normally obtained from magnetic resonance studies, for those photochemical reactions which show CIDEP and CIDNP. An example of this is those photochemical reactions in which the primary radicals react immediately to regenerate the starting compounds. The regenerated compounds may show CIDNP, and this is often the only evidence that this reaction has occurred. In the radical-pair mechanism, spin polarization is caused by the spin-selective reaction. While it is generally not possible to monitor by esr the selective reactivity of the radical pairs as a function of their nuclear spin states, CIDNP has proved to be a valuable tool to probe the small difference in reactivity of the nuclear spin states of the radical pairs. 

Another burst of activity in free radical research occurred in the 1960s and 1970s, after several reports of anomalous intensities in the EPR spectra of photochemically or radiolytically produced radicals, and in the NMR spectra of the products from free radical reactions in solution." " These so-called chemically induced magnetic spin polarization (CIDNP and CIDEP) phenomena provided a wealth of mechanistic, kinetic, dynamic, and structural information and were a cornerstone of carbon-centered free radical research for the better part of three decades. The umbrella term for this area of research is spin chemistry, which is defined as the chemistry of spin-selective processes. 

Here, RH is the organic substrate, R- is the alkyl radical which is produced by °Co irradiation, and R02- is the peroxy radical. The chain termination reaction includes the recombination of peroxy radicals which is expected to be spin selective and, therefore, an isotope-sorting reaction ... 

Electron-spin selective reactions of the radical pairs... 

Spin Selectivity. As a consequence of the preceding, reaction rates depend on the electron spin multiplicity. In many cases, the difference is so large that it practically amounts to a yes-no decision. As a typical example, consider an encounter complex, at the reaction distance, of two radicals, which can have either triplet or singlet multiplicity. In most cases, reaction will be impossible for the triplet complex because it would lead to an electronically excited product. For reaction of the singlet complex, there is no such restriction. 

Electron spin selective reactions of the radical pairs or biradicals (see Section II.A, item 2). 

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

F and F represent the flavin in the excited singlet and triplet state respectively. Nuclear spin polarization a-rises from the spin selective recondalnation of the radical pair (reaction 3) (14). In this way a strong emission effect has been observed for the 3,5 ring protons (ortho with respect to the hydroxyl group) of N-acetyl tyrosine and a weak enhanced absorption for the 2,6 protons and the 3 CH, protons (for a more extensive description of the ziethod see the contribution of Dr. R. Kapteln to this volume). 

Photo-induced Electron Transfer. Electron transfer is one of the most fundamental and widespread reactions in nature and has been extensively studied. In addition to the optical absorption spectroscopy widely used, TR EPR has become established as an appropriate method to study electron-transfer processes. In most of these investigations CIDEP effects are observed. The spin-polarization effects originate in the spin selectivity of chemical and physical processes involved in free-radical formation and decay, as well as in the spin-state evolution in transient paramagnetic precursors. For this reason, CIDEP constitutes a unique probe of the mechanistic details of electron-transfer processes. 

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