Radical pair reactions

Fig. 14.13 The oscillating magnetic field effect (OMFE) in the triplet state radical pair reaction.

Porter, N. A., Stereochemical Aspects of Radical Pair Reactions, 18, 97. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

Radical Pair Reactions, Stereochemical Aspects of (Porter and Krebs). 18 97... 

PET reaction of carbonyl compounds with olefins form either oxetanes (Paterno-Buchi reaction, Eq. 31) by direct coupling or a radical pair reaction leading to coupling product or reduction. The carbonyl-olefin radical pairs are formed by proton transfer within their radical ion pairs (Eq.32). Both these aspects of ketone-olefin photoreaction have been recently rationalized by Mattay et al. [167] from the photoreactions of 2,3-butanedione (208) with different olefins such as 209 and 210 as shown in Scheme 39. Photoprocesses of... 

There is a related effect seen in the EPR spectra of products arising from radical pair reactions. This effect is known as chemically induced dynamic electron polarization, or... 

Radical Pair Reactions in Micellar Solution in the Presence and Absence of Magnetic Fields... 

In this case, the micelle acts as a reaction vessel with molecular dimensions. Below we will describe some of our results on the effect of micelllzatlon on radical pair reactions. We will show as well that the effects of micellization can be dramatically altered by the application of small external magnetic fields. 

THE MAGNETIC FIELD DEPENDENCE OF RADICAL PAIR REACTIONS... 

The reactions analyzed here (and used to generate the initial radical pairs in the polymer media) can be separated into two distinct categories those that involve lysis of one molecule into two radicals (such as the Norrish Type 1, photo-Fries, and photo-Claisen reactions) and those that require bimolecular processes in which a part of one molecule is abstracted by another (e.g., H-atom abstractions from a phenol or an amine by the lowest energy triplet state of benzophenone). Each reaction produces either singlet or triplet radical pairs and, thus, allows the influence of spin multiplicity on radical pair reaction rates to be separated somewhat from other influences, such as the natures of the polymer matrices and the radical structures. Different methods for extracting rates of processes for the radicals from both static and dynamic data will be discussed. ... 

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

In a very interesting report of Molle and Bauer [7] using adamantyl halides and the hindered ketones hexamethylacetone and adamantanone, they observed no alcohol products when the hindered ketones were mixed with preformed adamantyl lithium at low temperature ( —20 C) in diethyl ether. However, when the ketones and halide were mixed with lithium powder under the same low-temperature conditions, substantial yields of the expected carbinols were isolated. They postulated a surface-mediated radical ion or radical pair reaction with the ketone or the ketyl radical. The reaction of a ketyl with the halide was ruled out because earlier researchers [4] had shown that no carbinol was obtained from the reaction of an alkyl halide and benzophenone ketyl. The authors also did not find any carbinol product when the ketyl of adamantanone was reacted with adamantyl halide. By extending their studies to other homologues of adamantane, the authors concluded that the stability of the cagelike radical species determined the extent to which the radical pathway is favored over the organometallic pathway. 

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