Radical pair CIDNP explanation

This chapter is concerned with chemical reactions that occur while the system is still in the paramagnetic world. After an explanation of the radical pair mechanism and a brief treatment of experimental details, three case studies are presented that illustrate the application of CIDNP to transformations of radicals into other radicals and to interconversions of biradicals. 

This report outlines the development and present status of CIDNP. Sect. 2 gives a brief account of the experiments so far reported and the generalizations of reactions and effects. Theoretical formulations of the current radical pair explanation of CIDNP are presented in Sect. 3, in particular for the so-called high-field case. Sect. 4 applies this theory to the interpretation of CIDNP phenomena in several illustrative examples, and, finally. Sect. 5 is devoted to the discussion of a few pertinent questions. Most of the work described in the literature is mentioned. However, we are not aiming at a broad and complete survey of all aspects and prefer to emphazise the basic facts and applications. 

From the start, these phenomena were recognized as spin polarizations (deviations of the populations of the nuclear spin states from the Boltzmann distribution) caused by radical reactions. As the first attempts to understand their generation erroneously focussed on Overhauser effects, they were christened "chemically induced d)mamic nuclear polarizations". Although only partially correct, that name has stuck, possibly because its acronym CIDNP (usually pronounced "kidnap") evokes the picture of radical scavenging. However, only 2 years later the now universally accepted quite different explanation, the hitherto unknown radical-pair mechanism, was found, again by two groups independently." ... 

Figure 5. Explanation of an S- T0-type CIDNP net effect with vector models (left), resulting schematic population diagram (center), and NMR spectrum (right). The example describes a radical pair with one proton in radical 1, triplet precursor, product of the singlet exit channel, gq > g2, and positive hyperfine coupling constant. For the vector models, a clockwise sense of precession has been chosen, the labels 1 and 2 designate the radical and a> and /i) the nuclear spin state, and the dotted vertical lines in the projections give the amount of singlet character. For further details, see the text.

Figure 7. Plots of CIDNP net effects (Eq. 67) and multiplet effects (Eq. 68) in a radical pair with two protons as functions of the quantity /, / = 2AgPBJat, with the ratio of hyperfine coupling constants a2/al as parameter t of the curves. For a detailed explanation, see the text. (Left) net effect I,z (right) multiplet effect 2IlzI2.. Curves a and d, t, = 0.3 curves b and e, t = 1 -0 curves c and f, = 3.

The controversy over the mechanism of CIDEP in radical reactions extends also to CIDNP. An elaboration of a theory for CIDNP based upon the radical-pair approach has been offered recently,424 425 and a brief note has been published which questions the need for an alternative to the radical-pair approach to account for CIDNP, and further shows that the same assumptions are necessary in both the radical-pair and Overhauser mechanisms, namely, that nuclear relaxation can occur in the escaping radicals.426 427 It would seem that, whereas in CIDEP the experimental observations require some alternative to the radical-pair treatment for satisfactory explanation, so far, CIDNP observations can be explained on the basis of the radical-pair theory, although this does not preclude the possibility that other mechanisms are also operative. 

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