Applications of CIDNP

Potential applications of CIDNP thus span the whole breadth of chemistry from organic reactions to the physical chemistry of the liquid state. A progress report seems timely. Other, rather more limited reviews have appeared (Gloss, 1971b Fischer, 1971 a Buchachenko and Zhidomirov, 1971 Iwamura, 1971 Ward, 1972 Lawler, 1972). 

For the purpose of discussing the applications of CIDNP, the foregoing account of the origin of the phenomenon may be summarized as follows. The observation of CIDNP in a chemical reaction requires... 

The thermolysis and photolysis of azo-compounds (R.N N.R) are well documented sources of radicals (for a review, see Strausz et al., 1972). As such they have been extensively used as radical sources in CIDNP studies (Fischer and Bargon, 1969 Gloss and Trifunac, 1969, 1970b, c Iwamura and Iwamura, 1970 Iwamura et al., 1970b Kasukhin et al., 1970). The application of CIDNP to unravel some of the mechanistic complexities of the decomposition of azo-compounds has now begun. 

Radical chemistry has undergone something of a renaissance in recent years. The phenomenon of CIDNP has played an important part in this. The growing interest in the role of radical processes in biological systems may stimulate the application of CIDNP in even wider fields in the future. The development of a practical device for radiofrequency amplification by the stimulated emission of radiation (RASER) may well be one such application. 

For a review of the application of CIDNP to rearrangement reactions, see Lepley, A.R. in Lepley Gloss Chemically Induced Magnetic Polarization, Wiley NY, 1973, p. 323. 

The applications of CIDNP to mechanistic studies of organic photochemical reactions are numerous, but only a few systems, such as the photoreduction of quinones, have been fully examined by both CIDEP and CIDNP methods. Instead of repeating some of the well-known CIDNP mechanistic studies summarized in other reviews, we shall go into a relatively new area of CIDNP studies involving metallorganic compounds. 

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. 

However, the instability of the simplest a-tin ketones precludes the application of CIDNP methods in studying their reaction mechanisms. Me3SnCOMe decomposes on attempted isolation if exposed to daylight. Therefore, to study the mechanisms of the photodecomposition of R3SnCOR it is reasonable to choose more steric hindered derivatives which are relatively stable under ambient conditions. Consequently, the regularities of the reaction mechanisms of a-stannyl ketones R3SnCOR have been studied with 2-methylpropanoyltripropylstannane" Pr3SnCOCHMe2 (9). 

Only a few applications of CIDNP with regard to the determination of radical properties have been published so far. Chapter 4.2. reports that a positive sign of Qu (CH3CX)) was found to agree with the observed CIDNP effects, whereas only the absolute value of this quantity was known from ESR studies. In Ref. the parameters... 

Recent Applications of CIDNP to Chemical and Biochemical Problems... 

RECENT APPLICATIONS OF CIDNP TO CHEMICAL AND BIOCHEMICAL PROBLEMS... 

Applications of CIDNP fo fhe sfudy of photoinitiators have recently been reviewed. ... 

There has been a strong but very selective interest in CIDNP on amino acids, strong because of the importance for the application of CIDNP to proteins (see Section 6.9), and very selective because only three amino acids (tr)q)tophan 10, tyrosine 11, and histidine 12, compare Chart 12 for their CIDNP spectra, see Ref. 185) are routinely useable for that purpose while two others, cysteine 13 and methionine 14, have received attention because of their putative role for long-range electron transfer across cell membranes or oxidative damage of cell components. 

Owing to their ease of use, Kaptein s rules (Eqs. 62 and 65) are the basis of most chemical applications of CIDNP. On the other hand, they represent the radical pair mechanism in a simplified form only, so the question as to the validity of this approximation is of importance. Salikhov [47a] was the first to show theoretically that these rules can be violated in systems containing more than one magnetic nucleus. This was later analyzed in more detail [47b] and verified experimentally [47b, 47c, 48],... 

Hydrogen abstractions in ketone/amine and quinone/amine systems continue to attract the attention of CIDNP spectroscopists [94] despite the facts that the application of CIDNP to these reactions dates back to 1974 [95] and that the basic mechanism — electron transfer from the amine DH followed by deprotonation of the resulting aminium cation DH + to give an a-aminoalkyl radical D" has already been cleared up in those early investigations [46]. CIDNP spectroscopy is very well suited to probe the microscopic details of such reactions that involve more than one radical intermediate. Polarizations can arise in both DH + and D, but the spin density distributions of these two radicals differ strongly. Hence, the polar-... 

Other examples of the application of CIDNP spectroscopy to radical additions and substitutions include the self-substitution of quinones in the presence of tertiary aliphatic amines [111], the photoreactions between hexamethyldisilane and quinones [112], and the allylation of quinones via photoinduced electron transfer from allylstannanes [113] (see also Section V.E). Cycloadditions via radical ions are treated in a separate section (V.D.3). 

Amino Acids, Peptides, and Proteins. The determination of the accessibility of amino acid residues is the standard application of CIDNP to proteins and larger peptides, the key idea being that only amino acids exposed to the surface can react with a photoexcited dye. The photoreactions must be reversible to avoid unwanted structural changes of the biopolymer that are induced by the experiment itself. This can be realized with cyclic electron-transfer (cf. Section V.A.2, Chart VI) or hydrogen-transfer reactions. Because of the photochemistry of amino acids, the only... 

For the chemical application of CIDNP it is important to be aware of the fact that in high magnetic fields the nonequilibrium population of nuclear spin sublevels is formed as a result of transitions solely between singlet (S) triplet zero (Tq) states or a radical pair (Figure 2). These transitions occur due to interaction of an electron spin with spins of magnetic nuclei in the radical pair and they are also dependent on the difference in g-factors of radicals in the radical pair. 

Gloss GL and Miller RJ (1978) Photoreduction and photodecarboxylation of pyruvic acid. Application of CIDNP... 

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