CIDNP effects

Observation of CIDNP effects in the resonances of either reactant or product species in the NMR spectrum of the reaction mixture. 

Becker et al., (1984) investigated the photo-CIDNP effect in the presence of crown ethers (see Sec. 11.2). CIDNP studies on the photolysis (Jiang et al., 1990) and on the NaBH4 reduction (Song et al., 1990) of arenediazonium ions showed that free radical intermediates are involved. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

The product is a yellow oil that reacts with benzylidenemalonodini-trile by 1,4-addition of the Bu and SnMeg fragments, and decomposes at 37°, giving a CIDNP effect (277, 278). 

Heesing and coworkers have reported the rearrangement of 0-alkylsulfinyl-iV-benzoyl-iV-phenylhydroxylamine (143) at — 70°C to the corresponding sulfonamide together with the o- and p-alkylsulfonyl derivatives 144 and 145 (equation 89). The reaction has been suggested to proceed by an intramolecular radical pair mechanism, as evidenced by experiments with oxygen-18 labeling and C-CIDNP effects. 

Weak CIDNP effects uere observed for the thermolysis of the azophosphate (37)102... 

The reactions with non-aromatic substrates follow a variety of mechanisms. In the reaction of Bu3SnK with s- and /-butyl bromides and iodides, a transient absorbing at 400 nm has been observed,90 and the Bu4Sn and Bu3SnSnBu3 that were formed showed a CIDNP effect, confirming the intermediacy of the Bu3Sm radical.91... 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

Scheme 6 Observation of a CIDNP effect on fragmentation of radical 5...

Gloss 83,84) developed a theory which quantitatively explains and predicts the CIDNP-effect. Kaptein 85) proposed qualitative rules to predict absorption or emission, i.e., the phase of the emission ( ) or absorption A) signal. His equation is as follows ... 

Such treatment of CIDNP results produced serious objections. Lippmaa et al. (1973), investigating the same reaction, revealed a strong N, C, and CIDNP effect. The C nuclei in the phenoxyl part of the azo dye were not polarized. At the same time, polarization of N nuclei of the azo bond and C nuclei at positions 1 and 2 of the free-of-hydroxyl phenyl ring connected with... 

As discussed earlier, CIDNP method is of great help to chemists. However, it cannot always give straightforward information, because the CIDNP effect may be masked and errors may creep into its interpretation. CIDNP requires strong chemical and physical professional skills (which are useful for all the method considered here). However, using CIDNP, a researcher can be compensated by the reliability of the conclusions. 

Exposure of several methyl-substituted derivatives to y-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, some of which had been previously identified on the basis of CIDNP results. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and /rans-l,2-diphenylcyclo-propane. ... 

The CIDNP effects for the spirocyclopropane signals of 18 are considerably weaker than those for 16. The homoconjugative and exchange interactions between the 17 SOMO and the cyclopropane A FMO must be relatively weak, which can be ascribed to the less than perfect match between the respective orbital energies. 

Figure 4.4. Spectra of radicals. Left ESR (top) and ENDOR (bottom) spectra of a triarylmethyl radical. [Reprinted with permission from B. Kirste, W. Harrer, and H. Kurreck, J. Am. Chem. Soc. 1985, 107, 20-28. Copyright 1985 American Chemical Society.] Right CIDNP effects in the vinyl proton absorbance region in a 60 MHz NMR spectrum from reaction of BuLi and BuBr before mixing (a), 30 s after mixing (b) and after completion of the reaction (c). [Reprinted with permission from H. R. Ward, and R. G. Lawler, J. Am. Chem. Soc. 1967, 89, 5518-5519. Copyright 1967 American Chemical Society.]...

Proton hyperfine couplings in the ESR spectmm of the methyl radical are —23 G, where the sign of the coupling is not important except for CIDNP effects. The... 

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