CIDNP studies

Roth H D, Weng H and Herbertz T 1997 CIDNP study and ab initio calculations of rigid vinylcyclopropane systems evidence for delocalized ring-closed radical cations Tetrahedron 53 10 051-70... 

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

One aspect of both EPR and CIDNP studies that should be kept in mind is that either is capable of detecting very small amounts of radical intermediates. This sensitivity makes both techniques quite useful, but it can also present a pitfall. The most prominent features of either EPR or CIDNP spectra may actually be due to radicals that account for only minor amounts of the total reaction process. An example of this was found in a study of the decomposition of trichloroacetyl peroxide in alkenes. 

Kasukhin et al. (1974) estimated a lifetime of 10-7 s for the phenyldiazenyl radical based on a CIDNP study. Later estimates of the decay rate constant were roughly in agreement with this 3.4 x 106 s 1 from the transient UV absorption in... 

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. 

The H and 13C CIDNP studies have shown that not only the sulfone 14, but also the sulfmic ester 15, is generated as cage products from the phenyl/p-toluenesulfonyl radical pair during the thermal decomposition of phenylazo aryl sulfone (13)54 (Scheme 2). The cross-termination of arenesulfonyl and triethylgermyl radicals was found to occur exclusively via the formation of germyl sulfinate, ArS(0)0GeEt333. 

One of the most attractive features of CIDNP studies is the ease with which experiments can be carried out. The basic requirement is only a high-resolution n.m.r. spectrometer. 

CIDNP studies of the decomposition have centred mainly on thermal decompositions photochemical decomposition has generally been less intensively investigated. While most reports of polarization refer to n.m.r. spectra, a number of papers have described polarization of other nuclei, (Kaptein, 1971b Kaptein et al., 1972), (Lippmaa el al., 1970a, b, 1971 Kaptem, 1971b Kaptein et al., 1972 Kessenikh et al., 1971), and F (Kobrina et al., 1972) contained in the peroxide reactant. Additionally, polarization of P has been reported in the products of decomposition of benzoyl peroxide in phosphorus-containing solvents (Levin et al., 1970). 

Few CIDNP studies on free radical reactions with olefins and related unsaturated molecules have been reported, and relatively little chemically useful information seems to have been derived, despite the potential relevance in polymerizing systems. Thus CIDNP has been reported in the decomposition of benzoyl peroxide in the presence of styrene and... 

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. 

The photolysis of carbonyl compounds is one of the most intensively studied areas of photochemistry. Since CIDNP studies have been concerned mostly with aldehydes and ketones we shall confine these brief introductory remarks to such compounds. More extensive reviews are available (e.g., Simons, 1971). 

On the basis of reaction-product structures, it might be expected that the reactions of organic halides with sodium naphthalene (Scheme 9) might resemble mechanistically the reactions of organic halides with lithium alkyls. CIDNP studies have shown that they are in fact quite different, in particular in the mechanism by which polarization occurs. The observations are as follows (Garst et al., 1970). 

This same CIDNP study also ruled out the existence of the bishomo-aromatic system [158c] by finding two distinct species, [158a] and [158b], for the intramolecular cycloreversion of the radical cation of quadricyclane to that of norbornadiene. 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

ESR and CIDNP studies intended to detect the radical intermediates failed [63], Conjugate addition of a vinylcuprate reagent to an enone takes place with retention of the vinyl geometry indicating that no vinyl radical intermediate is involved [64, 65], Kinetic isotope effects and substituent effects in cuprate addition to benzophenone indicate that C-C bond formation is rate-determining, which is not consistent with the involvement of a radical ion pair intermediate [66]. 

CIDNP studies indicate that back electron transfer competes directly with a-deprotonation (172). 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

The CIDNP studies show that a significant part of the overall reaction can involve the corresponding durene cation radical. 

CIDNP studies limited the lifetime of the putative intermediate to a fraction of a psee [262]. Most recently weak polarization was observed for a linked dimer (89), suggesting that the corresponding dimer radical cation is a discrete minimum, however short-lived it may be [263],... 

The first valence isomer to be investigated was the bicyclic (hexamethyl-, HM-) Dewar benzene. However, y irradiation of this substrate produced the electronic spectrum of HM-128 [360]. Similarly, nsec time-resolved laser spectroscopy failed to reveal evidence for the bicyclic radical cation [362]. The first indication for the existence of such a species as a discrete entity was provided by a CIDNP study [361]. These results are best discussed in connection with ab initio calculations carried out for the parent C6H6 system. At the 6-31 G level these calculations support the existence of two cationic states with the unpaired spin density either... 

According to earlier published data52 the primary act of the process is the homolytic decomposition of A-bromodisilazane 14 (Scheme 8, equations 17 and 18). Further reaction stages include the interactions of 11 with the (Me3Si)2N radical and bromine atom. The above-discussed CIDNP studies of the photoinduced interaction of 11 with CCI3B1 ... 

The insertion reactions of 16 into the C-Hal bond of alkyl halides were found to be the most convenient model for CIDNP studies. Since in accordance with Scheme 10 the photolysis of 17 leads to the formation of triplet germylene 16, we start with the first evidence of the reactions of triplet excited 1676. 

The proposed intermediacy of dirfidicals or zwitterions is supported by CIDNP studies. 

CIDNP study of the photocleavage of benzyl chloride and bromide in methanol/acetone only gives signals for the radical-related products and not for the ion-related ones145. For concentrated solutions of l-(iodomethyl)naphthalene in MeOH, yet another mechanistic pathway to photosolvolysis product occurs146. A complex formed between starting material and homolytically photogenerated Y reacts with methanol to yield the ether product and I2. ... 

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