Chemically induced dynamic nuclear radical pair dynamics

The chemically induced dynamic nuclear polarization (DNP) opened perspective to study products formed from free radicals [102], The basis of this study is the difference in NMR spectra of normal molecules and those formed from free radicals and radical pairs. The molecules formed from radicals have an abnormal NMR spectrum with lines of emission and abnormal absorption [102]. DNP spectra help to obtain the following mechanistic information ... 

Chemically induced dynamic nuclear polarization (CIDNP) is a very powerful tool for establishing the existence of radical pair intermediates and their spin. CIDNP has reinforced the view that singlet carbene undergoes direct insertion into C—H bonds and that the triplet abstracts hydrogen. 

Bargon J, Seifert KG (1974) A chemically induced dynamic nuclear polarization study of free-radical pairs and their reactions in aqueous solution. Ber Bunsenges Phys Chem 78 187-190... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

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]. 

CIDNP (chemically induced dynamic nuclear polarization) Non-Boltzmann nuclear spin state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion, or disproportionation of radical pairs, and detected by nuclear magnetic resonance spectroscopy by enhanced absorption or emission signals. 

Kaptein, R., Chemically Induced Dynamic Nuclear Polarization. VIII. Spin Dynamics and Diffusion of Radical Pairs, J. Am. Chem. Soc. 1972, 94, 6251 6262. 

In another class of experiments, hyperpolarized states are generated by spin-sensitive chemical reactions. These include para-hydrogen-induced polarization (PHIP) [3-5] and chemically induced dynamic nuclear polarization (CIDNP) [6-8]. The latter involves non-equilibrium nuclear spin state populations that are produced in chemical reactions that proceed through radical pair intermediates. CIDNP s applicability has been focused towards the study of chemical reactions and the detection of surface exposed residues in proteins [9], but has so far remained limited to specialized chemical systems. 

Consequently, all the alkene 12 detected under such conditions was formed by the Cope-type elimination (b). On the other hand, alkene formed in the absence of thiophenol was generated by both paths. Recently, Marque et al showed that thermolysis of alkoxyamines in the presence of the scavenger PhSH induces chemically induced dynamic nuclear polarization (CIDNP). The CIDNP effect, detected by H-NMR, indicates that the intermolecular H-transfer occurs, in contrast to earlier reports,already in the spin correlated geminate (cage) radical pair, formed immediately after the cleavage of the NO C bond. 

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

The NMR spectra of compounds being formed from reactions involving radical pairs often exhibit anomalous signals as the result of chemically induced dynamic nuclear polarization (CIDNP). While EPR signals are due to radicals themselves, CIDNP signals... 

The photolyses of dlbenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by mlcelllzatlon and hyperflne induced intersystem crossing In the geminate radical pairs. This latter effect is the basis of Chemically Induced Dynamic Nuclear Polarization (CIDNP), a phenomenon which is well known in homogeneous solution. 

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

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