Induced Dynamic Nuclear Polarization CIDNP

CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION (CIDNP)  

When two radicals are in close association as a pair surrounded by a cage of solvent molecules, the two odd electrons will interact with one another just as two electrons do within a molecule. The interaction will yield either a singlet state, if the two electrons have spins paired, or a triplet, if the spins are unpaired. If, for example, the caged pair arose by thermal dissociation of an ordinary ground state molecule, in which all electrons would have been paired, the state would initially be a singlet, S, whereas if the pair arose in a photochemical reaction from dissociation of an excited molecule in a triplet state, it would be initially a triplet, T. 

If the two radicals were identical, the precession frequency of the two electrons would be precisely the same, and a particular radical pair would remain in whichever of the four states it found itself initially. But if the two radicals are different, the two electrons will have slightly different precession frequencies. The precession frequency of an electron is characterized by a quantity called the g factor. If one were to observe just one of the radicals of the pair in an electron paramagnetic resonance spectrometer, the value of its g factor would determine the position of the resonance line in the spectrum g of an electron in a radical is thus analogous to chemical shift of a proton. 

The consequence for our radical pair of a difference in g is that the two electrons will precess at very slightly different rates, and so over the course of many revolutions the relative phases of the two will change. One spin will gain on the other, and a pair that started out in state S will change to To, while a pair that started out in T0 will change to S. 

If the two radicals of the pair are identical, there are still mechanisms for polarization. Consider, for example, the radical pair 4. Now there are four 

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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... 

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)... 

On the other hand, in accord with the free radical mechanism peroxynitrite is dissociated into free radicals, which are supposed to be genuine reactive species. Although free radical mechanism was proposed as early as in 1970 [111], for some time it was not considered to be a reliable one because a great confusion ensued during the next two decades because of misinterpretations of inconclusive experiments, sometimes stimulated by improper thermodynamic estimations [85]. The latest experimental data supported its reliability [107-109]. Among them, the formation of dityrosine in the reaction with tyrosine and 15N chemically induced dynamic nuclear polarization (CIDNP) in the NMR spectra of the products of peroxynitrite reactions are probably the most convincing evidences (see below). 

For more complex spin systems, a computer program PHIP+ has been developed [13, 45] which allows the expected PHIP spectra to be calculated from the chemical shifts and coupling constants of the products. Depending upon which proton pair in the product molecule stems from p-H2, different - but characteristic - polarization patterns result [14]. The patterns also depend on the sign of the coupling constants. Simple sign rules governing the relative sequence of the emission and absorption lines in the PHIP spectra (i.e., their phase ) can be formulated in similar manner to the Kaptein Rules of chemically induced dynamic nuclear polarization (CIDNP) [15]. 

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]. 

The PFR of para-cresyl para-chlorobenzoate (30) (Scheme 12) has been studied by means of chemically induced dynamic nuclear polarization (CIDNP) [46]. The... 

Chemically induced dynamic nuclear polarization (CIDNP),... 

Among NMR methods providing insight into radical ions, chemically induced dynamic nuclear polarization (CIDNP) has proved especially useful it results in enhanced transient signals, in absorption or emission CIDNP effects were first reported in 1967 their application was soon extended to radical ions. The method lends itself to modest time resolution. 

These assignments are consistent with PES data ° and supported by theoretical calculations " in C2v symmetry, IS " " has two low-lying radical cationic states, and Ai. " The state is the ground state of IS " " the calculated hyperfine coupling constants (B3LYP/6-31G //MP2/6-31G ) are compatible with chemically induced dynamic nuclear polarization (CIDNP) and ESR/ENDOR results. No spin... 

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. 

Chemically induced dynamic nuclear polarization (CIDNP) was first reported in 1967 in independent work from three different laboratories. The effects of free radicals on NMR spectra were revealed (Fig. 6) in studies of radicals from peroxides (equation 62) and azo compounds, as well as radicals generated from the reaction of alkyl halides and organolithium compounds. ... 

H. R. Ward, "Chemically Induced Dynamic Nuclear Polarization (CIDNP). I. The Phenomenon, Examples, and Applications, Accfs. Chem. Res. 5, 18 (1972). 

Chemically Induced Dynamic Nuclear Polarization (CIDNP) 529... 

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