CIDNP nuclear polarization

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... 

Carboxamides, formation from ArNj 243 2-Carboxybenzenediazonium zwitterion, see 2-Diazoniobenzenecarboxylate Carboxy-de-diazoniation 241 f. Carcinogenicity 319 Chemically induced dynamic nuclear polarization, see CIDNP Chloro-de-diazoniation 194, 230ff., 247, 270, 277... 

CIDNP chemically induced dynamic nuclear polarization... 

Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

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]. 

Some insight into the mechanism of these 1,3-N02 group shifts has been obtained from measurements of the 15N CIDNP effect85. There appear to be two mechanisms. During the reaction of 83 to give 84 there is a strong enhancement of 15N nuclear polarization indicative of reaction via a radical pair, whereas for the reaction of the isomer 85 to give 86 there is no such enhancement, indicative of a mechanism which does not involve a radical pair. 

Chemically Induced Nuclear Polarization (CIDNP)-Experiments... 

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

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. 

The products observed in the reactions of Me2Ge with CCI3X (X = Cl, Br), PhCH2X (X = Br, 1), and Ph2CHCl, by proton chemically induced dynamic nuclear polarization ( H CIDNP) are those of net insertion of Me2Ge into the C—X bond and Me2Ge2X2 (X = Cl, Br) (Scheme 14.29). A two-step radical reaction takes place... 

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