Dynamic nuclear polarization measurement

A significant step in the measurement of MFEs is the development of rapid field switching apparatusModem MOSFET transistors have made the possibility of field switching on the timescale of RP dynamics a reality. This technique was first used in a method called SEMF-CIDNP (switched external magnetic field chemically induced dynamic nuclear polarization). 

A minor product observed in the photolysis of DBK in aqueous HDTCl (Fig. 5) is the isomeric ketone 4-methylphenylacetophenone (PMAP, 3) 15 18). The question arises as to whether the formation of 3 is due to some special orienting feature of the micelle which operates on the ratio of coupling products 1 and 3. Although 3 has not been detected by the conventional analytical methods for product analyses, (e.g., vpc), some 3 is detectable via chemically induced dynamic nuclear polarization (CIDNP) measurements 28). Thus, it may be that the ratio of formation of 1 to 3 is similar in both micellar and homogeneous solutions, but that the absolute yield of both combination products is so much smaller in homogeneous solution that 3 is not detectable by conventional analyses. 

Berliner and Kaptein (1981) used another NMR method, induced dynamic nuclear polarization, to investigate the solvent accessibility of Tyr, Trp, and His residues in five species of a-lactalbumin. This method measures the access of the photo-excited flavin dye to the surface-exposed... 

As shown in Fig. 2-6, the field positions for ESR transitions are split to a doublet for 7=1/2 and a triplet for 1=7, respectively, from the original position (Bo). Such split structures are called HF structures. It is noteworthy that the sign of a can not be determined from ESR measurements at room temperature and even at 77 K. The sign can be obtained experimentally from chemically induced dynamic nuclear polarization (CIDNP) which will be explained in Chapter 4. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

Recendy, Song et al. measured the local water diffusion coefficient and proton diffusion coefficient Dp within 5—10 A of spin probes that are partitioned into selectively different local environments of the swollen NAFION using Overhauser dynamic nuclear polarization relaxometry (ODNP) for nuclei of water at 9.8 GHz. This experiment concluded that... 

One technique, Overhauser Dynamic Nuclear Polarization (ODNP), is based on the well-known chemical shift of water in NMR spectra. Ordinarily, the liquid water signal intensity is low however, intensity can be magnified 1000-fold by addition of a nitroxide spin label such as TEMPO. Precession of the unpaired electron in TEMPO at the Larmor frequency results in Nuclear Overhauser-mediated polarization of the protons in water. These get polarized within 15 A of the spin labels and then relax with a relaxation time determined by the local diffusivity, i.e. in bulk water, the diffusivity is high and so relaxation is rapid by contrast, in hydration layers, relaxation takes 10-fold longer than in bulk water. Next, the trick is to covalently tether spin labels to surfaces of interest and measure how relaxation rates in hydration layers change as adhesive proteins approach and locally dehydrate the surfaces. 

The direct proof that the Grignard formation reaction proceeds in this way was given by CIDNP (Chemical Induced Dynamic Nuclear Polarization) in 1972 by Bodewitz et al. [22,23]. They found the variation of peaks caused by the formation of the radical using CIDNP by measurement of the methylene proton in the Grignard formation reaction of CH3CH2I or (CH3)2CH2CH2Br. 

Dynamic nuclear polarization of water protons has been used to measure the relaxation time of Li at very low magnetic field. NMR data have also been reported for [Li2 Li(THF)2 2(Ph2Si02GaMe2)2 Ph2Si02GaMe(OH) 2-2THF, ( Li, Si), " [(2-HO-3,5-Bu 2CH2NHCH2CH2)2 AlH2Li(THF)2 2], ( Li), ... 

The existence of the biradicals and the multipHcity of the surfaces on which these are formed have not been demonstrated directly however, experimental results (stereochemistry of the reaction, CIDNP [chemically induced dynamic nuclear polarization], radical trapping experiments, and quantum yield measurements) support their existence. Recently, the mechanism of 1,3-migration and oxa-di-Jt-methane reactions in terms of potential energy surface and decay funnels has been described this also supports the aforementioned mechanistic impHcations. The detailed mechanism, however, depends, in a very subtle way, on the structure of the chromophoric system and the presence of the functional groups. 

Geophysics. Utilization of dynamic proton polarization phenomenon in nitroxyl radical solutions facilitated the improvement of spin-precision magnetometers by synchronizing the processes of nuclear polarization and precision measurement. Deuterated radical solutions serve as sensors in such instruments. Instruments of this type have already been produced in France. They find application in geophysical research, geological explorations and in searches for metallic objects under water. 

A developing approach measures by NMR the local hydration dynamics around nitroxides at room temperature via Overhauser d3mamic nuclear polarization (DNP). 

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