Dynamic nuclear polarization signal enhancements

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. 

S. Han, E. R. McCarney, B. D. Armstrong and M. D. Lingwood, Dynamic nuclear polarization-enhanced magnetic resonance analysis at X-band using amplified 1H water signal, in Magnetic Resonance Microscopy, S. L. Codd and J. D. Seymour (eds.),. Wiley-VCH, Weinheim, 2009. 

One of the most important phenomenon, chemically induced dynamic nuclear polarization (CIDNP), deserves more detailed consideration, since it forms the basis of one of the most powerful modem methods for the investigation of the structure and reactivity of short-lived (from nano- to microseconds) paramagnetic precursors of the reaction products. CIDNP manifests itself in the form of unusual line intensities and/or phases of NMR signals observed when the radical reaction takes place directly in the probe of the spectrometer. These anomalous NMR signals—enhanced absorption or emission — are observed within the time of nuclear relaxation of the diamagnetic molecule (from several seconds to several minutes). Later on, the NMR spectrum re-acquires its equilibrium form. 

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. 

Solid-state NMR signal enhancements of about two orders of magnitude (100-400) have been observed in dynamic nuclear polarization (DNP) experiments performed at high magnetic field (5 T) and low temperature (10 K) using the nitroxide radical 4-amino TEMPO as the source of electron polarization. ... 

Figure 6.30. Enhancement of the proton NMR signal amplitude (P/P -l) is shown as a ftinction of microwave frequency as taken by the dynamic nuclear polarization experiments, where P and / are the signal amplitude with and without microwave pumping, respectively [143,146,173,174], The Overhauser effect (OE) typical to the mobile spins makes a shaip peak at the Lannor frequency cve of the electron spin but the Solid State Effect (SSE) shows two extrema separated by o from (O charactcrictic with the fixed paramagnetic spins [8], The solid curves are a guide for the eye. (a). t a is-(CH), (A//pp 0.8 G) and , trs-rich (CH)> (Affpp 7.8 G) 1174]. (b) The temperature dependence of the peak-ratio in //ww-(CH), 173], y/x = 0 means pure Overhauser enhancement and y/x— 1, the solid slate effect [173]. Note the change from the mobile above 150 K to the trapped below 150 K, Most of the solitons are trapped below 10 K, (after (a) [174] and (b) [173])...

The recent renaissance of dynamic nuclear polarization (DNP) provides an extremely promising approach for sensitivity enhancement [87—89]. It borrows the large Boltzmann polarization from unpaired electron spins of free radicals to boost the NMR signal. DNP experiments have been performed on quadrupolar nuclei such as [90], " N [91], [92,93],... 

Hyperpolarization is a term which collects a set of dilferent signal-enhancement techniques in NMR spectroscopy. They have in common that they create population differences (polarizations) of the nuclear spin, which are far larger than the thermal equilibrium value (Boltzmann polarization). Following a proposal by Overhauser in 1953, which was experimentally verified by Carver and Slichter Dynamic Nuclear Polarization (DNP) schemes were developed. These techniques employ an auxiliary reservoir of fast and effectively polarizable spins to generate spin order which is subsequently transferred to the spin system of interest. In case of DNP experiments the strong polarization of electron spins is transferred to the nuclear spins by means of microwave irradiation. In the early days DNP experiments were restricted to relatively low fields ( H-frequency below 60 MHz) due to the technical limitations of klystrons used as microwave sources. 

Reynhardt, E.C., and High, G.L. Dynamic nuclear polarization of diamond. III. Paramagnetic electron relaxation times from enhanced C nuclear magnetic resonance signals. J. Chem. Phvs. 2000 113 744-750. 

Another technique for the study of reactions that is highly specific for radical processes is known as CIDNP, an abbreviation for chemically induced dynamic nuclear polarization." The instrumentation required for such studies is a normal NMR spectrometer. CIDNP is observed as a strong perturbation of the intensity of NMR signals in products formed in certain types of free radical reactions. CIDNP is observed when the normal population of nuclear spin states dictated by the Boltzmann distribution is disturbed by the presence of an unpaired electron. The intense magnetic moment associated with an electron causes a polarization of nuclear spin states, which is manifested by enhanced absorption or emission, or both, in the NMR spectrum of the diamagnetic product of a free radical reaction. The technique is less general than EPR spectroscopy because not all free radicals can be expected to exhibit the phenomenon. 

Fig. 5.2 Dynamic nuclear polarization experiment. Proton NMR signal enhancement as a function of the pumping frequency for /ran5-polyacetylene ( ) and ri5-polyacetylene ( ).

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