Chemically induced dynamic nuclear polarization spectrum

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 ... 

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. 

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. 

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). 

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]. 

A new and little understood technique that is worthy of attention is chemically induced dynamic nuclear polarization (CIDNP). A hexane-ether solution containing w-butyl-lithium, n-butyl bromide and diphenylacetylene, gives a transient spectrum resembling that of but-l-ene in which some of the lines are inverted. The authors ascribe the phenomena to the presence of free-radical intermediates, although the lines show no evidence of broadening due to paramagnetic induced relaxation. See also ref. 7a. 

Stoicheiometric hydroformylation of alkenes with [MnH(CO)s] has recently been reported. Treatment of 1,2-diphenyl 3,3-dimethylcyclo-propene in hexane under CO with [MnH(CO)s] at 55 °C gave a mixture of cis-(S7%) and trans-(l37o) aldehydes together with some alkane. The proposed mechanism is shown in Scheme 3. A chemically induced dynamic nuclear polarization (CIDNP) effect was observed in the H n.m.r. spectrum during the reaction and this was ascribed to the initial formation of an alkyl radical (10). The final step, reaction of a metal hydride with a metal acyl to give free aldehyde, is similar to that proposed in Co-catalysed hydro-... 

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. 

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP (chemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. If a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum ot the substrale/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emission). Note that the spectrum involved is not... 

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