In-situ NMR

Xu T, Munson E J and Flaw J F 1994 Toward a systematic chemistry of organic reactions in zeolites in situ NMR studies of ketones J. Am. Chem. Soc. 116 1962-72... 

J. E Stebbins and I. Farnan. Science. 245,257, 1989. Highlights in situ NMR applications at ultrabigh temperatures. 

This example should illustrate that in situ NMR spectroscopy can be a powerful tool with which to study catalysts dissolved in ionic liquids, if the signals of the metal complex can be detected in sufficient intensity independently from the signals of the ionic liquid. 

In addition to in situ NMR spectroscopy, other methods such as in situ IR spectroscopy, EXAFS, and electrochemistry should be very useful for the investigation of active catalytic species in ionic liquids. However, far too little effort has been directed to this end in recent years. 

In situ NMR measurements can be made in conjunction with down-hole fluid sampling [5, 6]. The NMR relaxation time and diffusivity can be measured under high-temperature, high-pressure reservoir conditions without loss of dissolved gases due to pressure depletion. In cases when the fluids may be contaminated by invasion of the filtrate from oil-based drilling fluids, the NMR analysis can determine when the fluid composition is approaching that of the formation [5, 6]. 

The reported in situ NMR of combustion [2] served largely as proof of concept work. It was demonstrated that despite the presence of paramagnetic oxygen and radicals, the xenon relaxation times are sufficiently long for gas exchange studies. 

In situ NMR Methods in Catalysis Volume Editors Bargon, J., Kuhn, L. T. 

Kuhn LT, Bargon J (2007) Exploiting Nuclear Spin Polarization to Investigate Free Radical Reactions via in situ NMR. 276 125-154... 

Figure 22 In situ NMR observation of the copper carbene (NPN)Cu=C(ph)co2Et. Reproduced with permission from Wiley.

Of course, other reaction types have been also investigated more recently, such as the Beckmann rearrangement [247,277,278] or ethylbenzene disproportionation [279, 280], just to name a couple. In situ NMR methods are expected to play a vital role in the future science of heterogeneous catalysis. 

This system shows an induction period of about six hours before constant activity is attained during which the Ru3(C0)12 undergoes complete conversion to another ruthenium carbonyl complex. In situ nmr studies suggest this species to be the HRu2(C0)e ion. Kinetic studies show complex rate profiles however, a key observation is that the catalysis rate is first order in Pco at low pressures (Pcohigher pressures. A catalysis scheme consistent with these observations is proposed. 

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

The fact that two entirely different phenomena can both yield nuclear spin polarization may cause confusion therefore, the appearance of intense emission and absorption lines during in-situ NMR investigations of hydrogenation reactions is not necessarily proof for free radical intermediates, and examples of erroneous conclusions do exist [5]. 

Integrated thermal conductivity cells (see Fig. 12.8) allow a quantitative determination of the corresponding ortho/para ratios of the dihydrogen. The enriched parahydrogen is well-suited for in-situ NMR studies of hydrogenation reactions that yield nuclear spin polarization due to symmetry breaking during the reaction. The same apparatus has also been used successfully to enrich ortho- and paradeuterium mixtures. 

There is only one detailed kinetic study of ruthenium enantioselective hydrogenation, in this case involving (BINAP)Ru(OAc)2, and MAC [65]. The extensive study involved reaction kinetics, isotopic analysis of reaction components and products, and in-situ NMR. The derived catalytic cycle is shown in Figure 31.15, differing from the Bergens studies described above in that the intermediates -both observed and assumed - are neutral rather than cationic. Right up to the formation of the alkylruthenium intermediate, the individual steps are revers-... 

Further information on the reaction intermediates is achieved by in situ NMR experiments. Because the signals in NMR spectra depend upon the concentration of the investigated species, a quantitative treatment is possible. Bianchini and coworkers investigated the hydroformylation of 1-hexene [62], using high-pressure NMR spectroscopy to evaluate the influence of synthesis gas on the equilibria of rhodium triphenylphosphine species. They were able to establish at least four resting states of rhodium (catalyst species that do not participate directly in the reaction). When synthesis gas interacted with... 

High-pressure in-situ NMR spectroscopy have been reported about reactions of carbon monoxide with cobalt complexes of the type, [Co(CO)3L]2. For L=P(n-C4H9)3, high pressures of carbon monoxide cause CO addition and disproportionation of the catalyst to produce a catalytically inactive cobalt(I) salt with the composition [Co(CO)3L2]+[Co(CO)4] . Salt formation is favoured by polar solvents [13],... 

In situ NMR studies on analogous Pt catalysts for the methoxycarbonylation reaction reveal CO trapping at every step in the catalytic cycle of the active intermediates (Figure 1.10) [27]. This explains the observed slow kinetics. Thus, 36 reacts with CO in CHjClj at 193 K to form only [Pt(L-L)(C2H5)( CO)]+, 37, which upon warming to ambient temperature in the presence of excess CO affords [Pt(L-L)( C(0)Et)( CO)]+, 38. This transformation is reversible, and both compounds have been detected by in situ C H NMR spectroscopy. 

Figure 7.3 Exploded view of the high pressure Figure 7.4 High pressure IR cell connected in situ NMR flow cell. (From J. A. Iggo, to an autoclave (constructed at ICCOM-CNR,...

In aprotic solvents, chain transfer occurs exclusively by fl-H elimination, unless a protic acid or water is present in the reaction mixture, in which case protonolysis may occur. Indirect evidence (for example, M, and M measurements) proves that P-H chain transfer in aprotic solvents is slower than methanolysis in protic solvents with comparable structures of the Pd" catalyst [5f, 17, 20, 21]. This effect and the possibility of using well-defined catalysts have remarkably favored the use of in situ NMR spectroscopy for the detection of intermediates during CO/copolymerisation in organic solvents. 

In situ NMR experiments in anhydrous CD2CI2 confirmed unequivocally that the p-OH complexes are just generated by reaction of water with the P-chelates [5f]. 

Using in situ NMR spectroscopy, Brookhart has also studied the activation barriers for the migratory insertion steps corresponding to chain growth in ethene/ CO copolymerisation catalysed by dppe-derived nickel(II) complexes [4a]. Activa-... 

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