High-pressure NMR spectroscopy

The typical active volume in 5 mm capillaries is 50 pL. Such a small amount of sample material within the NMR coils causes a poor signal-to-noise ratio, which limits useful applications of most of the multidimensional NMR experiments important in protein science. Therefore, a number of groups were investigating set ups 

The maximum pressure obtainable is a function of the tensile strength x of the material and the quotient outer and inner diameters do and d. It can be 

In Table 1, mechanical and physical parameters of different materials used for the production of high-pressure capillaries are listed. Quartz and sapphire display a much higher tensile strength than borosilicate glass. The effective tensile strength given in Table 1 includes the possible existence of faults in the material, which substantially reduces the maximum pressure the capillaries can resist, their number is also much dependent on the details of fabrication process. As the probability for faults decreases with size of the piece of material, small capillaries can endure much higher forces. Selected quartz capillaries with an outer diameter of 3 mm and an inner diameter of 1 mm were reported to sustain pressures up to 400 MPa. Yamada even reported that quartz capillaries made out of synthetic quartz can withstand pressures up to 600 MPa.  

The use of sapphire cells with 5 mm outer diameter and 0.8 mm wall was first suggested by Roe for pressures up to 41.5 MPa. Urbauer et al reported that sapphire cells with 5 mm outer diameter and 1 mm inner diameter tolerate pressures up to 100 MPa (US patent No. 5977772). 

In general, the sapphire cells can be used with a structure according to the descriptions of Price and Liidemann for borosilicate glass cells. Here, the pressurizing fluid (methyl cyclohexane methyl cyclopentane 50 50) is pressurized externally by a pressure bench which is connected to the autoclave by a 6m high-strength steel tube 

Many technically important reactions that require enhanced gas or liquid pressures (and activities) [41], can - up to now - not adequately be examined by NMR spectroscopy. Besides the considerations of reactions under high pressure, often also investigations of physicochemical effects under these conditions are desirable, which appear only with high pressure of several 100 MPa, for example, investigations of reaction and activation volumes - or structural changes in biomolecules and biocatalysts. 

The first high-pressure NMR experiments fall back to the 1950s [42]. Yet, nevertheless, only custom-made constructions have been described in literature, either as autoclaves or as capillaries. In the hterature, several examples for high-pressure reaction monitoring are described, as, for example, the observation of the isobutane oxidation [43], clarification of organo-metal reactions [44], or polymer reactions in supercritical carbon dioxide [45]. Some authors offer a good overview [4h]. 

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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... 

A review has been published on the applications of 103Rh NMR spectroscopy in structural chemistry.323 The complex [CpRh(dmpm)(HD)]+, where dmpm = bisdimethyldiphosphinomethane, gives a H 3IP) spectrum showing that it is a dihydrogen complex.324 High-pressure NMR spectroscopy was used to identify RhH(CO)(L)3 and RhH(CO)2(L)2 under 40 bar of CO/H2 (L = P... 

High-pressure NMR spectroscopy made possible to observe the true... 

High pressure NMR spectroscopy was used to detect RhH(CO)(L ) in SCCO2 solution, which indicated the high solubility of the complex and demonstrated that CO2 did not insert into the Rh—H bond under the reaction conditions. 

Kachel N, Kremer W, Zahn R et al (2006) Observation of intermediate states of the human prion protein by high pressure NMR spectroscopy. BMC Stmct Biol 6 16... 

The remarkable change in chemoselectivity shown in Scheme 12.16 results from the reversible interaction of secondary amines with CO2 to give the corresponding carbamic acids, as shown by high-pressure NMR spectroscopy [30]. We have discussed several examples above, where this equilibrium played a role for catalytic... 

The first high-pressure NMR probe was developed by Benedek and Purcell in 1954 [4]. Their probe, like many of those in use today, was constructed from beryllium-copper alloy (Berylco 25) and was used in hydrostatic pressure experiments at pressures up to 10000 bar. As research in high-pressure NMR spectroscopy progressed, a large variety of increasingly specialized probes have appeared. Thus, in 1979 de Fries and Jonas reported the first... 

Unlike the reaction catalysed by [RuCl2(PPh3) (5)-biphemp ], a dimeric complex has been reported to play a direct role in the enantioselective conversion of acetylacetone to (i )-(/2)-2,4-pentanediol with the catalyst precursor [(BDPzP)(DMSO)Ru(p-C1)3RuC1(BDPBzP)] (BDPBzP = (R)-(R)-3-benzyl-2,4-bis(diphenylphosphino)pentane) (MeOH, 50 bar Hj, 50 °C) [68]. The binuclear complex [(BDPzP)C1(ti2-H2)Ru(h-H)2Ru(ti2-H2)(BDPBzP)] was actually detected by high-pressure NMR spectroscopy as the only ruthenium compound in the course of the catalytic reaction yielding (/ )-(/ )-2,4-pentanediol (Scheme 15). 

In our laboratory, we have used high-resolution, high-pressure NMR spectroscopy to investigate simple molecular liquids for over two decades. Early on, we predicted that this high-pressure technique would be applied... 

R 130 K. Akasaka, High Pressure NMR Spectroscopy Characterizes Higher Energy Conformers of Proteins , p. 9... 

The application of high pressure is capable of delivering a wealth of important information about the physicochemical properties of proteins, especially folding as well as the dynamics and structure of folding intermediates. In combination with NMR spectroscopy, the stability of proteins can be studied at atomic resolution without chemical perturbations, such as denaturants (for a review see ref. 2). Thus, high-pressure NMR spectroscopy can yield local information about mechanical and... 

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