High pressure NMR of supercritical

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

High-Pressure NMR of Polymers Dissolved in Supercritical Carbon Dioxide (Dardin et al., 1997)... 

Dardin A, Cain JB, DeSimone JM, Johnson CS Jr, Samulski ET. High-pressure NMR of polymers dissolved in supercritical carbon dioxide. Macromolecules 1997 30 3593-3599. 

Thurecht, K., Hill, D. and Whittaker, A. (2006). Investigation of spontaneous microemulsion formation in supercritical carbon dioxide using high-pressure NMR, J. Supercrit. Fluid, 38,... 

In the following, we will present a selected choice of high-pressure cells developed for high-resolution NMR. We have divided the applications of high-pressure NMR into three sections. First, we will describe high pressure equipment used to study liquids under hydrostatic pressure up to 1000 MPa then, we present some special approaches used to study supercritical fluids which, besides moderate pressure, often need high temperatures. Finally, we discuss NMR cells to study solutions under gas pressure, a situation which is quite common in catalysis. 

High-pressure NMR studies for catalysis and with supercritical fluids will lead to a much broader application of sapphire NMR cells and to special applications of toroidal probes. The sapphire tube technique can today be considered as a standard, cheap and easily applicable technique to study samples under medium gas pressures, up to 100 MPa. 

In addition to these acidity/exchange effects, side reactions with supercritical CO2 have also been studied [72]. The authors reported experiments in a high-pressure NMR cell withstanding pressures up to 1000 bar. For methanol/ IL mixtures, NMRs showed signals of bicarbonafe and mefhyl carbonate ... 

Yonker, C. R. and Linehan, J. C., A high-pressure NMR investigation of reaction chemistries in a simple salt hydrate,. Supercrit. Fluids, 29, 257 2004. Mehnert, C. R, Supported ionic liquid catalysis, Chem. Eur. ]., 11,50,2005. Giernoth, R. and Bankmann, D., Transition-metal free synthesis of perdeuter-ated imidazolium ionic liquidsby alkylation and H/D exchange, Eur. J. Org. Chem., 2008 (in print). DOT 10.1002/ejoc.200700784. 

Sapphire tubes introduced by Roe [17] paved toe way for more routine applications of high-pressure NMR involving gas-liquid interphases. Systems similar to the original one have been extensively used for numerous applications [2,18-20,72,73]. For applications with supercritical media such as CO2, CHF3, and CF3CI, a modified cell equipped with a pressure sensor and a 10-mm sapphire tube has recently been described (Figure 3.2-27) [74]. 

In order to illustrate the scope and range of high pressure NMR experiments, the next part of the article discusses our recent results on relaxation and transport behavior of compressed supercritical water. The article finishes with a brief account of application of high pressure NMR to studies of the dynamic structure of various disordered systems. 

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]. 

T. Gross and H. -D. Ludemann,/ , T c- dependence of intradiffusion in supercritical solutions of ammonia and carbon dioxide studied by high pressure NMR... 

Supercritical Fluids as Solvents and Reaction Media, ed. G. Brunner, Elsevier B.V., Amsterdam, Netherlands, 2004 R 271 T. Gross, E. Chen and H.-D. Euedemann, p, T - Dependence of Molecular Mobility in Supercritical Fluids Studied by High Pressure NMR , p. 343... 

We hope this chapter encourages and inspires researchers to use high-pressure NMR to directly investigate chemical reactions and solution dynamics in supercritical fluids. An overview of this type, by its nature, cannot be considered totally inclusive it is hoped that practitioners will gain a better appreciation... 

In a second series of experiments by Dardin et al. (27), using a foldedcapillary high-pressure NMR cell, the authors investigated the H and chemical shifts of hexane, perfluorohexane, and 1,1-dihydroperfluorooctylpropionate in supercritical CO2 as a function of pressure and temperature. The nuclear... 

Diffusion coefficients measured by the spin-echo technique provide a means of investigating the translational motion of molecules under the extremes of temperature and pressure. There have been numerous smdies of the self-diffusion coefficients of high-pressure liquids and supercritical fluids by NMR. As an illustration of the potential of these physicochemical measurements, we will focus on CO2 (3,28,33,38,39). The availability of a wide range of diffusion coefficients and viscosities allows one to test the Stokes-Einstein equation at the molecular level. From hydrodynamic theory,... 

NMR spectroscopy is routinely used in today s organic synthesis laboratories to identify and structurally characterize reaction products. Yet despite the enormous structural information content of NMR and the availability of a variety of high-pressure NMR techniques (3-13), it is little used for studying in situ chemical reactions and solvent effects on chemical reactions in supercritical media. 

Hoffmann and Conradi (49) studied the deuteration of 4-ethylphenol in supercritical D2O at 460°C and 250 bar using high-pressure NMR. The methyl protons were found to be stable under these conditions. The temperature was raised to 500°C, and the deuteration of the methyl protons in conjunction with decomposition reactions was monitored. From this investigation, the selective deuteration of the different protons in 4-ethylphenol in supercritical D2O could be demonstrated. 

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