High Pressure NMR of Supercritical Fluids

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. 

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,... 

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,... 

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. 

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. 

This article treats the benefits, possibilities and drawbacks of supercritical fluid chromatography (SFC) and supercritical fluid extraction (SFE) coupled to nuclear magnetic resonance spectroscopy. After a general overview and consideration of the motivation for such techniques, the design of high-pressure flow probes, as well as the principle experimental set-ups, are described. By means of several applications and comparison to HPLC-NMR, the utility of these hyphenated techniques is demonstrated. 

The study of supercritical fluid systems by Nuclear Magnetic Resonance (NMR) has been accomplished in two ways the high-pressure probe method and the high-pressure cell method. At this present stage, no paper on the study of polymerization reactions in SCF by NMR could be found. Nevertheless, other reactions have been studied in supercritical media and at extreme conditions [13], and these can be compared with polymerization reactions. 

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... 

CR Yonker, JC Linehan, JL Fulton. The use of high pressure NMR for the determination of phase behavior for selected binary solvent systems. J Supercrit Fluids 14 9-16, 1998. 

JC Linehan, SL Wallen, CR Yonker, TE Bitterwolf, JT Bays. In situ NMR observations of the photolysis of cymantrene and methylcymanirene in supercritical fluids a new technique using high-pressure NMR. J Am Chem Soc 119 10170-10177, 1997. 

NMR techniques can be used to study supercritical fluids as the employed pressures range from 0.1 to 80 MPa. The reason for renewed interest in the properties of supercritical fluids can be traced to the great promise of supercritical fluid extraction techniques. High pressure NMR spectroscopy can not only be used to investigate transport and intermolecular interactions in compressed supercritical... 

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... 

This method has a number of positive features it may be applied to most supercritical fluids with critical temperatures close to ambient deposition of the solid product occurs in a controlled manner, if necessary under an inert atmosphere and the high pressure "stabilizing" conditions are maintained right up to the point of precipitation. The precipitated solid product may then be analysed and characterised by other off-line spectroscopic techniques. In our example, the 13C-NMR spectrum of the solid material, redissolved in d8-toluene, shows the same resonances as those observed with a genuine sample of Cr(CO)4(C2H4)2. 

The drawbacks already described must be taken into account in the acquisition process of SFC-NMR spectra. One is the increase in the spin-lattice relaxation times T due to the reduced viscosity of the supercritical mobile phase. The other is the fact that with increasing pressure the susceptibility of the supercritical fluid changes, thus causing a high-field shift of the NMR signals. For the latter problem, up until now only the number of transients can be reduced to gain acceptable spectra, while for the problem of the long spin-lattice relax-... 

Notes LOD, limit of detection MeOH, methanol EtOH, ethanol ACN, acetonitrile MTBE, methyl tert-butyl ether DCM, dichloromethane THF, tetrahy-drofuran KOH, potassium hydroxide SFE, supercritical fluid extraction MS, mass spectrometry HPLC, high-performance liquid chromatography DAD, diode array detector PDA, photodiode array detector FD, fluorescence detector ECD, electrochemical detector ESI, electrospray ionization APCI, atmosphere pressure chemical ionization TLC, thin layer chromatography FAB, fast atom bombardment NMR, nuclear magnetic resonance BHT, butylated hydroxytoluene SPE, solid phase extraction. 

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