High pressure NMR tubes

Fig.3 High-pressure NMR tube and NMR spectrum of the IL [BMIM][BTA] under a pressure of 30 bar of hydrogen without CO2 (lower trace) and in the presence of an additional 80 bar of CO2 (upper trace). The signal at 4.3 ppm results from dissolved hydrogen, indicating the increase in solubility in the presence of CO2 by the increase in intensity relative to the signal at 4.0 ppm from the IL solvent...

Recently, a new (and now commercially available) methodology was reported for measuring in-situ high pressure NMR spectra up to 50 bar under stationary conditions. The instrument uses a modified sapphire NMR tube, and gas saturation of the sample solution and exact pressure control is guaranteed throughout the overall measurement, even at variable temperatures. For this purpose, a special gas cycling system is positioned outside the magnet in the routine NMR laboratory [51]. 

Figure 6.9 An example of a sapphire NMR tube suitable for high pressure NMR Photograph by Gabor Laurenczy...

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. 

Figure 7.2 Fast spinning, high pressure NMR sapphire tube with safety and charging device (constructed at ICCOM-CNR, 2003).

High-pressure NMR [53, 54, 56] typically utilizes either pressurizable NMR tubes or a high-pressure probe head. The greatest advantage of the pressurizable glass tubes is that they ean be used with commercial probes which makes it possible to spin the sample and thus improve the resolution. On the negative side, the thiek tube walls reduce the sensitivity and lead to imperfect temperature control. Also, the pressures are usually lower than those obtained with high-pressure probe heads. 

High-pressure NMR experiments have been performed for more than 40 years using techniques ranging from hydrostatic methods employing hydraulic presses and titanium alloy vessels to the use of glass and sapphire tubes. In this article we will focus on sapphire tubes because they represent the only high-pressure NMR tool we are utilising. 

Fig. 4.20 A schematic representation of two different approaches to high-pressure NMR (a) a pressurizable sample tube, and (b) a dedicated pressurizable NMR probe.

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]. 

Figure 3.2-27 Modified cell for high-pressure NMR measurements with supercritical fluid A, titanium valve assembly B, titanium pressure sensor C, needle valve D, sapphire tube E, glue clearance ...

Table 6. High-pressure NMR studies using sapphire tubes.

Miscellaneous high-pressure NMR studies of inorganic systems High-pressure NMR studies using sapphire tubes Various high-pressure studies of polymers and complex liquids High-pressure studies of liquids confined to porous media High-pressure NMR studies of solids... 

High temperature-high pressure NMR techniques provide useful information on the intermolecular interactions of SCW. The NMR signal of water observed at 30°C was sharp and shifted toward the low field side with temperature at constant density (p = 0.4 g cm at high temperatures and pressures the pressure was changed in an NMR tube with a constant volume) [62, 63]. The signal was broadened with the increase in the temperature as seen in Fig. 17a. The chemical shift cr of water is plotted against temperature in the liquid and gas phases in Fig. 17b. 

Another investigation describes a high-pressure NMR fiow cell for the in situ study of homogeneous catalysis (60). This fiow cell was constructed from a sapphire tube and reaction intermediates from a ruthenium-rhodium organometallic complex with CO were detected for the first time. 

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