Sapphire NMR tubes

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...

Figure 2.16 Electronic pressure gauge to follow/control the pressure in sapphire NMR tubes. The small dead volume allows the pressure in the tube to be measured precisely and also allows the pressure variation to be monitored with time.

A more recent in situ P H HP NMR study of styrene/CO copolymerisation by the same [(R,S)-BINAPHOS]Pd-acyl complex applying both diffusion-controlled (non-spinning sapphire NMR tube) and reaction-controlled (flow NMR cell) conditions has provided evidence for other intermediate species as well as information on the relative rates of CO insertion into isomeric Pd-acyls [6b, 7gj. Figure 7.15... 

Applications of metal NMR are no longer limited to static characterization in terms of sample homogeneity or trends in chemical shift. Examples are drawn to emphasize structural characterization and dynamic studies. A novel sapphire NMR tube is described which permits routine high resolution operation up to 2,000 psi. The tubes can be used in any spectrometer, and have allowed the study of fundamental reactions of transition metal carbonyl complexes under conditions which would otherwise bring about sample decomposition. 

Construction and Operation of the Sapphire NMR Tube. Sapphire was chosen as the material for tube construction because its excellent tensile strength characteristics can be uniformly retained in a tube grown intact as a single crystal. Tubes 5 mm in o.d., with 0.8 mm wall and sealed at one end, were chosen for our prototype and were purchased from Saphikon, Inc. (51 Powers Street, Milford, NH 03055) at the time of our original purchase, the cost of the tubes was approximately 300 each. 

Performance of the Sapphire NMR Tube. The tube and valve are sufficiently light weight and symmetrical that they spin and achieve a typical resolution of about 1 Hz. In hydrostatic pressure testing, the burst pressure of one tube was found to be approximately 14,500 psi, and it is therefore believed that operations up to 2000 psi may be carried out with a reasonable margin of safety. Obviously however, all due caution must be exercised and operator exposure to a pressurized tube must be avoided. It is as yet unclear whether extended use will eventually affect the performance of these tubes. In our hands, the tubes have been operated without incident, over a period of two years, from -140 to +150°C at nominal pressures up to 1200 psi. 

NMR studies under high pressure have increased markedly in the last two decades. Technically, these measurements are most frequently carried out using sapphire NMR tubes, and this methodology has been modified over the years. These pressure experiments are usually carried out with the joint aims both of determining activation volumes and of shifting chemical equilibria. Occasionally, details with respect to the pressure dependence of NMR parameters are published. ... 

DC Roe. Sapphire NMR tube for high-resolution studies at elevated pressure. 

Laurenczy G, Dyson PJ (2008) Determination of the viscosity of the ionic liquids [Bmim] [PFs] and [BmimllTFjN] under high CO2 gas pressure using sapphire NMR tubes. Zeitschrift Fur Naturforschung Section B-a. J Chem Sci 63 681-684... 

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