High-pressure NMR instrumentation

Nuclear magnetic resonance (NMR) spectroscopy has proved to be a versatile and powerful experimental technique for structural and dynamical studies in chemistry and physics. Although the concentration of species, the temperature, and the magnetic field are often varied in NMR studies, the pressure is normally left constant at its ambient value. This is mainly because apparatus must be constructed specially for high-pressure measurements since it is not commercially available. None the less, there is increasing interest in the development of high-pressure NMR instrumentation. 

In this chapter we review the literature in the period from 1993 to the present several detailed review articles dealing with high-pressure NMR appeared in 1992 and 1993. In view of our own interest, the main emphasis of this report is on high-resolution, high-pressure NMR instrumentation and applications to studies of biochemical systems. 

This report is organized as follows. The next section (Section 2) deals with high-pressure NMR instrumentation and covers the main design features of high-pressure, high-resolution NMR. In Section 3, we include a discussion of two specific applications of high-pressure NMR for the study of model membranes. Section 4 includes several examples of high-resolution two-dimensional (2D) NMR experiments on chemical and biochemical systems performed recently in our laboratory. In order to inform the reader about... 

Since high resolution NMR spectroscopy represents a spectroscopic technique of major importance for chemistry and biochemistry the overview of high pressure NMR instrumentation focuses on high resolution, high pressure NMR techniques. 

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]. 

The application of NMR to the study of chemical reactions has been expanded to a wide range of experimental conditions, including high pressure and temperatures. In 1993, Funahashi et al. [16] reported the construction of a high pressure 3H NMR probe for stopped-flow measurements at pressures <200 MPa. In the last decade, commercial flow NMR instrumentation and probes have been developed. Currently there are commercially available NMR probes for pressures of 0.1-35 MPa and temperatures of 270-350 K (Bruker) and 0.1-3.0 MPa and 270-400 K (Varian). As reported recently, such probes can be used to perform quantitative studies of complicated reacting multicomponent mixtures [17]. 

Since the data-acquisition time for a given S/N scales inversely with the square of the number of molecules sampled, high pressures and large tube volumes are desirable. NMR tubes are produced in a range of diameters, and the largest of these that is compatible with the instrument probe design should be chosen. Tubes of 5-, 10-, and 15-mm diameter with an in-line Teflon valve, which permits easy attachment to a vacuum line via i-in. or -in. Ultra-Torr fittings and which can be spun in the normal way, are available from various suppliers. 

A high-pressure probe in combination with a NMR instrument was used recently in a study of self-exchange reactions for several cyanometalate complexes, Os(CN)6 -/Os(CN)6 -, Mo(CN)g3-/Mo(CN)8 , and W(CN)8 7W(CN)8 - [57]. The rate constant /cexch was found to be strongly influenced by added cations. Moreover, the values of AV were inconsistent with Marcus theory. It was concluded that partially desolvated cations (Li+, Na+, K+, etc.) bridge the reacting anions in the transition state. 

INDOR) however, direct detection is practical with modern instrumentation/ The concentrations and time required for these studies make extensive use of Rh nmr by conventional Fourier Transform methods relatively unattractive for catalytic study applications. The development of a polarization transfer technique (as opposed to an Over-hauser enhancement) known as INEPT,however, promises to allow detection under reasonable concentration and time conditions. " Perhaps one of the more interesting developments has been the development of sample systems that allow the determination of nmr spectra under high pressure, such as 1000 atm of Even without exceptional equip-... 

The human nose neither tries to break the aroma into different constituents nor to quantify the constituents. Unfortunately, there is no effective instrumental analysis to replace the human sense. Analytical instruments such as gas chromatography (GC)/mass spectrometry (MS), high pressure liquid chromatography (HPLC) and NMR spectroscopy could be used to monitor the particular compounds present in a variety of samples. However, their use is... 

Hyphenated TLC techniques. TLC has been coupled with other instrumental techniques to aid in the detection, qualitative identification and, occasionally, quantitation of separated samples, and these include the coupling of TLC with high-pressure liquid chromatography (HPLC/TLC), with Fourier transform infra-red (TLC/FTIR), with mass spectrometry (TLC/ MS), with nuclear magnetic resonance (TLC/NMR) and with Raman spectroscopy (TLC/RS). These techniques have been extensively reviewed by Busch (1996) and by Somsen, Morden and Wilson (1995). The chemistry of oils and fats and their TLC separation has been so well established that they seldom necessitate the use of these coupling techniques for their identification, although these techniques have been used for phospholipid detection. Kushi and Handa (1985) have used TLC in combination with secondary ion mass spectrometry for the analysis of lipids. Fast atom bombardment (FAB) has been used to detect the molecular species of phosphatidylcholine on silica based on the molecular ion obtained by mass spectrometry (Busch et al, 1990). 

The use of pressure as an experimental variable in NMR studies of chemical and biochemical systems leads to added complexity in instrumentation but the unique information gained from the combination of high pressure and NMR techniques justifies fully its use. There are several fundamental reasons for carrying out NMR experiments at high pressures. 

Solids characterization may be performed by Si 9 NMR or XRD. With Si NMR, changes in the crystal dimensions due to sorption have been observed (refs. 1-3). Sorbed phase characterization can be studied by volumetric sorption. Thermodynamically simple molecules (e.g., spherical and small) at low temperatures arc used to study pore volume and sizes. A novel, automated, low pressure sorption instrument (Omicron Tech., Berkeley Heights, NJ, USA) has greatly simplified the analysis of zeolites. The technique has recently been named High Resolution Adsorption (HRADS). 

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