High-pressure NMR measurements

The material for any high-pressure NMR vessel must be non-magnetic and of high mechanical strength. The most commonly used materials for this purpose are beryllium copper (Berylco) and titanium alloys which allow building pressure vessels up to pressures of about 8-10 kbar. In such an 

4K X 512 real data points with zero-filling applied in the dimension to 2 K data points. For eaeh inerement 16 or 32 FID s were aeeumulated with a relaxation delay of 2 s. 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings 

Here 6 is the instantaneous angle between a given C-D bond vector and the axis of rotational symmetry of the molecules, i.e., the bilayer normal. The brackets denote an average over the time scale of the experiment 10 s) so that Sen is the time-averaged orientation of the particular C—D bond with respect to the bilayer normal. 

Furthermore, in all phases studied the first spectral moment Mi of the H NMR spectra can be calculated and the weighted mean splitting of the H NMR spectrum can be obtained, which is proportional to the average chain orientational order parameter of the lipid, using  

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Recently, high-pressure NMR measurements have enabled us to identify an intermediate conformer of the hamster prion protein, which has been suggested to be the PrP factor responsible for the PrP /PrP ° conversion or a closely related precursor. ... 

In order to carry out high-pressure NMR measurements it is necessary to generate and maintain a pressure and to transfer it through the pressure-... 

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 ...

R640 Y. Kohori, High Pressure NMR Measurements at Low Temperatures , Seramikkusu, 2011, 46, 413. 

From Eq. (6), the self-diffusion coefficient can be related to the viscosity of the solution in a simple manner. The measurement of solution viscosities at high pressures and temperatures is a difficult, time-consiuning task. Using high-pressure NMR measurements of the diffusion coefiicient under these extreme conditions, the solution viscosity can be easily estimated from Eq. (6). Therefore, high-pressme NMR not only provides a very good estimate of solution viscosity under extreme conditions of pressure and temperature but also provides an experimental method to test the different microscopic physicochemical models of translational dynamics in solutions. 

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]. 

Determination of reaction mechanisms by combining the observed intermediates in a catalytic cycle. To do this, it is often necessary to measure under different conditions - that is, variable temperature NMR. The use of high-pressure NMR cells is crucial in order to measure under the real catalytic conditions. The EXSY experiment helps to unravel exchange pathways, both intra-and intermolecular. 

H. Yamada, in Glass Cell Method for High-Pressure, High-Resolution NMR Measurements. Applications to the Studies of Pressure Effects on Molecular Conformation and Structure, Ed. 

Recently, we have observed a similar inapplicability of the SED law to a much larger molecule (benzene) in water in a high pressure NMR relaxation measurement The initial compression does not decrease but increases the rotational correlation time for a benzene molecule in water at 30 C. In other words, the viscosity exponent defined by Eq. (3) is negative at lower pressures. The exponent is turned over to a large positive at higher pressures. 

Almost every imaginable spectroscopic and electrochemical method have been used for the determination of A The development of a transportable high pressure unit [52] made such measurements accessible to many laboratories. High-pressure NMR work, however, is more difficult to carry out and requires special equipment [53, 54]. 

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. 

Much work has been carried out on pressure-dependent studies of properties (including diffusion measurements and spin-lattice relaxation times) in water and aqueous solutions. There has been particular interest in supercooled metastable states. Heterogeneous nucleation is dependent on the sample volume. Consequently, a small volume capillary cell is required for high-pressure NMR studies on aqueous solutions under these conditions. This was discussed in Section 5.1.2.3. In order to stabilize aqueous solutions under... 

With the advent of Fourier-transform NMR and powerful superconducting magnets, spin-echo NMR is now the method of choice for high-pressure intradiffusion measurements. Individual measurements take of the order of few (2-30) minutes to collect rather than the 24-50 h needed for diaphragmcell measurements. Because of the higher sensitivity and the faster collection of data it is now possible to study by NMR systems with diffusion coefficients which are as low as 10 s whereas the time scale for diaphragm-cell mea-... 

Fig. 1.8. Design features of a probe head for high-pressure NMR (400 MHz) measurements 1 - O-ring 2 - probe jacket 3 - thermal insulation 4 - polyvinyl chloride 5 - O-ring ...

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