High pressure NMR spectra

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]. 

Deuterium gas experiments, continuous NMR and GC/MS analysis, in situ high-pressure NMR spectra and the isolation of some intermediates provided Fish with sufficient information to propose the mechanism shown in Scheme 16.16 for the hydrogenation of quinoline to THQ, catalyzed by [Rh(NCMe)3Cp ]2+ (40°C, 33 bar H2, CH2C12) [55]. 

Studies of high-pressure NMR spectra have been carried out in by one of two ways, known, respectively, as the high-pressure probe method and the high-pressure cell method. High-pressure probes usually show a better sensitivity, due to the better filling factor. 

In this chapter we will discuss both applications, but first we will discuss the different experimental approaches used to produce high pressure NMR spectra. 

Figure 5 The time-resolved high-pressure NMR spectra of the back reactions for Cp Re(CO)(r -C2H4)2 and CO at 345 bar and 30°C in supercritical ethylene (A) Cp Re(CO)3, (B) Cp Re(CO)2(Ti -C2H4) and (C) Cp Re(CO)(Ti2-C2H4)2. The spectra are spaced about 80 min apart from bottom to top.

Catalysis studies are also the goal in a paper by Bianchini and co-workers dealing with the in situ high-pressure NMR spectra of the hydroformyla-... 

Further information on the reaction intermediates is achieved by in situ NMR experiments. Because the signals in NMR spectra depend upon the concentration of the investigated species, a quantitative treatment is possible. Bianchini and coworkers investigated the hydroformylation of 1-hexene [62], using high-pressure NMR spectroscopy to evaluate the influence of synthesis gas on the equilibria of rhodium triphenylphosphine species. They were able to establish at least four resting states of rhodium (catalyst species that do not participate directly in the reaction). When synthesis gas interacted with... 

In Vivo Proton NMR Studies in Skeletal Musculature is covered by J. Machann, G. Steidle, C. Thamer, I. Mader and F. Schick this is followed by an account on Quality Assessment of Horticultural Products by NMR from B. P. Hills and C. J. Clark Applications of NMR to Thermostable Proteins is reviewed by E. Alberti, R. Consonni and L. Zetta High Pressure NMR Studies on Lyotropic Lipid Mesophases and Model Biomembrances are reviewed by R. Winter W. S. Veeman discusses Diffusion in a Closed Sphere finally L. Griffiths covers Automatic Analysis of NMR Spectra. It is a pleasure for me to record my gratitude to all of the authors and to the production team for this volume. 

Fig. 5.12 High-resolution NMR spectra for the n-alkyl moieties of A/,A/-di-r -propyl-1-naphthoamide as a function of temperature and pressure.

Multlnuclear NMR studies at atmospheric pressure, i.e., under less extreme conditions, have yielded valuable structural and mechanistic information on transition metal carbonyl clusters and both the above limitations suffered by infrared spectroscopy have now been overcome by measuring high resolution NMR spectra at high pressure. Well-resolved NMR spectra, with excellent slgnal/nolse, have been obtained on the catalytic system involved In the formation of ethylene glycol. 

There is yet another important origin of observed pressure changes in the NMR spectra of biomolecules in aqueous solutions. NMR studies of protein denaturation using temperature or chemical means are well established but pressure also leads to reversible denaturation as discussed in more detail in the section on applications. In the case of model membranes the use of pressure leads to very rich barotropic behaviour and high pressure NMR techniques can establish pressure-temperature phase diagrams for the membrane system studied. 

SFC-NMR is available from 200 to 800 MHz, and is suitable for all common NMR-detected nuclei. SFC/SFE-NMR requires dedicated probe-heads for high pressure (up to 350 bar) and elevated temperature (up to 100 °C). SFC-NMR is carried out with conventional packed columns, using modifier, pressure and temperature gradients. The resolution of 1H NMR spectra obtained in SFE-NMR and SFC-NMR coupling under continuous-flow conditions approaches that of conventionally recorded NMR spectra. However, due to the supercritical measuring conditions, the 111 spin-lattice relaxation times 7) are doubled. 

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