High temperature NMR measurement

In the high-temperature spectra, where the solvent as well as the dissolved investigated substance are solely ionic in character, an extremely fast mutual exchange of nuclei. 

high-temperature spectra bands of all structural units could not be observed, as it is the case of low-temperature spectra in molecular systems, and moreover, only one band averaging contributions from all structural units that can be expressively wide-spread. In the case of bands with higher half-width, it is thus necessary to use for their mathematical description, the weighted combination of the Lorentzian and Gauss functions. 

Nucleus Frequency/ MHz (9.4 T) Number of scans Pulse length/ xs Recycle delay/s Reference substance 

In alkali fluoroaluminate solid compounds, aluminum is present only in octahedral coordination with fluorine. According to Spearing et al. (1994) and Smith and Van Eck (1999), their Al chemical shifts range between -13 and -1.4 ppm and are typically more shielded than the AlOe octahedrons in oxide compounds. Only a few studies report lower coordination numbers for Al in fluorides. Kohn et al. (1991) have described the Al MAS NMR spectra of glasses of jadeite mixed with cryolite in terms of the 5-fold and 6-fold coordination of aluminum at 22 and -5 ppm, respectively. Herron et al. (1993) reported a Al chemical shift at 49 ppm for the tetrahedral anion AIFJ in a [l,8-bis-(dimethylamino) naphtathalene H ] [AIFJ] saturated solution. 

In these liquids, for all the observed nuclei, the high-temperature NMR spectrum consists of a single, narrow line, characterized by its position (isotropic chemical shift) and its line width. This single sharp line reflects rapid exchange between the different available environments (rapid as compared to NMR time scales ranging from 10 to 10 Hz). Consequently, the observed peak position is the average of the chemical shifts of individual species, weighted by their respective populations. 

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Characterization Techniques for Pitch Materials. Among a number of characterization techniques developed in Japan, the technique due to the members of the Society of Heavy Oil, led by Kunugi, stands out as particularly useful. The analytical data are treated by computer methods to construct average molecular structures for the carbonaceous materials. Sanada s group in Hokkaido University used high-temperature NMR and ESR data obtained by in situ measurements of pitch materials in molten salt (21). Much information on mesophase behavior during the heat-treatment process was obtained in this way. 

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

A new high temperature probehead has been constmcted for NMR measurements up to 900 K. The rf-coil made firom molybdenum wire of 0.5 mm diameter is fixed on a boron nitride cylinder which is located inside the furnace tube. The sample volume which is detected by the NMR coil is 10 mm in diameter and 15 mm in height. The furnace consists of a molybdenum wire resistance heater noninductively wound on an alumina tubing. A dc power supply has been used to avoid parasitic signals. The whole system is placed inside a water cooled vacuum recipient fitting into the cryomagnet. For recent reviews on high temperature NMR techniques see e.g. [11,12]. 

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. 

H-NMR analyses were performed on an IBM Instruments AF2S0 spectrometer and IR analyses were obtained fi om an IBM FTER Mcxlel 44. High temperature FTIR measurements were made in-situ on films whicdi were solution spun onto NaQ plates. The sample cell was coimnected to a temperature controller for isothermal and step-ramp temperature profiles. The relative amount of imidization was determined from the ratio of the 1776 cm imide peak area to the peak area of the final, fully cured film (350 °C, 30 minutes) at each isothermal temperature to remove the temperature effect on the peak area. 

In situ NMR measurements can be made in conjunction with down-hole fluid sampling [5, 6]. The NMR relaxation time and diffusivity can be measured under high-temperature, high-pressure reservoir conditions without loss of dissolved gases due to pressure depletion. In cases when the fluids may be contaminated by invasion of the filtrate from oil-based drilling fluids, the NMR analysis can determine when the fluid composition is approaching that of the formation [5, 6]. 

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. 

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