Toroid probes

High-pressure NMR studies for catalysis and with supercritical fluids will lead to a much broader application of sapphire NMR cells and to special applications of toroidal probes. The sapphire tube technique can today be considered as a standard, cheap and easily applicable technique to study samples under medium gas pressures, up to 100 MPa. 

Commercially available instruments, one of which is shown in Figure 10, have useful ranges extending from 50 pS cm to 2 S cm with relative accuracy of a few tenths of a percent of full-scale, after-temperature compensation. A temperature sensor is incorporated in the toroid probe, and a compensation circuit corrects readings to the standard reference temperature of 25 °C. 

Specially designed NMR probes capable of withstanding high internal pressures have been described. Improved sensitivity and resolution have been obtained with toroidal probes. They consist of a toroidal-shaped RF detector inside a pressure vessel with RF and gas connectors mounted on the top and bot tom, respectively, and are useful for studying pressurized reactor effluent. 

During the course of the study on the simplest archetypal twin-walled structures in PCNT material described here, the observation of lijima et al.[5 on related multi-walled hemi-toroidal structures in ACNTs appeared. It will be most interesting to probe the differences in the formation process involved. 

The toroidal pressure probe, introduced in 1989 by Rathke and coworkers [249, 250], has been modified by Woelk and coworkers [249, 251], who have used a toroid cavity NMR autoclave for high pressure PHIP NMR experiments. Figure 1.43 shows the PHIP spectrum of the [Rh(norbornadiene)(PPh3)2]PF6-catalyzed hydrogenation of 1,4-diphenylbutadiyne with 40 bar of 50% emiched para-H2 [252]. The spectrum from the same reaction at ambient H2 pressure is shown in the inset [253]. The two absorption/emission PHIP patterns in both spectra indicate that para-H2 is transferred pairwise during the catalytic cycle. 

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

Another class of toroidal NMR probes is formed by toroid cavity detectors (TCD) [31]. These TCD have been mounted into cylindrical metallic autoclaves to study spin relaxation effects of gases under pressure [31]. If the toroid cavity detector is the metallic pressure vessel itself it is called a toroid cavity autoclave (TCA) probe [32, 33]. These probes can be tuned to higher resonance frequencies than TCDs and also show better spectral resolution [29]. Figure 2.12 shows the design of a TCA where the autoclave body is built from phosphorus bronze [33] keeping the field distortions of the magnetic field Bq, induced by susceptibility mismatches... 

Figure 2.11 High pressure NMR probe using a toroid detector (from Ref [29]). SP-1 Vespel...

Figure 2.12 Exploded (left) and assembled (right) view of a toroid cavity autoclave probe for in situ investigations under high gas pressures or in supercritical fluids. Autoclave base (A) and autoclave body (P-bronze, B) thermocouple (C) coaxial heater (D) PTFE ring (E) central conductor (Cu/Be ring, F) nonmagnetic pin from male coaxial connection (G) RF feedthrough (from Rathke [28], H) base-plate (MACOR, I) fixing screws (P-bronze, J) PEEK capillary (l<) ceramic ball (Si3N4, L) PTFE seal (M).

Figure 2.19 High pressure NMR probe and toroid detector equipped with stir coii [57].

Figure 8.1.5 Schematic of a continuous-flow 13 C probe employing an additional proton toroidal coil...

Measurements of the poloidal distribution of the ELM energy deposition at the divertor target, such as those in Fig. 3.18, are usually taken at only at one toroidal location. Few experiments have been carried out with measurements of the divertor ELM energy and particle fluxes (with infra-red cameras and Langmuir probes) at various toroidal positions. These experiments show that the divertor ELM energy flux is, approximately, toroidally... 

Autoclave Cells. The NMR probe is constmcted inside a high pressure autoclave. Autoclave cells are principally used for volume of activation studies. Toroid cells have been used for studies of organometallic reactions. ... 

Figure 5 Schematic drawing of a toroidal detection probe as used by Rathke [16]. (Reproduced with permission)...

Pressure probe design using a toroid NMR detector. 

More recent research has investigated a stirring device which can easily be adapted to toroid cavity probes, as shown in Figure 3.2-13. The device comprises a copper coil form wound with several turns of fine Teflon-coated cop-... 

Figure 3.2-13 Stirrer for dissolving solids in a toroid cavity probe.

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