NMR cells

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. 

Figure 6.34 Some typical HPLC peak widths compared with LC-NMR cell active volume for a 4-mm probe. For a peak 90 s wide only 10% of the peak is in the active volume of the flow cell. Reproduced from [57] with permission from John Wiley and Sons Ltd.

In the following, we will present a selected choice of high-pressure cells developed for high-resolution NMR. We have divided the applications of high-pressure NMR into three sections. First, we will describe high pressure equipment used to study liquids under hydrostatic pressure up to 1000 MPa then, we present some special approaches used to study supercritical fluids which, besides moderate pressure, often need high temperatures. Finally, we discuss NMR cells to study solutions under gas pressure, a situation which is quite common in catalysis. 

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. 

A more recent in situ P H HP NMR study of styrene/CO copolymerisation by the same [(R,S)-BINAPHOS]Pd-acyl complex applying both diffusion-controlled (non-spinning sapphire NMR tube) and reaction-controlled (flow NMR cell) conditions has provided evidence for other intermediate species as well as information on the relative rates of CO insertion into isomeric Pd-acyls [6b, 7gj. Figure 7.15... 

Nuclear magnetic resonance (NMR) measurements are usually considered to be slow processes, but recent advances in the design of flow-through NMR cells have allowed the method to be applied in combinatorial chemistry 97). These technological improvements were applied to the development of two NMR-based high-throughput ee assays for evaluating the products of enzyme- or transition metal-catalyzed reactions 98). 

J.C. Metcalfe, T.R. Hesketh, G.A. Smith, Free cytosolic Ca measurements with fluorine labelled indicators using F NMR, Cell Calcium 6 (1985) 183-195. 

In addition to these acidity/exchange effects, side reactions with supercritical CO2 have also been studied [72]. The authors reported experiments in a high-pressure NMR cell withstanding pressures up to 1000 bar. For methanol/ IL mixtures, NMRs showed signals of bicarbonafe and mefhyl carbonate ... 

Flow-through NMR cells for, e. g., BEST (Bmker Biospin GmbH, Rheinstet-ten, Germany) or VAST (Varian, Palo Alto, CA, USA) spectrometer (300 MHz)... 

Several flow-through NMR cells are commercially available, for example, the BEST -NMR system (Bruker Biospin) described here or the VAST NMR system (Varian) (Figure 9.6). In addition to the flow-through cell and the NMR spectrometer (300 MHz), the system requires an autosampler, for example, aGilson215 autosampler [21]. 

The typical peak width with analytical columns of 4.6 mm i.d. and a 1 ml/min flow rate is of the order of 10-30 s. The acquisition of NMR spectra with a short relaxation delay and an acquisition time of below 1 s allows the acquisition of 8-24 transients for one spectrum during the presence of a peak in the NMR cell. This low number of transients limits the detectable amount of sample to 5-10 xg per compound. 

For the on-line SFE-NMR experiments, the set-up shown in Figure 7.2.17 can be used. A main pump serves an HP supercritical fluid chromatograph (G1205A), with analytical HPLC columns being used as extraction cells. The continuous-flow NMR cell is connected between the column outlet and the back-pressure regulator. 

The second alternative is to direct specific samples to the NMR that are of particular interest. The sample can then be trapped in the cell and data acquired from an adequate number of pulses to provide the required resolution. Subsequently, the sample can be expelled from the cell using solvent supplied directly from the chromatography pump. The third alternative is to direct the eluent from the column to a sample loop where it can be stored until the spectrometer is available to take data. If necessary, a number of solutes can be stored in different loops and they can be examined when convenient. When the data has been acquired from one sample, the solute stored in the next loop can then be displaced into the NMR cell. Samples that have been examined can either be displaced to waste or collected for further examination. A photograph of the Varian flow control device for the LC/NMR system is shown in figure 41. 

Studies of the catalytic reactions in the adsorbed phase require conditions of controlled atmosphere, which become especially challenging if the adsorbate is only weakly adsorbed or catalyst-adsorbate samples are air/moisture sensitive and require preparation on a vacuum line, including loading into the MAS rotor. The NMR cells must thus be carefully sealed. On the other hand, to achieve a sufficiently high rotation speed the NMR ceils must be well balanced (83). 

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