NMR tube

Note 2. Traces of oxygen will induce polymerization of the cumulene. The NMR tube must therefore be filled with nitrogen before putting the sample in it. Low-temperature NMR gives the most representative results. 

The role of complexes 23-28 as catalyst precursors in the ring closing metathesis reactions was investigated. Three different diene substrates diethyldiallyl-malonate (29), diallyltosylamine (30). and dielhyldi(2-methylallyl)malonate (31) were added to the NMR tubes containing a solution of 5 mol% of catalyst precursor in an appropriate deuterated solvent. The NMR tubes were then kept at the temperatures reported in Table X. Product formation and diene disappearance were monitored by integrating the allylic methylene peaks in the H NMR spectra and the results are presented in Table X and the catalytic transformations are depicted in Scheme 3. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

Why is degassing of sample solutions in NMR tubes essential before nOe experiments, and why are aqueous solutions (solutions in DgO) not generally degassed in the NMR tubes ... 

Fig. 9.6 Photograph of cross-linked polymer stick in different states of swelling for the potential use as alignment medium. From left to right unswollen polymer stick in a standard 5-mm NMR tube, the unswollen polymer stick after polymerization, the polymer stick swollen in chloroform, and the...

Fig. 1.26 I maging of 129Xe using (a) regular imaging and (b) chemical shift selective imaging sequence along the cross section of three NMR tubes containing toluene, water...

Next we examined the reaction between the Mo complex 70 and linear carbonate L-C in an NMR tube [26]. The result was quite interesting, as summarized in Scheme 2.21. 

We have chosen the PVC diad and triad compounds 2,4-dichloropentane (DCP) and 2,4,6-trichloroheptane(TCH) as subjects for our attempt to obtain quantitative kinetic data characterizing their (n-Bu)3SnH reduction in the hope that they will serve as useful models tor the reduction of PVC to E-V copolymers. Unlike the polymers (PVC and E-V), DCP and TCH are low molecular weight liquids whose high resolution 13C NMR spectra can be recorded from their concentrated solutions in a matter of minutes. Thus, it is possible to monitor their (n-Bu)3SnH reduction directly in the NMR tube and follow the kinetics of their dechlorination. 

Significant evidence for the extreme stability of the carbenium ions derived from isopropenylferrocene was also provided by its -H-NMR spectrum in trifluoroacetic acid at 20°C which was similar to that reported earlier. However, repeated scans of this sample at 1 h intervals showed no changes until a period of 10 h had elapsed. Thus it seems that the formation of the carbenium ion is very facile and once formed it is extremely stable in a non-nucleophilic environment. On pouring the contents of the NMR tube into methanol, no precipitate was seen. 

We have already mentioned that by simply adding deuterated water to the chloroform solution and shaking the NMR tube leads to H-D exchange, so that the OH signal disappears. 

A long capillary with a computer-controlled switching valve (the instruments must be separated by 2-3 metres because of the strong magnetic field) connects the exit from the HPLC with the probehead. The latter is completely different in its construction from conventional probeheads instead of the NMR tube there is a small flow cell, the volume of which is 40-100 pi. The transmitter and receiver coils are attached directly to the cell in order to maximize the sensitivity. 

Fig. 37 Nitrogen-15 spectra of two aminophosphonates (structures as shown). 10-mm NMR tube, concentration 25% in CDC13, proton decoupling, relaxation delay 15 sec, measurement time 12 hours...

Silicon is one of the most abundant elements in the earth s crust. We find silicon in sand and quartz, and in our NMR tubes. Of course we also find it in the computers which run our NMR spectrometers. 

Reaction rate experiments were conducted in NMR tubes sealed with Teflon valves. In an inert atmospere glovebox, catalysts and internal standard, TMS4C, were weighed into the tube, followed by addition of solvents and reactants. The tube was immediately inserted in the preheated (50 °C) probe of a 500 MHz Varian Unitylnova spectrometer. To acquire spectra the sample was irradiated twice with a 30° pulse, 5 sec acquisition time, and 120 sec delay. 

Kinetic Studies. Self-condensation of A in pyridine solution was conducted at 100°C in 5 mm NMR tubes and the 60 MHz -NMR spectra of the reaction mixtures were recorded at room temperature after various reaction times. The relative concentrations of o- and p-methylol groups and of methylene ether linkages were determined from the relative intensities of the resonances observed at 6 5.25 (o-methylol), 5.0 (p-methylol), 4.8-4.9 (methylene ether) and 2.3 (CH3) ppm, respectively. Resonance areas were measured by cutting and weighing expanded spectra. Initial estimates of koo were obtained by considering the reaction to be a second order process... 

Of course, there is no point in overfilling your NMR tubes. This can make shimming more difficult (but certainly not impossible as in the case of too low a sample depth) but more importantly, it merely wastes materials and gives rise to unduly dilute samples giving reduced signal/noise. Any sample outside the receiver coils does not give rise to signal. 

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