Sample in NMR

There are many possible variations on this simple pulse sequence. For example, in Chapter 4 we learned that it is possible to transmit two signals into the sample. In NMR spectroscopy, a pulse sequence similar to that shown in Figure 10.1 is transmitted at the absorption frequency of the nuclei. At the same time, a second transmitter, tuned to the frequency of the hydrogen ( H) nuclei in the sample, transmits a broad band of frequencies to decouple the hydrogen nuclei from the nu-... 

Coupling between peak width and number of sampling points (i.e., experimental time) is the main reason for the use of non-uniform sampling in NMR. 

There are many possible variations on this simple pulse sequence. For example, in Chapter 6 we learned that it is possible to transmit two signals into the sample. In NMR spectroscopy, a pulse... 

To compare the concentration of material which the techniques of NMR and IR require, let us first discuss sample size. There is no system such as a beam condenser in NMR spectroscopy as there is in IR, so one cannot discuss running micro amounts of samples in NMR. On the other hand, in NMR by operating at higher frequencies and larger magnetic fields a smaller amount of sample can be used than that required for lower fields and frequencies. 

The spectroscopic methods, NMR and mass spectrometry for predicting cetane numbers have been established from correlations of a large number of samples. The NMR of carbon 13 or proton (see Chapter 3) can be employed. In terms of ease of operation, analysis time (15 minutes), accuracy of prediction (1.4 points average deviation from the measured number), it is... 

Lahajnar, G., Zupancic, L, Rupprecht, A. Proton NMR relaxation and diffusion study of water sorbed in oriented DNA and hyaluronic acid samples. In Biophysics of Water (Franks, F., Mathias, S., eds) Wiley, New York (1982) 231-234... 

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 syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

Sample preparation requirements in solid state NMR are strikingly simple because the measurement is carried out at ambient temperature and pressure. Wide-line NMR experiments can be carried out on solid samples in any form, as far as the sample dimensions fit those of the coil in the NMR probe. MAS experiments require the material to be uniformly distributed within the rotor. 

UV spectrum changes dramatically. The salt causes maxima at 251 (4.16), 286 (4.16), 387 (3.56), and 473 (3.37) nm. NMR data for a sample in CF3COOD were presented. It is interesting to note that the nonmethylated derivatives of 130 and 131 are present in solution above pH 6.5 as betaines, forming a protonated pyridine ring and an olate group (91MI3). 

Figure 7. MAS NMR spectrum of a 7-alumina sample in a 0.1 M KH2P0 solution at pH 5.

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

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 ... 

Figure 7.21 One-dimensional NMR imaging. When a magnetic field gradient is applied across a sample, it gives a spectrum that is a profile of the sample in the direction of the gradient.

The use of NMR for studying chemical reactions began about 30 years ago. In 1972, Asahi and Mizuta [11] reported a study of the performance of five different types of flow cells used in NMR studies of chemical reactions (a) a straight glass tube, (b) a pipette-type tube, (c) a spiral capillary in a conventional sample tube, (d) a jet in the base of a conventional tube, and (e) a conventional spinning tube with an inlet at the base and an outlet at a height of about 50 mm. The best results were... 

D. G. Gadian, F. N. H. Robinson 1979, (Radiofrequency losses in NMR experiments on electrically conducting samples),/. Magn. Reson. 34, 449. 

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