Continuous wave, NMR

Although continuous wave NMR is sufficient for naturally abundant nuclei with strong magnetic moments such as hydrogen, fluorine and phosphorous, the study of low abundance nuclei and/or weak magnetic moments such as carbon 13 or silicon 29 requires pulse NMR. 

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

We see from Table 1 that the only observable nuclide for oxygen, 0, has a very low natural abundance, even in comparison with those of popular nuclides like (1.108%) and N (031%). Moreover, its quadrupole moment prevents any practical utilization of polarization transfer techniques like INEPT or DEPT, now widely used in and N NMR spectroscopies. A range of chemical shifts much wider than those of and N is an important point in favour of utilization of 0. All these properties did not prevent important applications of O NMR spectroscopy in organic chemistry, even from the times of continuous wave NMR spectroscopy. Interesting examples of such pioneering works can be found both at natural abundance as well as with enriched samples . However, also in the case of O NMR spectroscopy, FT NMR proved to be decisive for its development. 

Figure 8.3 gives the basic layout of a continuous wave NMR spectrometer. These intruments were the original type of instrument and have largely been replaced by Fourier transform instruments. However, the principles of operation are broadly similar ... 

In a continuous-wave NMR (CW NMR) experiment the HY frequency is slowly scanned and each nucleus resonates at its specific frequency, which is a function of the examined nucleus and the electron density that surrounds it. This experiment can be long and tedious because if high resolution is desired, the frequencies must be swept very slowly. If the concentration of the examined nucleus is low, a large number of repeated sweeps may be needed to obtain a detectable signal. 

We define an experimental quantity, which we call the quadrupole splitting (13), as the distance (in Hz) between two adjacent peaks obtained in the CW (continuous wave) NMR spectrum, corresponding to the distance A-B or B-C in Figure 2. For an oriented liquid crystalline sample one obtains from Equation 6 ... 

For a pulse-type NMR experiment, the assumption has a straightforward interpretation, since the pulse applied at the moment zero breaks down the dynamic history of the spin system involved. The reasoning presented here, which leads to the equation of motion in the form of equation (72), bears some resemblance to Kaplan and Fraenkel s approach to the quantum-mechanical description of continuous-wave NMR. (39) The crucial point in our treatment is the introduction of the probabilities izUa which are expressed in terms of pseudo-first-order rate constants. This makes possible a definition of the mean density matrix pf of a molecule at the moment of its creation, even for complicated multi-reaction systems. The definition of the pf matrix makes unnecessary the distinction between intra- and inter-molecular spin exchange which has so far been employed in the literature. 

Constant K and the exponent a of the Mark-Houwink equation, 255 Constitutive properties, 60 Contact angle, 232 Continuous chain model, 489-90 Continuous wave NMR, 365 Contour length, 247, 490 Controllability, 800... 

As noted earlier, NMR lines from solids are very broad, but as early as 1958 Raymond Andrew and Irving Lowe independently showed that rapid spinning of the sample under certain conditions can reduce the line width. During the period 1965—1975 a number of clever new methods were developed (primarily in the laboratories of John Waugh, Alex Pines, Peter Mansfield, and Robert Vaughn) to employ both pulsed and continuous wave NMR methods to obtain narrow lines in solids. We shall describe the basic ideas behind these techniques and give some illustrations of their application. 

Partial saturation can be an impediment in continuous wave NMR studies (as described in Section 2.9), because the reduced magnitude of the magnetization causes a reduced NMR signal, but it is of little consequence in modern pulse experiments, in which the full magnetization is manipulated, as we shall see, in a time that is normally much less than Tx. In some instances in which there is no component of magnetization along the z axis parallel to B0, but there is a component in the xy plane, the term saturation is occasionally (but inappropriately) used. 

CONTINUOUS-WAVE NMR IMAGING IN THE SOLID STATE 117 number of harmonics of wm ... 

Figure 9.26. Schematic diagram of a continuous wave nmr spectrometer...

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