Continuous Wave NMR Spectroscopy

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

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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. 

The first application of NMR for the study of humic substances in soil was made by Neyroud and Schnitzer(7) using continuous wave NMR spectroscopy. Later, Gonzalez-Vila et al.(2) were the first to apply Fourier transform NMR for the analysis of dissolved humic substances extracted from soil. The development of... 

The free induction decay of the excited slice contains spatial information, which can be revealed by Fourier transformation as in most other conventional NMR techniques. If the sample is a thin film with a thickness less than the width of the excited slice, the STRAFI image obtained in one spatial dimension is the image across the whole film. Larger samples, however, must be scanned to get the whole ID projection. One way is to sweep the frequency of the pulse, but the older STRAFI method is to move the sample through the field. The first possibility is analogous to the method of frequency sweep in continuous-wave NMR spectroscopy. Field sweep in STRAFI-MRI has also been tried. 

The normal method of scanning NMR spectra (or, for that matter, other spectra) consists of altering the wavelength of the electromagnetic energy supplied and observing absorption whenever the Larmor relation for a particular nucleus is met. This is known as continuous-wave (CW) spectroscopy. 

Figure 14.24 Diagram of a continuous wave NMR spectrophotometer. (Friebolin, H. 1991. Basic One- and Two-Dimensional NMR Spectroscopy, 8, Figures 1-6. Weinham, Germany VCH Publishers. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)...

With the development of Fourier transform (FT) techniques in NMR spectroscopy (early 1970s), the first major advance in the NMR technology was made. A significant increase in the sensitivity, as compared to the conventional continuous wave method, resulted in the NMR spectroscopy of rare nuclei, particularly 13C NMR, which is essential for polymer studies. The 13C NMR analysis of swollen lightly crosslinked polymers was made possible. The relaxation measurements, based on the different pulse sequences, provided additional information on the network dynamics. 

I assume that you are conversant with basic principles of XH or proton NMR spectroscopy as applied to small molecules. In particular, I assume that you understand the concepts of chemical shift (8) and spin-spin coupling, classical continuous-wave methods of obtaining NMR spectra, and decoupling experiments to determine pairs of coupled nuclei. If these ideas are unfamiliar to you, you may wish to review NMR spectroscopy in an introductory organic chemistry textbook before reading further. 

The NMR spectrum can be recorded in various ways. The earliest commercial NMR spectrometers operated in continuous-wave mode, in which the sample is irradiated at constant frequency v while the magnetic field is swept through a range of values. The rf power absorbed by the sample is recorded at each value of H. When the value of H satisfies the resonance condition, a peak appears in the spectrum. Newer instruments rely on Fourier transform (FT NMR) spectroscopy, in which a sample held in a fixed magnetic field is irradiated with a short, intense burst of... 

The first NMR spectrometers developed were continuous-wave (CW) instruments and they are still in use for proton and fluorine NMR spectroscopy. In these instruments the irradiation frequency is fixed and the magnetic field strength of the magnet is slowly and continuously changed. When the correct magnetic field for the fixed frequency is reached for a proton in a particular chemical environment, then an absorption peak appears in the recorder of the instrument. The area of this absorption peak is a function of the number of protons in the sample that are in this particular chemical environment. 

Proton NMR was the first type of NMR spectroscopy to become available to the researcher. The high natural abundance of the NMR-active H-nucleus (-100%) and its relatively high sensitivity allowed early low-field continuous-wave (CW) spectrometers, with little in the way of signal-averaging capabilities, to successfully produce spectra from milligram quantities of small organic molecules. Such instruments,... 

A major consequence of the introduction of pulse (FT) NMR spectroscopy has been ready access to C data - limited prior to 1970 by factors which render this magnetic nucleus relatively insensitive to continuous wave methods of recording NMR spectra (1 % natural abundance, and low value of the nuclear magnetic moment compared with that of a proton). C-NMR spectra are generally much simpler than corresponding H spectra. When run under conditions where all couplings to protons are removed (by simultaneous wide-band irradiation of proton resonances), a C-NMR spectrum consists of a series of sharp lines, each of which corresponds to the resonance of a nucleus (or nuclei) of specific magnetic environment. Further, since the chemical shift spread of C nuclei (0-200 ppm) is about 20-times that of protons. 

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