CW-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]. 

It would be pertinent to point out (25,27) that the trisubstituted isomer of the enamine of 2-aIkylcyclohexanone reacts in a quantitative manner with ethyl azodicarboxylate to give the addition product (35). This reaction in Conjunction with NMR spectroscopy can thus be employed for the determination of the amount of the trisubstituted isomer. According to the authors, hydrolysis of 35 furnishes the corresponding cw-2,6-disubstituted cyclohexanone (36) this seems unlikely since it would involve the stereo-electronically unfavored equatorial protonation of the enamine. 

The principal advantage of FT NMR spectroscopy is a great increase in sensitivity per unit time of the experiment. A CW scan generally takes of the order of one hundred... 

Fourier transform NMR spectroscopy, on the other hand, permits rapid scanning of the sample so that the NMR spectrum can be obtained within a few seconds. FT-NMR experiments are performed by subjecting the sample to a very intense, broad-band, Hl pulse that causes all of the examined nuclei to undergo transitions. As the excited nuclei relax to their equilibrium state, their relaxation-decay pattern is recorded. A Fourier transform is performed upon this relaxation-decay pattern to provide the NMR spectra. The relaxation-decay pattern, which is in the time domain, is transformed into the typical NMR spectrum, the frequency domain. The time required to apply the Hl pulse, allow the nuclei to return to equilibrium, and have the computer perform the Fourier transforms on the relaxation-decay pattern often is only a few seconds. Thus, compared to a CW NMR experiment, the time can be reduced by a factor of 1000-fold or more by using the FT-NMR technique. 

Fukui, K., K. Sato, D. Shiomi, T. Takui, K. Itoh, K. Gotoh, T. Kubo, K. Yamamoto, K. Nakasuji, and A. Naito. 1999. Electronic structure of a stable phenalenyl radical in crystalline state as studied by SQUID measurements, cw-ESR, and 13C CP/MAS NMR spectroscopy. Synth. Metals 103 2257-2258. 

Database, which contains as much data on CWC-related chemicals as possible. Since on-site analysis in particular by MS is planned for future verification activities, library of mass spectra is mandatory. Retention times of chemicals are collected for chromatography analyses. Although NMR spectroscopy is not suitable for on-site analysis, it is nevertheless considered an essential technique (as may be also IR) for laboratories specialized in the detection and identification of CW agents and related chemicals. Qualified laboratories from all parts of the world were therefore asked to submit their mass, NMR, and IR spectra and retention time data to be included in the OCAD. 

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