Pulse Fourier Transform Method

Radiofrequency spectroscopy (NMR) was introduced in 1946 [158,159]. The development of the NMR method over the last 30 years has been characterised by evolution in magnet design and cryotechnology, the introduction of computer-based operating systems and pulsed Fourier transform methods, which permit the performance of new types of experiment that control production, acquisition and processing of the experimental data. New pulse sequences, double-resonance techniques and gradient spectroscopy allow different experiments and have opened up the area of multidimensional NMR and NMRI. 

The very high accuracy which may be obtained by the pulsed Fourier transform method has been demonstrated using o-phenylene phosphorochloridite.1... 

For NMR spectra, it is known that if s(t) is the free induction decay following a pulse, S((o) represents the slow passage spectrum. We shall also use the FT relationship in other ways—for example, for data processing (in Section 3.4) and for analyzing random molecular motions (in Chapter 8). We return to the use of pulse Fourier transform methods in Section 3.6. 

Carbon is the defining element in organic compounds, but its major nuclide ( C) has a spin of zero. The advent of pulsed Fourier transform methods in the late 1960s made the examination of the low-abundance nuclide (1.11%) apractical spectroscopic technique. The low probability [(0.0111) = 0.0001, or 0.01%] of having two adjacent nuclei in a single molecule removes complications from carbon-carbon couplings. When... 

Since the first report (5) of a pronounced configurational dependence for the anomeric protons of monosaccharides in aqueous solutions, studies in this laboratory have shown (6), (7 ), (8), (9) that proton Ri-values have a variety of stereospecific dependencies and reflect the overall steric environment of a proton. In the following discussion, a brief introduction to the phenomenon of spin-lattice relaxation and to the measurement of proton Ri-values by pulse Fourier transform methods, will be... 

Just as important as these developments in magnet design has been the introduction of pulsed Fourier transform methods, for these permit the performance of new types of experiment by the computerised systems that control the production, acquisition and processing of the experimental data. New pulse sequences increasingly made available by instrument manufacturers within their software suites permit the routine performance of these new experiments an early example is the distortionless enhancement polarisation transfer, or DEPT, experiment to identify the number of protons attached to a carbon by controlling the final... 

Extensive and often difficult chemical degradations were required to determine the distribution of isotopes when C-labeled radiotracers were used in early biosynthetic studies 13,19,56,89). The more recent use of cmr spectroscopy employing the pulsed Fourier transform method has made the study of the ansamycins easier, especially that of their aromatic chromophores, which are not easily degraded. Carbon-13 occurs in 1.1 % natural abundance and natural abundance cmr spectra of many ansamycins have been reported, as described in Section IV. When highly enriched C-labeled compounds are used as precursors in ansamycin-producing cultures, cmr data reveal the extent and site of incorporation of C-labeled precursors 106). 

As an example of applying the Fourier transform method to a non-periodie funetion, eonsider the loealized pulse... 

The introduction of additional techniques such as Pulsed Fourier Transform NMR spectroscopy (PFT-NMR) has considerably increased the sensitivity of the method, allowing many magnetic nuclei which may be in low abundance, including 13C, to be studied. The additional data available from these methods allow information on polymer structure, conformation and relaxation behaviour to be obtained (1.18.20). 

I. P. Gerothanassis, Methods of avoiding the effects of acoustic ringing in pulsed Fourier transform nuclear magnetic resonance spectroscopy. Prog. Nud. Magn. Reson. Spectrosc., 1987,19, 267-329. 

In order to identify the spin multiplicity of the tris(carbene), field-swept two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy was used. This technique is based on pulsed fourier transform (FT) EPR spectroscopic methods and is capable of elaborating straightforward information on electronic and environmental strucmres of high-spin species even in amorphous materials, information that conventional CW EPR cannot provide. The nutation spectra unequivocally demonstrated that the observed fine structure spectrum is due to a septet spin state. 

The most economical and efficient method of sensitivity enhancement in 13C NMR of organic molecules is the pulse Fourier transform technique (PFT) in combination with decoupling methods such as proton broad band decoupling and polarization transfer. These methods will be described in the following sections. 

Fourier transform techniques are used throughout the whole spectroscopic region, particularly in the infrared and visible. As we pass from the microwave region to the far-infrared, Fourier transform methods are still used, but based now on interferometry rather than pulsed methods. Perhaps this region of the spectrum will, in... 

There are two basic methods to determine the resonance condition and to record a NMR spectrum, viz. a) the continuous wave technique (CWNMR) and b) the pulsed NMR or pulsed Fourier Transform technique (FTNMR). 

Another major development in the field of pulsed ESR is the use of Fourier-transform methods. It is this combination that has led to major improvements in NMR spectroscopy however, unfortunately, it is still of relatively limited use to ESR spectroscopists because of the difficulties involved in covering a wide enough range of field. In this technique, the high-resolution spectrum is recovered by Fourier transformation of the entire time-domain signal after one or several pulses (Angerhofer et al., 1988). 

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