Pulse Fourier transform

Like NMR spectrometers some IR spectrometers oper ate in a continuous sweep mode whereas others em ploy pulse Fourier transform (FT IR) technology All the IR spectra in this text were obtained on an FT IR instrument... 

From the Heisenberg uncertainty principle as stated in Equation (1.16) estimate, in cm and Hz, the wavenumber and frequency spread of pulsed radiation with a pulse length of 30 fs, typical of a very short pulse from a visible laser, and of 6 ps, typical of pulsed radiofrequency radiation used in a pulsed Fourier transform NMR experiment. 

Fourier transform spectroscopy in the radio frequency region has been applied most importantly in pulsed Fourier transform NMR spectroscopy, which is not a subject which... 

The isotope has a nuclear spin quantum number I and so is potentially useful in nmr experiments (receptivity to nmr detection 17 X 10 that of the proton). The resonance was first observed in 1951 but the low natural abundance i>i S(0.75%) and the quadrupolar broadening of many of the signals has so far restricted the amount of chemically significant work appearing on this rcsonance, However, more results are expected now that pulsed fourier-transform techniques have become generally available. 

Why are pulse Fourier transform (PFT) NMR experiments preferred over continuous wave (CW) NMR techniques ... 

Greatly enhanced sensitivity with very short measuring time is the major advantage of PFT (pulse Fourier transform) experiments. In the CW (continuous wave) experiment, the radiofrequency sweep excites nuclei of different Larmor frequencies, one by one. For example, 500 s may be required for excitation over a 1-KHz range, while in a PFT experiment a single pulse can simultaneously excite the nuclei over 1-KHz range in only 250 jits. The PFT experiment therefore requires much less time than the CW NMR experiment, due to the short time required for acquisition of FID signals. Short-lived unstable molecules can only be studied by PFT NMR. 

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. 

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... 

Table 5.16 Main characteristics of high-resolution pulse Fourier-transform 1-NMR spectroscopy...

OES Optical emission spectrometry PFT-NMR Pulse Fourier-transform NMR... 

So far, we have shown where the signal comes from, but how do we measure it There are two main technologies continuous wave (CW) and pulsed Fourier transform (FT). CW is the technology used in older systems and is becoming hard to find these days. (We only include it for the sake of historical context and because it is perhaps the easier technology to explain). FT systems offer many advantages over CW and they are used for all high field instruments. 

Broadband proton-decoupled pulse Fourier transform C n.m.r. were recorded in deuterochloroform at 20 MHz using a Varian CFT-20 spectrometer. 

NMR spectra were obtained in continuous wave mode on a Varian T-60, and in the pulsed Fourier transform mode on a Varian HR-220 with Nicolet TT-100 Fourier transform accessory, a Nicolet NT-300 wide bore system, and a Bruker WM-500. 13C T1 data were obtained... 

Instrumentation. The NMR Process. Chemical Shift. Spin-spin Coupling. Carbon-13 NMR. Pulsed Fourier transform NMR (FT-NMR). Qualitative Analysis - The Identification of Structural Features. Quantitative Analysis. Applications of NMR Spectrometry. 

A considerable improvement in speed and sensitivity can be achieved with a pulsed Fourier transform (FT) spectrometer. Here the sample is subjected to a series of short duration high intensity RF pulses (1-100 us)... 

NMR has been used comparatively little for quantitative analysis although peak areas are directly proportional to concentration. The principal drawbacks are the expensive instrumentation and a lack of sensitivity. The latter can be improved with the aid of computers to accumulate signals from multiple scans or by using a pulsed (Fourier transform) technique. Relative precision lies in the range 3-8%. 

The NMR spectrum of calcitriol, recorded on a Varian XL-100/Nicolet TT-100 pulsed Fourier Transform NMR spectrometer, with internal deuterium lock, is shown in Figure 2 (2). The spectrum was recorded using a solution of 0.84 mg of sample dissolved in 50 microliters of CD OD (100%D) containing 1% v/v tetramethylsilane in a 1.7 mm capillary tube. The spectral assignments are given in Table I. 

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). 

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

Suppose that a pulse Fourier transform proton NMR experiment is carried out on a sample containing acetone and ethanol. If the instrument is correctly operated and the Bq field perfectly uniform, then the result will he a spectrum in which each of the lines has a Lorentzian shape, with a width given hy the natural limit 1/(7tT2). Unfortunately such a result is an unattainable ideal the most that any experimenter can hope for is to shim the field sufficiently well that the sample experiences only a narrow distribution of Bq fields. The effect of the Bq inhomogeneity is to superimpose an instrumental lineshape on the natural lineshapes of the different resonances the true spectrum is convoluted by the instrumental lineshape. 

Spectra.—Pulsed Fourier-transform n.m.r. spectroscopy of labelled steroids,... 

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. 

Designations such as upfield/highfiekT or "downfield/lowfield" are meaningless in a pulse Fourier transform experiment, because, in contrast to the older Continuous Wave ( Sweep-") technique all nuclei experience exactly the same pulse frequency, i.e., the same excitation field. Thus, it is better to say that a nucleus is shielded or deshielded, respectively, and that a signal is shifted to lower or higher frequency, respectively487. Since, however, the term .vyu-upheld rule is well established in the literature, it is used in this section. 

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 newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

Fluxionality. The fluxional behavior of unsaturated organic ligands in mono- and oligonuclear organometallic complexes was well documented 109a) as a result of extensive NMR spectroscopic investigations. The advent of pulse Fourier transform... 

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