Free induction decay processing

Although the idea of generating 2D correlation spectra was introduced several decades ago in the field of NMR [1008], extension to other areas of spectroscopy has been slow. This is essentially on account of the time-scale. Characteristic times associated with typical molecular vibrations probed by IR are of the order of picoseconds, which is many orders of magnitude shorter than the relaxation times in NMR. Consequently, the standard approach used successfully in 2D NMR, i.e. multiple-pulse excitations of a system, followed by detection and subsequent double Fourier transformation of a series of free-induction decay signals [1009], is not readily applicable to conventional IR experiments. A very different experimental approach is therefore required. The approach for generation of 2D IR spectra defined by two independent wavenumbers is based on the detection of various relaxation processes, which are much slower than vibrational relaxations but are closely associated with molecular-scale phenomena. These slower relaxation processes can be studied with a conventional... 

When the pulse is switched off, the excited nuclei return slowly to their original undisturbed state, giving up the energy they had acquired by excitation. This process is known as relaxation. The detector is switched on in order to record the decreasing signal in the form of the FID (free induction decay). You can observe the FID on the spectrometer s computer monitor, but although it actually contains all the information about the NMR spectrum we wish to obtain, it appears completely unintelligible as it contains this information as a function of time, whereas we need it as a function of frequency. 

At the very beginning of our discussion in 1.1.1, we mentioned that any pulse experiment begins with a delay period. This is necessary so that the spins can return to equilibrium before they are excited. After excitation (when the pulse is turned off) we observe the FID, the free induction decay What decays The induced magnetization of the spins, and this process is known as relaxation. It may be slow or fast, as we shall see, and can also occur via a number of processes, which are discussed in detail in the monographs we have recommended for further reading. We will only treat relaxation very briefly here. 

Fourier Transformation. Mathematical process of converting the interference free induction decay into a spectrum. 

A pre-requisite for the successful extraction of key NMR parameters from an experimental spectrum is the way it is processed after acquisition. The success criteria are low noise levels, good resolution and flat baseline. Clearly, there are also experimental expedients that can further these aims, but these are not the subject of this review per se. In choosing window functions prior to FT, the criteria of low noise levels and good resolution run counter to one another and the optimum is just that. Zero filling the free induction decay (FID) to the sum of the number acquired in both the u and v spectra (in quadrature detection) allow the most information to be extracted. 

The corrected free induction decay Sc t) will transform to a spectrum Sc i ) in which not only the acetone signal but also all the ethanol signals have had the instrumental contributions to their lineshapes removed. Provided that the reference region lui to wr gives a complete and accurate representation of the experimental acetone lineshape, our deconvolution process should allow us to obtain a clean corrected spectrum even when the shimming is far from ideal. There are of course limitations on this process. If the experimental lineshape is very broad, it will clearly not be possible to obtain a corrected spectrum in which the lines are very narrow without some sort of penalty. Here the limiting factor is signal-to-noise ratio since S u>) is much sharper than Se u>), the ratio of their inverse Fourier... 

MAS NMR experiments characterizing catalysts in reaction environments in flow systems may be carried out under conditions close to those of industrial processes. The formation of catalytically active surface species and the cause of the deactivation of catalysts can be characterized best under flow conditions. When flow techniques are used for the investigation of reactions under steady-state conditions, a continuous formation and transformation of intermediates occurs. The lifetime of the species under study must be of the order of the length of the free-induction decay, which is ca. 100 ms for " C MAS NMR spectroscopy. 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

Unlike the sample condition, the experimental parameters have only a minor effect on the NMR spectral parameters. Experimental parameters such as spectral width, flip angle, repetition time, number of points in the free induction decay (FID) and in the real spectrum, number of scans, and processing parameters need to be comparable to those used for the acquisition of the database spectrum or spectrum of the authentic... 

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