Truncation artefacts

To ensure tmncation of the time-domain data and thus attenuate truncation artefacts,... 

As is shown clearly in the figure, a truncated FID leads to oscillations around the base of the peak these are usually called sine wiggles or truncation artefacts - the name arises as the peak shape is related to a sine function. The more severe the truncation, the larger the sine wiggles. It is easy to show that the separation of successive maxima in these wiggles is l/tacq Hz. 

Highly truncated time domain signals are a feature of three- and higherdimensional NMR experiments. Much effort has therefore been put into finding alternatives to the Fourier transform which will generate spectra without these truncation artefacts. The popular methods are maximum entropy, linear prediction and FDM. Each has its merits and drawbacks they all need to be applied with great care. 

How can truncation artefacts be suppressed Mention any difficulties with your solution to the problem. 

Truncation artefacts occur at the interface of tissues of significant different signal intensities. This results in... 

The reliability of this method for the evaluation of (vertical) electronic spectra has been clearly established in the present work, and further calculations on other molecules (ethylene, vinylydene... for example) have confirmed the very promising potentialities of such an approach that avoids the possible artefacts brought in by any arbitrary truncated CIs when dealing with excited states [49]. We also emphasize that this methodology is able to give reliable splittings between states ranging from 10 keal/mol to more than 10 eV. 

Sometimes the FID doesn t behave as we would like. If we have a truncated FID, Fourier transformation (see Section 4.4) will give rise to some artefacts in the spectrum. This is because the truncation will appear to have some square wave character to it and the Fourier transform of this gives rise to a Sine function (as described previously). This exhibits itself as nasty oscillations around the peaks. We can tweak the data to make these go away by multiplying the FID with an exponential function (Figure 4.1). 

Processing of time domain data may cause artefacts in the frequency domain. One example for these distortions are truncations at the beginning or at the end of the FID which could lead to severe baseline artefacts which can be reduced by an appropriate filter. Undesired resonances leading to broad lines in the final spectra can be more easily eliminated in time domain by truncating the first few data points. Furthermore, the model functions in time domain are mathematically simpler to handle than the frequency domain analogues, which leads to a reduction of computation time. The advantage of the frequency domain analysis is that the quantification process can be directly interpreted visually. 

Fig. 4.15 Illustration of how truncation leads to artefacts (called sine wiggles) in the spectrum. The FID on the left has been recorded for sufficient time that it has decayed almost the zero the corresponding spectrum shows the expected lineshape. However, if data recording is stopped before the signal has fully decayed the corresponding spectra show oscillations around the base of the peak.

Whether leurosine, Catharine, and their congeners are true alkaloids, or artefacts derived from anhydrovinblastine, the fact remains that the aerial oxidation of anhydrovinblastine is a facile process which does not need to be enzyme-mediated, and a further examination of this reaction has revealed that all the alkaloids of the vinblastine group are produced. The oxidations were performed in acetonitrile solution, and in one experiment, conducted at 26 for 48 hours, the composition of the alkaloid mixture obtained was roughly similar to the relative abundances of the dimeric alkaloids isolated from Catharanthus species. In the oxidation the lone electrons on Nb are presumably involved, since anhydrovinblastine Nb-oxide is inert towards oxidation by air, and while the presence of moisture promotes the reaction, oxygen from the water is not incorporated into the oxidized alkaloids. On the basis of the available evidence, a mechanism, shown in truncated form in Scheme 37, was proposed for the oxidative transformation of anhydrovinblastine into the various alkaloids iso-lated. "°... 

In any case, a disorder must be chemically reasonable. Not every significant residual electron density peak is caused by disorder. High residual electron density can also be caused by inadequately corrected absorption, Eourier series truncation errors (for example when strong reflections are missing) or radiation damage. Such artefacts often lead to the accumulation of spurious electron density at special positions. 

When we analyze the list of residual electron density peaks in the file zr5-08.res, the final model of this refinement, we find that Q( 1) and Q(2) are significantly higher than all the other maxima. Q(l) corresponds to 1.38 electrons and is located 0.66 A away from Zr( 1), not far from the location of the deepest hole. This could be an artefact arising from Fourier truncation, but absorption effects can also lead to spurious electron density close to heavy atom positions. It is difficult to decide which of the two effects holds responsibility for Q(l), but it is clear that we cannot do anything about it. Q(2) represents 1.13 electrons and sits on the crystallographic fourfold. It... 

As the side lobes do not correspond to actually measured information but rather represent an artefact due to the abrupt truncation, it is desirable to reduce their amplitude. The process that attenuates the spurious feet in the spectral domain is known as apodization (originating from the Greek word aTToSoo , which means without feet ). In other words, apodization is the removal of the side lobes by multiplying the interferogram by a suitable function before the Fourier transformation is carried out. The price paid for suppressing the side lobes is that one has to accept a broadening of spectral lines. 

There are four types of artefacts related to the magnetic field gradients truncation, aliasing (wrap around), chemical shift and susceptibility (Rawson and Siegel 1996). 

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