Editing Experiment Parameters

Pulse programs are written independently of a particular spin system or spectrometer configuration such as magnetic field strength or transmitter pulse power so that, for instance, the same HMQC pulse sequence can be used for a 13C/19F or a i N/lR 

HMQC experiment. To simulate an experiment it is necessary to select not only, the spin system and pulse sequence, but also the experimental parameter relating to the simulation. The configuration file and job file features in NMR-SIM enables tested simulation parameters to be edited and stored for use in later sessions. Configuration files have been used extensively in the previous sections in this chapter and are used extensively throughout this text. The NMR Wizard can also be used to set up the necessary simulation parameters this is achieved by searching the spin system and the pulse sequence and, provided that the pulse sequence has been written following the rules in section 4.1.2, the parameters relevant to the simulation are extracted automatically. In the current version the NMR Wizard is restricted to homonuclear spin systems and to the following types of experiments  

The GolOptimize parameter command can be used to determine the optimum parameters for use in a particular experiment. A typical example would be to determine the delay necessary to generate antiphase coherence for several coupled spins, which have very different coupling constant values. 

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Normally the experiment parameter dialog box displays the pulse program and a list of parameters with a brief comment beside each parameter. This scroll window can be changed into a more compact form by disabling the options Show pulse program and One column edit. The option Start Win NMR allows either ID WIN-NMR or 2D WIN-NMR to be started automatically as soon as the calculation is completed with the current simulated data loaded into the main display window. 

Carbon NMR is a valuable tool for the study of organic materials. The NMR parameters are directly correlated with the structural properties of the various carbon sites. Modern spectrometers make carbon spectroscopy on samples containing feasible at natural abundance. With the development of modern NMR technology, a host of techniques may be applied to obtain a wide variety of information about the molecule under study. In particular, spectral-editing experiments can provide information on carbon-proton connectivity that aid in the assignment of resonances and the elucidation of structure. 

All the spectroscopic approaches applied for structural characterization of mixtures derive from methods originally developed for screening libraries for their biological activities. They include diffusion-ordered spectroscopy [15-18], relaxation-edited spectroscopy [19], isotope-filtered affinity NMR [20] and SAR-by-NMR [21]. These applications will be discussed in the last part of this chapter. As usually most of the components show very similar molecular weight, their spectroscopic parameters, such as relaxation rates or selfdiffusion coefficients, are not very different and application of these methodologies for chemical characterization is not straightforward. An exception is diffusion-edited spectroscopy, which can be a feasible way to analyze the structure of compounds within a mixture without the need of prior separation. This was the case for the analysis of a mixture of five esters (propyl acetate, butyl acetate, ethyl butyrate, isopropyl butyrate and butyl levulinate) [18]. By the combined use of diffusion-edited NMR and 2-D NMR methods such as Total Correlation Spectroscopy (TOCSY), it was possible to elucidate the structure of the components of this mixture. This strategy was called diffusion encoded spectroscopy DECODES. Another example of combination between diffusion-edited spectroscopy and traditional 2-D NMR experiment is the DOSY-NOESY experiment [22]. The use of these experiments have proven to be useful in the identification of compounds from small split and mix synthetic pools. 

The major breakthroughs, however, have come from the use of high magnetic fields and further from the use of different multiple pulse sequences to manipulate the nuclear spins in order to generate more and more information time domain NMR spectroscopy, that is used to probe molecular dynamics in solutions. The latter made it also possible to "edit" sub-spectra and to develop different two-dimensional (2D) techniques, where correlation between different NMR parameters can be made in the experiment (e.g. SH versus 813c, see later). Solid state NMR spectroscopy is used to determine the molecular structure of solids. 

Experiment with the program for a while, trying out different parameters. For example, try producing the same sound with a smaller loop rate. Press fl to enter edit mode, then press RETURN after the first seven prompts. Now enter 0.75 for the loop rate. Pressing RETURN at a prompt preserves the old value, so you need to type in only the parameters you want to change (however, you must always enter the loop rate for a falling tone). 

Molecular diffusion coefficients are parameters that are not related directly to NMR spectral intensities under normal conditions. However, molecular diffusion can cause NMR signal intensity changes when pulsed field gradients are applied during the FT NMR experiment. A number of pulse sequence developments, particularly the LED sequence, have meant that measurement of diffusion coefficients is relatively routine. The editing of H NMR spectra of biofluids based on diffusion alone or on a combination of spin relaxation and diffusion has been demonstrated. This has been termed the Diffusion and Relaxation Editing (DIRE) pulse sequence. This approach is... 

A very elaborately produced book with exceptional color plates of the chromatograms. Enghsh translations of four monographs are provided, and these give a very good description of the derivation of the chromatographic parameters, with comprehensive data. Unfortunately, at least 95 % of the experiments were performed with TLC plates prepared in-house, so that full evaluation of these would be problematical. The Enghsh edition is in preparation. 

Signal enhancements were obtained in H — O cross-polarization experiments without spinning, and reliable second-order quadrupolar powder patterns were observed [1291. Relaxation parameters involved in cross-polarization transfer were shown to be characteristic of the various sites, so that they can be used for signal assignment. In addition, in some cases the differences in crosspolarization rates were used to edit spectra by a selective enhancement of protonated oxygen resonances, such as those from surface hydroxyl groups in amorphous silica. The latter method can be applied to complicated systems, provided dipolar H — O interactions for the various sites are different. We illustrate this procedure by using a static H — O CP spectrum [126] of talc. 

The first edition of an operating manual often contains excessive technical information because it is written while the facility is still in the design or constmction phase. These procedures are typically written by design engineers who know the process and equipment but who do not necessarily have much operating experience. Hence they may describe the material of construction and the design parameters of a pump in some detail, but provide little or no information on how to actually operate that pump. 

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