Selective decoupling

For zero-order decoupling selection rules these k values should not be equal to -UK, resulting in the decoupling condition... 

A new alkaloid (7) is the first example of a Gelsemium alkaloid having an A a-methoxyindole moiety in the molecule. This alkaloid may be an early biogenetic intermediate to the A a-methoxyoxindole alkaloids and their related compounds. Full assignments of the IH- and l C-NMR spectra of A7 a-methoxy-19(Z)-anhydrovobasinediol (7) were conducted mainly by CSCM ID (decoupled selective population transfer experiment) (18) and selective INEPT (insensitive nuclei enhanced by polarization tansfer) (19) experiments. The structure was finally determined by single crystal X-ray analysis (11). 

Once the resonances of each unit were identified, the attachments were determined by identifying correlations in selective COSY spectra like those shown in Figure 24.3. In the/i-decoupled selective COSY spectra sufficient chemical shift dispersion and spectral resolution was achieved to identify most of the stereo-sequences present in the oligomers. A similar approach enabled them to identify the structures of all the chain ends in these oligomers and to assign their resonances [48]. 

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

Fig. 2.—A. Normal, H-N.m.r. Spectrum of Asperlin (1) in Benzene-

Nonselective polarization transfer, as implied by the term, represents a process that allows simultaneous polarization transfer from all protons to a//X nuclei. In sefectmepolarization transfer, however, the population of just one nucleus is inverted at any one time. The selective polarization transfer sequence therefore cannot be used to generate a proton-decoupled C-spectrum containing all sensitivity-enhanced C resonances. 

Figure 5.14 Pulse sequence for selective indirecty-spectroscopy. The three proton pulses at the center of the evolution period flip attached protons selectively, resulting in decoupling between distant and attached protons. (Reprinted from J. Magn. Reson. 60, V. Rutar, et ai, 333, copyright (1984), with permission from Academic Press, Inc.)...

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

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