Inverted peaks

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

Section 13 18 One of the special techniques for distinguishing carbons according to the number of their attached hydrogens is called DEPT A series of NMR measurements using different pulse sequences gives normal nulled and inverted peaks that allow assignment of primary secondary tertiary and quaternary carbons... 

A number of different equilibria may be present in the solution of a chiral substrate and the added chiral auxiliary compound. When all equilibrium processes are fast on the NMR timescale at ambient temperature, an averaged spectrum of shifts is observed. This is also the reason why peak coalescence of the anisochronous nuclei is observed when a racemic auxiliary compound is used5,81. The observed anisochrony of the enantiomers AR sS is highest when the chiral auxiliary compound is enantiomerically pure. AR sS decreases as the enantiomeric purity of the chiral auxiliary compound is reduced. AR sS changes its sign when the chirality of the chiral auxiliary compound is inverted (peak reversal). 

Metabolic information of infarcted tissue can be retrieved by selecting a single cuboid volume of the pathological area for the spectroscopic examination using one of the above described SVS spectroscopy sequences. Results from an SVS MRS examination at 1.5 T with the PRESS sequence applying a TE of 144 ms are shown in Fig. 11.2. As expected, the infarcted area shows lactate which is visible as two inverted peaks (doublet) around 1.33 ppm while the contralateral tissue exhibits the spectrum of a healthy white matter. To achieve sufficient S/N for quantifiable SVS data from a 3 ml volume (approximately 1.5x1.5x1.5 cm3), the data acquisition time will take approximately 4 min. This leads to a rather... 

The AFD, in addition to its already temperamental behavior, will often show negative response (inverted peaks). Such peaks have been observed by many authors, who probably considered them an interesting nuisance (24, 25, 31, 32, 33, 43). There appeared only two reports in the literature which suggested or made some use of negative response Dressier and Jandks paper on sulfur compounds (55) and our own on chlorinated hydrocarbons (75). 

The DEPT spectrum (Figure 4.15) immediately confirms the seven-saccharide repeat, since there are seven anomeric resonances such a confirmation of the results of methylation analysis by NMR is important, because incomplete methylation can give the appearance of additional branch points. Moreover, the hydroxymethylene carbons at high field show inverted peaks, which identifies them as the C6 carbons of the various residues. 

The baseline does not undergo depletion even if the reagent carrier stream absorbs (or fluoresces) at the selected wavelengths. Inverted peaks are therefore not usually observed in confluence systems. 

The two methyl carbons (numbered 1) can be seen as the tallest peak (at 22.3 ppm), while the methyl group on the acetyl function (numbered 6) is a shorter peak at 20.8 ppm. The methine carbon (2) is a still smaller peak at 24.9 ppm. The methylene carbons produce the inverted peaks carbon 3 appears at 37.1 ppm, and carbon 4 appears at 63.0 ppm. Carbon 4 is deshielded since it is near the electronegative oxygen atom. The carbonyl carbon (5) does not appear in the DEPT spectrum since it has no attached hydrogen atoms. 

At time (2//) = 1/7, the vectors have realigned along the Y axis but in the negative direction. An inverted peak would be produced if we collected signal at that time. So, if r = l/J, a methine carbon should show an inverted peak. 

An analysis of the precessional frequencies of these vectors shows that after time t = 1/7, the methyl carbon should also show an inverted peak. 

In this type of experiment, we should see a normal peak for every quaternary carbon and methylene carbon and an inverted peak for every methine carbon and methyl carbon. We can thus tell whether the number of hydrogens attached to the carbon is even or odd. 

Fig. 14 - Saturated spectrum of the sodium line. The four theoretical peaks are shown on the bottom of the figure. The inverted peaks in the middle of the experimental curves are the parasitic crossover (reference 118 I).

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