Z-COSY

Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

COSY can be combined with a z filter (see Section 9.5) into z-COSY, which is occasionally used to produce absorption phase spectra. A 90°, T, 90° sequence replaces the second 90° pulse in the usual COSY sequence, just as in DQF-COSY (see Fig. 12.6a). In this case, however, pulse 2 rotates in-phase magnetization to the z axis, where it is stored for the short r period and restored to the xy plane by pulse 3. Suitable phase cycling or imposition of a pulsed field gradient eliminates the coherences remaining in the xy plane during r, so that the final... 

R163 R. C. R. Grace and N. Syryaprakash, Application of Z-Cosy Technique for the Analyses of NMR Spectra of Oriented Molecules , p. Pl/441... 

An alternative approach to tailor the cross peaks is the z-filtered COSY spectrum (z-COSY) [5.130]. In Check it 5.4.1.7 the "small-flip angle COSY" spectrum of 2,3-dibromopropionic acid, the basic sequence of the z-COSY spectrum, is simulated. As such a z-COSY spectrum can not be calculated because the randomly changing delay which is the major part of the z-filter can not be simulated in the current version of NMR-SIM. A comparison of the results of Check it 5.4.1.7 and the E.COSY spectrum of the same spin system calculated in Check it 5.4.1.6 shows that due to the small flip angles the diagonal peaks of the z-COSY spectrum are reduced in intensity while the cross peaks are very similar. 

The reader should note that, in case of centrosymmetric structures the term z=0, and the Kirkwood factor is 1. In this case Ac provides no information about the average orientational order. However, that does not occur for our samples, because poliols ar linear chains and very unlikely they can form centrosymmetric stmctures via hydrogen intermolecular bondings... 

No general studies have been carried out for these compounds, but there are several reports in which the stereochemistry of the final product has been elucidated by NOESY, correlation spectroscopy (COSY), or heteronuclear single quantum correlation (HSQC) experiments. For example, intensive NOESY experiments were used to establish the exact nature of each of the three cycloadducts 151a-c generated by the cycloaddition of a substituted nitrone to dimethyl (Z)-diethylenedicarboxylate <2000EJ03633>. 

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H-13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the ,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13. 

The second new compound, pseudopterosin Y (2) was also purified a yellow gum. Its UV and IR spectra were identical to those of compound 1, indicating the presence of same n system and functional groups in 2. The HREIMS of 2 showed the M+ at m/z 516.2420 corresponding to the molecular formula A combination of H-, C-NMR, COSY, HSQC and HMBC spectral data revealed that compound 2 was C-3 acetyl derivative of compound 1. Based on these spectral data, stmcture 2 was assigned to this new compound. 

In the pulse sequence for a homonuclear COSY experiment, the first 90° pulse flips the z magnetization into the x direction and into the xy plane. Considering an AX spin system (one in which the nuclei have very different chemical shifts) having two doublets due to spin-spin coupling, the magnetization will include four components processing at different frequencies. During a... 

Fig. 4. Bottom the Double-Band-Filtered COSY spectrum obtained by selection through DANTE-Z of the H region (prior to the evolution interval) and by the selection through SPlN-PlNGING [11] of the amide region (before the acquisition interval) of toxin 7. Top the corresponding region of a standard COSY spectrum. Note, in the bottom diagram, the considerable increase in spectral resolution as well as the occurrence of additional crosspeaks (indicated with asterisks). Experiments were performed at 360 MHz (Bruker AMX360) in H2O at 318 K. The 50 W class C amplifier of the proton channel was used as transmitter.

An example of the complex set of spectroscopic information required in natural products research is afforded by the mixture of hydroperoxides 207, 208, the endoperoxide 209 and other minor products, obtained by photosensitized addition of O2 to phytol (206), according to Scheme 10. The scheme is complicated by the fact that 208 is a mixture of the E and Z isomers. After separation by HPLC, each fraction can undergo characterization by FTIR, H and NMR, also applying the HSQC, HMBC, H- H COSY and NOESY techniques, and EI-MS . ... 

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