Delayed COSY spectra

Figure 18. A 0.2-s delayed COSY spectrum of the aliphatic region of 10 (2mg, CDCLj). Long-range "W-type" coupling of 19 and 21 axial protons to 30-CHj and coupling across the gem dimethyls from I9eq to 21 eq establish the position of oxidation at C-22. The spectrum was obtained under conditions similar to those in Figure 1, except that 32 transients were acquired for each of 128 x 512 data point spectra (17).

Figure 5.56. (a) The conventional COSY-90 spectrum and (b) the delayed-COSY spectrum of 5.6. Additional A delays of 200 ms were used in (b) whilst all other parameters were as for (a). The small (1 Hz) long-range couplings are not apparent in the conventional COSY experiment but the correlations in (b) unambiguously provide proton assignments. 

Figure 3.10 Effect of different window functions (apodization functions) on the appearance of COSY plot (magnitude mode), (a) Sine-bell squared and (b) sine-bell. The spectrum is a portion of an unsymmetrized matrix of a H-COSY I.R experiment (400 MHz in CDCl, at 303 K) of vasicinone. (c) Shifted sine-bell squared with r/4. (d) Shifted sine-bell squared with w/8. (a) and (b) are virtually identical in the case of delayed COSY, whereas sine-bell squared multiplication gives noticeably better suppression of the stronger dispersion-mode components observed when no delay is used. A difference in the effective resolution in the two axes is apparent, with Fi having better resolution than F. The spectrum in (c) has a significant amount of dispersion-mode line shape.

Figure 7.2.8 shows the contour plot of one constituent of the phthalate separation. Here the dead volume between the UV detector and the SFC flow cell was determined before the separation. After an adequate delay after the occurrence of the UV signal of bcnzyl-n-butylphthalate in the UV detector, the SFC separation was stopped and the two-dimensional acquisition was started. The pressure proved to be stable for several hours, which was sufficient for the acquisition of the two-dimensional COSY spectrum. Despite the intense... 

The modulation part is the main part to create n dimensional experiments. During this period coherences evolve under free precession due to the chemical shift or coupling of a particular kind of nucleus like nucleus in an IH, COSY experiment. If only chemical shift evolution is allowed a shift resolved COSY spectrum results whilst a J-resolved experiments is obtained if only coupling evolution is permitted. Incrementing the delay between experiments modulates this evolution which after Fourier transformation generates the second frequency dimension. In a further step several modulation units can be implemented to create n dimensional spectra where each dimension is assigned to an individual nucleus or a particular type of coupling. 

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. 

Fig. 2.42. a Long-range COSY spectrum of derivative 41 from hausterium-inducing component (40), isolated from Lespedeza sericea showing four-bound coupling from H-21 (eq) to 3O-CH3 and H-19 (eq), utilizing a delay A = 0.02 sec b COSY spectrum without delay, A - 0.0 (363)... 

These long-range couplings were not detected in the COSY spectrum without the delay A inserted in the pulse sequence (Fig. 2.42b). 

RELAY spectrum with appropriate delay 2r in contrast to the COSY cross peaks at and intensity of this peak depends on the magnitude of... 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

Fig. 14. ID COSY-RELAY spectra of two terminal glucoses of oligosaccharide 5. (a) Partial H spectrum of 5 at 600 MHz and 27°C. Spectra (b) and (c) were acquired using the pulse sequence in fig. 13(a) (k = 3) with the initial polarization transfer from overlapping anomeric protons of terminal glucoses. Duration of the Gaussian pulse was 50 ms, to = 39 ms, T] = 50 ms, A = 9.09 ms, T2 = 50 ms, T3 = 40 ms, number of scans was 64, relaxation delay and acquisition times were 2 and 1.4 s, respectively. AT = 0 for the first and N = 1 for the second spectrum, (d) is the sum of (a) and (b), (e) is the difference between (a) and (b). (Reprinted with permission from ref. [38]. Copyright 1993 ESCOM Science Publisher...

Similar spectra can be obtained more rapidly and with less sample if the data are acquired through the proton signals, which are much more intense. Basically, the H NMR data are acquired and the H- C coupling constant used as the delay in a pulse sequence, which enables us to obtain the carbon spectrum. This method of obtaining the data is called inverse-mode , since the carbon atoms are detected through their attached hydrogen atoms rather than by direct detection, with obvious benefits in the sensitivity and the time taken to obtain a spectrum. HMQC and HMBC are both examples of inverse-mode spectra and this method is so much quicker than CH COSY that an entire HMQC spectrum can be obtained in much less time than it takes to obtain the proton-decoupled C... 

Standard correlation spectroscopy (COSY) experiments were run on the same four samples and the results are displayed in Figure 8.2.16. Since the acquisition time was approximately an order of magnitude less than the recycle delay, a full factor of four for improvement in throughput was achieved in fact, the number of coils could be increased for yet further improvements in temporal efficiency. No signal bleedthrough was observed from one spectrum to another. Similar results were reported in this paper with a two-coil probe used at 500 MHz [19]. 

COSY and a C.H-HSC. With only an edited DEPT spectrum and a 2D INADEQUATE spectrum the atomic sequence (structure without stereochemistry) of most molecules can be determined. However, there is a down side the time and effort required to generate the 2D INADEQUATE spectrum. Because we are looking at very weak sidebands of weak signals, it can take days of pulse sequence repetitions to generate the desired information. Moreover, the recycle delay between pulses must be carefully set to exceed (by 1.5- to 3-fold) the longest carbon T, value in the molecule. Also, the experiment... 

---