Delayed COSY

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.

When is the delayed-COSY experiment advantageous over the standard COSY experiment ... 

Fiffire 5.38 Pulse sequence for delayed COSY—a modification of the COSY experiment. The fixed delays at the end of the evolution period t and before the acquisition period <2 allow the detection of long-range couplings between protons. 

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).

Delayed-COSY Correlating coupled homonuclear spins through small couplings. Often used to identify proton correlations over many... 

Delayed-COSY Enhances detection of small and long-range couplings (<2 Hz) such as between protons in allylic systems or those in w-relationships. Requires magnitude-mode presentation. Crosspeaks due to larger couplings can be significantly attenuated. 

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. 

Most 2D NMR methods in solids have low sensitivity because of fast transverse relaxation. In particular, for the COSY experiment the system must evolve for sufficiently long time periods in both dimensions before satisfactory intensities of cross-peaks can be obtained. Accordingly, Fyfe et al. [30,31 ] introduced two extra deiays into their solid-state COSY pulse sequence, a concept originally conceived for so-called long range or delayed COSY in liquids [40]. If transverse relaxation is too fast on the time scale of the required evolution and acquisition periods, there will be no cross-peak magnetization to detect. Furthermore, rapid transverse relaxation leads to wide lines, so that diaganol peaks can overlap with adjacent cross-peaks, which are already very weak because of the destructive interference of their broad antiphase components. 

The addition of delays before and after the mixing pulse in the COSY sequence gives a relative enhancement to correlations arising from small coupling constants. This technique is called "delayed COSY" (or "small J COSY") it should be noted that the term "long-range COSY" is inappropriate for homonuclear proton experiments because use of it can encourage people to overlook the... 

Returning now to the Cleistopholis compounds, NOE difference experiments where the 1-Hs are irradiated are shown in Fig. 8(i)-(iv). Discrimination between incompletely resolved multiplets, e.g. lb- and Ic-H, is possible by restricting irradiation to only one line of each doublet. Here we see NOEs across the glycosidic links (5%) which confirm the delayed COSY results and also NOEs of the 2-Hs (3%) but not of 3-H or 5-H. Hence we conclude that the glycosidic links are axial (a in the L sugar). 

The sixth compound 9 was a trirhamnoside with four acetates and again with the dodecanyl ether, [M + Na]" " - 815, and we deduced the structure without needing the delayed COSY experiment. FAB MS showed that three acetates were on the terminal ring (m/z 273), and one on the central ring (m/z 461, 273 + 188). The proton shifts showed that the acetylated positions were C-2, -3, and -4 (twice). 

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