COSY spectra pulse sequence

Figure 5.42. The 2D C, H COSY NMR spectrum of erythromycin in CDCIj as a contour plot beneath the corresponding ID C spin-echo NMR spectrum. The spin-echo spectrum (pulse sequence 90°-(r-I80°-x) -data acquisition) was acquired with broadband proton decoupling during the second period and data accumulation. With x = 8 ms CH2 and C resonances are inverted relative to CHj and CH resonances. The small triangles (A) indicate the two outer lines of the solvent triplet. The contour plot levels are higher than the cross-peaks due to methylene moieties.

The most common two-dimensional NMR experiment is COSY (pronounced cozy ), which is an abbreviation for correlated. spectroscopy. This experiment exploits internuclear coupling to establish relationships among peaks in the spectrum an example will be discussed in Sec. II.H.4. In homo-nuclear COSY, the pulse sequence is essentially the same as that depicted in Fig. 16 the heteronuclear version utilizes a somewhat more complex scheme. The whimsically named INADEQUATE (mcredible atural-abun-dance do xb e-quantam transfer experiment) reveals a molecule s carbon skeleton. Similarly, /zeteronuclear wultiple-ftond correlation (HMBC) estab-... 

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

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... 

The pulse sequences for HMQC-COSY and HMQC-NOESY experiments are presented in Fig. 6.10. The 3D HMQC-COSY spectrum of a N labeled tripeptide is shown in Fig. 6.11. Since the coherence transfer involved in this experiment is N([Pg.362]

Figure 3.22 Pulse sequence used to produce 1H-I3C heteronuclear COSY spectrum. (Adapted with permission of Nelson Thornes Ltd. from Figure 8.9 of Akitt, J. W. NMR and Chemistry, 3rd ed., 1992.)...

As with the COSY experiment, the sequence starts with a pulse followed by an evolution period, but now the mechanism that couples the two spins (which must be in close proximity, typically <6 A) is the Nuclear Overhauser Effect (NOE). The second pulse converts magnetization into population disturbances, and cross-relaxation is allowed during the mixing time. Finally, the third pulse transfers the spins back to the x-y-plane, where detection takes place. The spectrum will resemble a COSY spectrum, but the off-diagonal peaks now indicate through-space rather than through-bond interactions. 

A deuterium 2D Q-COSY experiment has recently been proposed,112 using a n/2 — t — n —12 pulse sequence (Fig. 8(a)), to study weakly oriented systems. The ID deuterium NMR spectrum obtained in a simple one-pulse experiment... 

Figure 3.16 Pulse sequence used to produce a heteronuclear COSY spectrum.

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...

In the case of isoorientin 6"-0-caffeate isolated from Gentiana arisanensis, the C NMR spectrum was assigned by H-decoupled spectra, DEPT pulse sequence, H- C COSY spectrum, long-range C- H COSY, and NOESY experiments the H NMR spectrum was analyzed with the aid of H- H COSY and H- C COSY. 

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... 

In principle, all the combinations of homonuclear 2D spectroscopies can be performed to originate a 3D spectrum (COSY-COSY, NOESY-COSY, NOESY-TOCSY, etc.). The considerations made in this chapter for the most basic experiments can be easily extended to their combinations. The general guideline should always be that the more complex the pulse sequence is, the more the experimental sensitivity will suffer from fast nuclear relaxation. 

Figure 1. [H NMR spectrum (500 MHz) of [Ru(bipy)2(l)]2+ complex in CDCI3 at 20 °C with partial signal assignment based on COSY pulse sequences.

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... 

The COSY pulse sequence can be modified to provide a 2D spectrum that is decoupled in the a)t dimension while retaining coupling in crowded spectra, as the chemical shifts are clearly revealed. Although this experiment is not widely used as such, it serves as a prototype for a building block in other more complex 2D and 3D experiments. 

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