Alternative COSY sequences

There are an enormous range of experiments that are, in essence, variations on the simple COSY sequence described above. In this section we take a look at a selection of these and examine the benefits these modified sequences provide the practising chemist. Many of the COSY variants proposed in the literature offer little gain over the simpler sequences whilst being rather more complex to execute or process, or are beneficial for a rather limited class of compounds and lack wide applicability. Whilst the experiments presented below represent only a fraction of the proposed sequences, they have established themselves over the years as the most widely used and informative methods. 

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Note that both pathways with p = 1 during the spin echo and with p = 2 during tx are retained. There are a number of possible phase cycles for this experiment and, not surprisingly, they are essentially the same as those for DQF COSY. If we regard the first three pulses as a unit, then they are required to achieve the overall transformation Ap = 2, which is the same as that for the first two pulses in the DQF COSY sequence. Thus the same cycle can be used with these three pulses going 0123 and the receiver going 0202. Alternatively the final pulse can be cycled 0123 with the receiver going 0 3 2 1, as in section 9.5.5.4. 

An alternative to the H-H-X relay COSY experiment is the X-H-H relay COSY experiment [5.148], which has the advantage that it is based on the detection of the sensitive nucleus [5.3]. In Check it 5.4.1.15 the X-H-H relay COSY sequence is created and then various aspects of the phase cycling examined to illustrate how the original phase cycling proposed in the literature must be adapted to give quadrature detection in fl [5.148] using NMR-SIM. 

Alternatively, we may consider a heteronuclear COSY experiment (HETCOR) shown in Fig. 2c. It begins in the same way as the homonuclear COSY sequence shown in Fig. 2a, except the read pulse followed by acquisition is usually applied to the X-nuclei. Of course, on a multi-receiver system we may choose to detect signals from both the A and X nuclei (see Fig. 2c). With a proper phase cycle in place and with A = and X = C, both H- H and correlation spectra can be... 

As before, we use the table to adjust the phase according to the reference axis for each scan. Now we see that the 2IaIb terms alternate sign and cancel as we move from first scan, first term to second scan, second term to third scan, first term and finally to fourth scan, second term. Likewise, the 2IbIa terms alternate sign and cancel as we move down. So the ZQC, which exists between the second and third pulses of the DQF-COSY pulse sequence (Fig. 10.28) does not contribute anything to the observed FID after four scans. Just for completeness, we can show that all of the other terms present at the end of the 90S-fi-90j sequence are also destroyed by the phase cycle... 

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 HOHAHA or TOCSY experiment [5.150, 5.151] has proved a popular alternative in many applications to the main homonuclear correlation experiment for sensitive nuclei, the basic COSY experiment. Both the HOHAHA and the TOCSY experiment are based on the principal of isotropic mixing but differ in the type of spinlock sequence used. Nevertheless they may be considered together and for convenience in the following discussion the expression TOCSY experiment will be used for both sequences. The TOCSY experiment uses cross polarization for the coherence... 

For samples of sufficient strength to require only a single scan per increment, gradient versions of TOCSY may be attractive alternatives for the rapid recording of spectra. By analogy with the previous COSY discussions, the absolute-value TOCSY sequence simply requires equal gradients to be placed on either side of the spin-lock sequence (Fig. 5.69a) to... 

In the work by Carnevale et al. [56], a combination of 2D-NMR experiments were used to study the stereo-sequences in the alternating copolymer of poly(chlorotrifluoroethlene-alt-ethyl vinyl ether) (PCTFE-EVE). Structures 9 and 10 show two possible diastereoisomeric backbone dyad units. Their mirror images are not shown here as the enantiomers cannot be distinguished from each other based on NMR data obtained in achiral media. In each structure, there are two stere-ogenic centers, and the terms meso and racemic are used loosely here since the substituents on each stereo-center are different. They used a combination of H- H COSY (with 1 F decoupling), 1h 1 F HMQC, J-resolved, and 1h 13c HSQC 2D-NMR experiments to identify two sets of resonances from two distinguishable isomers. 

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