Modified COSY experiment

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

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... 

As an example and to clarify the principle, the acquisition and the processing schemes for the modified heteronuclear inverse detected ID COSY experiment (pulse sequence IVa in fig. 1) are shown below (tables 1 and 2). 

Homonuclear correlation spectroscopy (COSY) experiments (see Chapter 9) substantiate the theoretical predictions, based on molecular orbital calculation, of the pattern of spin delocalization in the 3e orbitals of low-spin Fe(III) complexes of unsymmetrically substituted tetraphenylporphyrins [46]. Furthermore, the correlations observed show that this n electron spin density distribution is differently modified by the electronic properties of a mono-orf/io-substituted derivative, depending on the distribution of the electronic effect over both sets of pyrrole rings or only over the immediately adjacent pyrrole rings [46]. No NOESY cross peaks are detectable, consistently with expectations of small NOEs for relatively small molecules and effective paramagnetic relaxation [47]. 

This then finishes this part of the chapter that has essentially laid the foundations for understanding 2D (or more generally nD) NMR. Throughout this the basic COSY sequence, either in its absolute-value or phase-sensitive forms, has been used as an illustrative 2D sequence. Not only is COSY a simple sequence, but the spectra are rather easy to interpret and require relatively little explanation. Beyond this basic sequence there are a variety of other COSY experiments which provide the chemist with new or modified information. These are, in fact, more widely used in the laboratory than the experiment described thus far, and it is these that are now addressed before progressing to consider other homonuclear correlation techniques. 

The DQF-COSY sequence (Fig. 5.40) differs from the basic COSY experiment by the addition of a third pulse and the use of a modified phase-cycle or gradient sequence to provide the desired selection. Thus, following tj frequency labelling, the second 90° pulse generates multiple-quantum coherence which is not observed in the COSY-90 sequence since it remains invisible to the detector. This may, however, be reconverted into single-quantum coherence by the application of the third pulse, and hence subsequently detected. The required phase-cycle or gradient combination selects only signals that existed as double-quantum coherence between the last two pulses, whilst all other routes are cancelled, hence the term double-quantum filtered COSY. 

Load the configuration file ch54113.cfg an6 run the simulation of the 2D 130-1H relay COSY experiment of the crotonaldehyde type spin system. In this modified spin system the coupling between H-1 and H-2 is excluded to show the relay transfer from H-1 to H-2 trough C-1. Compare the result with the basic 1H COSY spectrum. Repeat both simulations using the spin system relcspsy.ham. Note however that in this spin system the 13C nucleus is 100% abundant. 

Figure 31 Contour and stacked plots of a two-dimensional COSY experiment on ZSM-39 at 373 K using a modified COSY pulse sequence with fixed evolution delays. The data were acquired using 128 experiments, 64 scans in each experiment, 5 kHz sweepwidth, 256 data proints for acquisition, and a fixed delay of 5 ms. Sine bell apodization was used, and the data are presented without symmetrization or smoothing. The total experimental time was approximately 23 h. (From Ref. 71.)...

To identify which protons couple to each other, the coupling interaction is allowed to take place during ti. During the same period, the individual nuclear magnetization vectors spread as a result of spin-coupling interactions. These interactions modify the signal that is observed during t2. Unfortunately, the mechanism of the interaction of spins in a COSY experiment is too complex to be described completely in a simple manner. A pictorial description must suffice. 

This section introduces the most important experimental aspects relating to two-dimensional data sets and again uses the COSY experiment to illustrate these. Many of the discussions are simply extensions of what has already been discussed in Chapter 3 for the ID experiment and familiarity with this is assumed. No new concepts are introduced here, although a modified approach to experimental set-up and to data processing is essential if 2D experiments ate to he successful. 

A modified COSY (two-dimensional correlated spectroscopy) revamped with an asymmetric Z-gradient echo detection (CRAZED) sequence was designed to obtain a better CEST contrast image based on the inter-molecular double quantum coherence method." Experiments were performed on an agar-glucose phantom, and the results demonstrate the feasibility of this method. 

In a ID COSY-RELAY experiment [38] (fig. 13(a)) a multistep relay transfer is applied after the filtration. If the filtration is performed on the H-2 proton, the CSSF is incorporated into the first spin-echo. If there is not sufficient chemical shift separation between H-2 protons, the filter is shifted to the second spin-echo. The method is illustrated for the separation of the spin systems of two terminal /3-glucopyranose residues of a modified LPS (5) containing a total of nine saccharide units [76]. The anomeric proton resonances of the two /3-glycopyranoses overlapped almost completely, with a chemical shift difference of only 1.9 Hz, while the corresponding H-2 resonances were separated by 55.0 Hz. The length of the filtration interval, Ti, was adjusted to yield a maximum antiphase magnetization of H-2 pro-... 

The two-dimensional INADEQUATE experiment also suffers very much from low sensitivity given by the low natural abundance of carbon-13 (about 10 2), so that only 0.01 % or 10-4 of all carbon-carbon bonds contribute to the satellite signals. In fact, the basic experiment can be modified in order to reduce the data matrix and to save measuring time [72], giving COSY-like square correlations as shown in Fig. 2.60. Nevertheless, the two-dimensional INADEQUATE experiment requires several hours of measuring... 

For an effective measurement of "/(SiH) (1 < n < 4) values in silylated silyl enol ethers, a selective population transfer experiment was modified so that selective decoupling was applied during acquisition. 1H-29Si heteronuclear COSY confirmed the presence of silylated groups in the molecule6. 

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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