COSY pulse sequence

Figure 7.19 Soft H,C-COSY pulse sequence with two soft pulses having the same excitation frequency. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

What makes 2-D different is that it uses this evolution time to allow something to happen to the spins in the molecule. This can be seen graphically in a simple COSY pulse sequence (Figure 8.2). 

Fig. 9.1 Basic HNN COSY pulse sequence. Narrow and wide pulses correspond to flip angles of 90° and 180°, respectively, whereas low-power (water flip-back) 90° l-l pulses are illustrated as smaller narrow pulses. Delays 8 = 2.25 ms 7=15 ms (can be shorter or longer) (a = 2.5 ms fb = 0.25 ms fc= 2.25 ms fd = 0.5 ms. Unless indicated, the phase of all pulses are applied along...

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.

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... 

As is true for many of the advanced 2D NMR techniques, there are several variations of the basic COSY pulse sequence, each of which serves to improve the resolution and signal-to-noise ratio of the 2D signals, remove long-range couplings,... 

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. 

Figure 6-3 The solid line slanting upwards at frequency on the horizontal axis serves as the baseline for a series of H spectra of chloroform, according to the COSY pulse sequence for a series of values of t. Each peak results from one cycle of 90°-/i -90° followed by Fourier transformation during ti of Figure 6-1 to give frequency on the axis labeled V2 (corresponding to the time domain tj). The period is ramped up after each cycle. Fourier transformation in the fi dimension has not been carried out.

The sequence below shows the gradient-selected DQF COSY pulse sequence modified by the inclusion of extra 180° pulses to remove phase errors. Note that although the extra 180° pulses are effective at refocusing offsets, they do not refocus the evolution of homonuclear couplings. It is essential, therefore, to keep the gradient pulses as short as is feasible. 

ID selective COSY pulse sequence with DANTE-Z element. 

In Check it 5.4.1.11 the improved selective ID COSY experiment using a selective refocusing n pulse are calculated. These category of selective COSY experiments based on a gradient flanked selective spin echo generate less artefacts and are superior to the ID COSY pulse sequences with a selective excitation pulse. Essentially the flanking gradients cancel the artefacts in a similar manner as a "perfect EXORCYCLE" scheme [5.140]. 

By simply adding a third ir/2 pulse immediately following the second W2 pulse in our simple COSY pulse sequence and changing nothing else, we have the pulse... 

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.)...

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 COSY pulse sequence does not lend itself to explanation with visual images and spin gymnastics. To arrive at a better imderstand-ing of coherence selection in NMR pulse sequences, we can study the product operator formalism. Fortimately, an intimate grasp of how the COSY pulse sequence works is not required to use the method. 

The homonuclear TOCSY experiment [77, 79] utilizes the fundamental COSY pulse sequence and evolution time followed by a delay and then an isotropic mixing period the pulse sequence is shown in Fig. 8.12. Homonuclear vicinal... 

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