Gradient-selected COSY

In Check its 2.33.7 and 2.3.3.8 a phase-cycled COSY magnitude spectrum is converted into a gradient selected COSY spectrum. The Check its also show how the choice of either the n- or p-type CT pathways influences the frequency display in the fl dimension. 

The homonudear 2-D NMR experiments that use I-coupling indude the correlation spectroscopy (COSY, and variants induding gradient-selected COSY or gCOSY, double-quantum filtered COSY or DQF-COSY) experiment, the total correlation spectroscopy (TOCSY) experiment, and the incredible natural abimdance double quantum transfer experiment (INADEQUATE) [3]. 

In many cases, we find that our sample is concentrated enough to obviate the need to collect more than one scan per tj time increment, yet the minimum phase cycle dictates the collection of at least four scans per h time increment. Fortunately, the use of pulsed field gradients allows the collection of COSY spectra with just one scan per tj time increment through a process called coherence selection. The gradient-selected COSY (gCOSY) experiment often allows us to collect a 2-D ff- ff gCOSY experiment with 256 FlDs (256 tj time increments) in 6 minutes. Another favorable feature of the gCOSY experiment is that it is tolerant of poorly calibrated pulses. 

Pulsed field gradients were introduced in Section 9.3. The COSY method can be combined with the use of pulsed field gradients to produce a result that contains the same information as a COSY spectrum but that has much better resolution and can be obtained in a shorter time. This type of experiment is known as a gradient-selected COSY (sometimes known as a gCOSY). A gCOSY spectrum can be obtained in as little as 5 min by contrast, a typical COSY spectrum requires as much as 40 min for data acquisition. This spectrum of citroneUol was obtained by this method. 

Figure 5.41. The gradient-selected DQF-COSY experiment and coherence transfer pathway. No phase-cycling is needed as the required pathway is selected with gradient ratios of 1 2. Both gradient pulses are applied within spin-echoes for phase-sensitive presentations. Note only one pathway is retained from the double-quantum filter.

Simple pulse sequence for DQF COSY with gradient selection and retention of symmetrical pathways in f,. The second gradient is twice the area of the first. 

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. 

Devise a gradient selected version of the triple-quantum filtered COSY experiment, whose basic pulse sequence and CTP was given in E9-7. Your sequence should include recommendations for the relative size of the gradients used. The resulting spectrum must have pure phase (i.e. p = 1 must be preserved in /,) and phase errors due to the evolution of offsets during the gradients must be removed. 

The homonuclear 2D gradient Gradient commands selected COSY(mc) experiment. 

Fig. 4.7 An example of a pulse program a gradient selected magnitude COSY...

Select the file cosygmfq.seq (File I Pulse Program...). Open the pulse program viewer using the command Utilities I Show Pulse program. The pulse program for the gradient selected double quantum filtered homonuclear COSY experiment is displayed in the main window. 

In Check it 4.2.1.1 a job file is used to simulate a typical experimental combination used for routine structure analysis a standard ID IH spectrum and a 2D gradient selected iH COSY experiment. 

One class of selective ID COSY experiments relies upon a selective excitation pulse corresponding to the non-gradient ID selective COSY experiments (left hand scheme in Fig 5.24). 

Listed below a number of different types of ID COSY experiment with and without gradient selection are summarized. By necessity the following discussion can give only a short description of the development of the selective ID COSY experiment starting with the basic experiment and ending with the ID DPFGSE selective COSY experiment. 

In Check it 5.4.1.10 gradients are used in the ID selective COSY experiment to try and improve the selectivity of the selective 90° excitation pulse. 

ID selective COSY experiment with a selective excitation pulse and gradient pulses. 

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]. 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

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