Homonuclear COSY Experiments

The standard homonuclear IH COSY experiment is probably the most popular 2D experiment and is used for the detection of scalar coupling interaction in spin systems of abundant NMR active nuclei. From a IH COSY experiment it is possible to assign signals to individual IH substituents in a molecular structure and using the dominant 2J, 

Purpose/ Homonuclear correlation of signals using scalar homonuclear 3j 

Homonuclear COSY experiments may be broadly classified as follows, although as will become apparent from the following discussions some experiments fall into both categories  

In these types of experiment the selection of the correlated nuclei depends upon the magnitude of the homonuclear coupling constant involved. 

In these types of experiment the cross and diagonal peak structure are tailored to correlate the relative signs of coupling constants and to suppress passive coupling or unwanted coherences. In addition from the appearance of the cross peaks it is possible to determine the coupling constants. 

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Many variations of the basic homonuclear COSY experiment have been devised to extend its range. A brief guide to some classes of experiment follows, along with a few of the connnon acronyms. 

In the pulse sequence for a homonuclear COSY experiment, the first 90° pulse flips the z magnetization into the x direction and into the xy plane. Considering an AX spin system (one in which the nuclei have very different chemical shifts) having two doublets due to spin-spin coupling, the magnetization will include four components processing at different frequencies. During a... 

A variety of sequences exist, which differ with respect to the detected interaction ( J, or Jx ) and the mode of detection ( C or H detected, magnitude or phased mode, phase cycling or gradients for coherence selection). In view of the reduced sensitivity of heteronudear experiments with respect to homonuclear COSY experiments and the steadily decreasing sample amounts submitted for NMR experiments, there is no doubt that the inverse ( H) detected, gradient enhanced experiments are currently the best methods to apply. However on older type spectrometers, not equipped for inverse detection the old-fashioned direct C detected experiments are still in use. 

In a homonuclear decoupling experiment a particular multiplet is irradiated suppressing the coupling interaction between the irradiated nucleus and its coupling partners. A comparison of the standard coupled ID spectrum and the selectively homonuclear decoupled spectrum reveals which nuclei are coupled. Whether a homonuclear decoupling experiment or a 2D homonuclear COSY experiment would be the best solution for multiplet analysis in a one-dimensional spectrum depends very much on the nature of the problem under investigation. If a large number of multiplets need to be irradiated then a two-dimension approach may be preferable. 

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. 

The jump-and-return method can also be implemented in 2D experiments in a similar manner to the presaturation method. Check it 5.23.4 shows the jump-and-return method as part of a 2D homonuclear COSY experiment for peracetylated p-D-glucose. 

Prior to the development of 2D homonuclear COSY experiments, the SPT experiment was used to unravel coupling networks. The use of SPT experiments to trace eoupling interactions of a spin system has two advantages whieh even today makes this technique superior to the selective ID COSY experiment. 

Spin pinging [5.86] has been proposed as a method of using rectangular pulses in 2D homonuclear COSY experiments for tailored excitation. Using spin pinging it would be possible to select regions of overlapping multiplets which are not defined sufficiently in a standard 2D COSY spectrum because of the restricted number of data points. Probably... 

Homonuclear correlation experiments are not just restricted to the standard COSY experiment, but also include the TOCSY and INADEQUATE experiments. The separation of TOCSY and INADEQUATE from the homonuclear COSY experiment is based on the different coherence evolution and transfer processes involved. Thus the TOCSY experiment is based on cross-polarization in contrast to the polarization transfer used in the homonuclear COSY experiments. INADEQUATE experiments are characterized by the double quantum state of two scalar-coupled nuclei during the tl period such that the second dimension (fl) is scaled into a double quantum frequency. Nevertheless these experiments can all be considered together because they are based on homonuclear scalar coupling and the fl and f2 dimension of the corresponding 2D spectra are related to the same nucleus. 

In contrast to homonuclear COSY experiments in 2D heteronuclear correlation experiments the two frequency dimensions are assigned to different nuclei. The term COSY is often used to imply homonuclear correlation and in particular H, IH correlation. Consequently heteronuclear correlation experiments either have a specific name related to the processes involved such as IH HMQC or simply have the words "COSY experiment" appended to the detected nuclei as in IH, l c COSY experiment. 

The correlation peaks in a basic homonuclear COSY experiment arise from both directly and remotely connected transitions and as such does not allow the measurement of coupling constants in complex spin systems [5.130, 5.134]. To try and rectify this problem the COSY45 experiment can be extended to include a DQF. In addition to... 

Eor example, a homonuclear COSY experiment (see Table 3.4) is used to map the proton-proton J coupling in a molecule. Consider a simple system in which two protons are coupled to each other, CH—CH. The basic pulse sequence for a 2D... 

Definitive assignments of all and signals in NMR spectra of the lolines were reported in 1989 (22). Two-dimensional homonuclear (COSY) experiments, DEPT experiments, and heteronuclear correlation ex-... 

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