COSY experiment

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. 

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

Population transfer experiments may be selective or nonselective. Selective population transfer experiments have found only limited use for signal multiplicity assignments (SSrensen et al, 1974) or for determining signs of coupling constants (Chalmers et al., 1974 Pachler and Wessels, 1973), since this is better done by employing distortionless enhancement by polarization transfer (DEPT) or Correlated Spectroscopy (COSY) experiments. However, nonselective population transfer experiments, such as INEPT or DEPT (presented later) have found wide application. 

When is the delayed-COSY experiment advantageous over the standard COSY experiment ... 

Fiffire 5.38 Pulse sequence for delayed COSY—a modification of the COSY experiment. The fixed delays at the end of the evolution period t and before the acquisition period <2 allow the detection of long-range couplings between protons. 

H/ C One-Bond Shift Correlation by Hetero-COSY Experiment... 

Fi and F. The off-diagonal peaks (cross-peaks) represent the direct coupling interactions between protons. Working through cross-peaks, one can easily correlate protons that are coupled to each other. Several versions of the COSY experiment have been designed to get optimum performance in a variety of situations (such as DQF COSY, COSY-45°, and COSY-60°). 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single 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... 

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

The semisoft COSY experiment (Cavanagh et al, 1987a BrOschweiler et al, 1988) results in increased resolution in F] so that better cross-peak... 

Figure 7.12 (A) Pulse sequences for semisoft COSY experiments. (B) Pulse se-...

Figure 7.16 When soft pulses are used for excitation and mixing in a 2D experiment, it becomes a 2D soft experiment. The spectrum of the 2D soft experiment has reduced frequency ranges in F, and Fj. The excitation ranges of the selective pulse depend on the type of experiment. For example, in a soft COSY-COSY experiment, one multiplet is excited, while in the soft NOESYexperiment the whole resonance region of a group of signals is excited. (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.)...

The most important difference in the spectrum as compared with RGs released by RG-hydrolase action (Colquhoun et al., 1990) was a doublet at 5 5.81 (J = 3.4 Hz). From the COSY experiments this doublet was found to belong to a four-proton spin-coupling network... 

Gelsevirine (2) was first isolated in 1953 from G. sempervirens as a minor component (3). Its structure was later elucidated on the basis of mass spectrometry as well as H-NMR and 13C-NMR studies (4). Gelsevirine has been found to be the predominant alkaloid in G. rankinii (24), and it was claimed that some of the previously reported 1 H-NMR and 13C-NMR data should be revised. Thus the previous assignments of H-16, H-15, H-14a, H-14e, and H-6 for gelsevirine should be changed to H-15, H-14a, H-16, H-6, and H-14e, respectively, from the evidence of the more accurate homonu-clear 2D COSY experiments. Similarly, from the heteronuclear 2D correlation spectrum, the assignments for C-16, C-15, C-6, and 1V-CH3 should be revised to C-15, C-16,1V-CH3, and C-6, respectively. 

The H-NMR spectra of 21-oxogelsemine and gelsevirine have also been assigned by a 2-D COSY experiment (6). The absence of H-21, the downfield shifts of H-18, H-5, H-14, H-15, A-CH3, and upfield shifts of H-19 and H-16 also support the presence of the 21-oxo group. 

The proton assignments of H-17e, H-17a, H-6a, and H-6e are made by examination of the dihedral angles with Dreiding models and the J values. These and the other proton assignments are confirmed by 2D COSY experiments. 

In this case we pulse at the beginning of the evolution time and then wait before doing our acquisition pulse. If we vary this wait by incrementing it for each successive cycle, we can change what we see in the FID. This is what generates our second dimension. In the case of the COSY experiment, we allow the coupling information to evolve during this period and then read what has happened to it with the acquisition pulse. 

On the basis of their 13C NMR assignments (see below), 111—111 correlation spectroscopy (COSY) and H-13C COSY experiments allowed to assign the H NMR data of a series of sparteine analogues and derivatives (compounds 24-27). These data are collected in Table 2 <2003JST275>. Detailed 111 NMR assignments for other sparteine derivatives are also available in the literature (see, for instance, <2005JST75>). 

The 1H NMR spectrum of (DL)-penicillamine in D20 was obtained on a Bruker 500 MHz instrument, and the resulting spectrum is shown in Fig. 3. Confirmation of the spectral assignments was derived from a COSY experiment (see Fig. 4), and these assignments are summarized in Table 3. 

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