COSY-45 method

The signs of 3/c-i,c-6 and 3Jc-3,c-6 coupling constants have been assumed to be positive and were used to determine the relative signs of 2Jq-i,c-3 and 2Jc-i,c s by application of a 13C-13C COSY-45 method to triply 13C-substituted, D-aldopyranoses.113 These sign determinations provided experimental confirmation of the relative signs of 2/Ccc and 2Jcoc predicted by an empirical, projection resultant method.114,115... 

The total correlation spectroscopy (TOCSY) techniques, which come in both 1- and 2-D versions, offer an alternative to 1-D spin decoupling and COSY methods for establishing through-bond connectivities. The important difference between the two is that TOCSY methods allow easy identification of isolated spin systems. For example, using our trusty morpholine compound once more, you can see that it is possible to identify the -CH2-CH2- spin system between the nitrogen and the oxygen atoms, these hetero-atoms, effectively isolating the protons from all others in the molecule. 

In particular, DP9 presented ambiguities associated with the proton assignments from which the 13C assignments were derived. Thus, it was necessary to use the COSY method to assign the proton absorptions first. Homonuclear COSY NMR spectroscopy allowed unambiguous assignment of proton chemical shifts in all cases. 

Two-Dimensional NMR—Basically, the two-dimensional NMR techniques of nuclear Overhauser effect spectroscopy (NOESY) and correlation spectroscopy (COSY) depend on the observation that spins on different protons interact with one another. Protons that are attached to adjacent atoms can be directly spin-coupled and thus can be studied using the COSY method. This technique allows assignment of certain NMR frequencies by tracking from one atom to another. The NOESY approach is based on the observation that two protons closer than about 0.5 nm perturb one another s spins even if they are not closely coupled in the primary structure. This allows spacial geometry to be determined for certain molecules. 

TOCSY (Total Correlation Spectroscopy) is another important homonuclear 2D correlation experiment where correlations arise due to the presence of homonuclear scalar coupling.In the standard COSY experiment, crosspeaks appear for spins in which the scalar coupling occurs over typically two to four bonds. In the TOCSY experiment crosspeaks can appear for spins separated by many more bonds as long as they are part of a contiguous network of coupled spins. The correlations are effected by the application of a series of low-power rf pulses termed the spin-lock. The duration of the spin-lock period determines the extent to which the correlations are propagated through the spin system. The TOCSY experiment is a useful complement to the COSY methods for the elucidation of complex structures. 

The selective COSY method yields the same connectivity information as the homonuclear decoupling technique. In contrast to the latter, however, the multiplets due to the coupling partners are still visible and can be easily evaluated. Because this is a ID experiment, it can be performed at high resolution. The method works well even when the J-coupled spin overlaps with other signals, but the selectively pulsed signal must be well resolved[68]. 

You may have noticed that each of the COSY spectra shown in this section contains additional spots besides the ones examined in our discussion. Often these extra spots have much lower intensities than the principal spots on the plot. The COSY method can sometimes detect interactions between nuclei over ranges that extend beyond three bonds. Besides this long-range coupling, nuclei... 

A few alternative COSY methods have been developed to improve the quality of spectra with large numbers of overlapping correlations or with large spectral windows. Selective COSY utilizes long-shaped pulses to excite a small window within the field of correlations in a spectrum. The smaller spectral window of the selective COSY experiment enhances digital resolution and reduces experiment time due to the reduced number of 2D increments required to obtain high digital resolution. The... 

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. 

Fig. 16 Two-dimensional spectra from different authors, all using D-DNP and fast acquisition schemes, (a) C- H-HSQC NMR spectra of a 1 1 1 1 mixture of o-, m-, and p-xylene and toluene, comparing a spectrum recorded by Frydman s ultrafast method (left) with a conventionally recorded spectrum (right) [150]. (Copyright Wiley-VCH. Reproduced with permission), (b) DNP enhanced proton COSY spectrum of 1.5 mM Ala-Gln at 9.4 T using an ultrafast COSY method by Kockenberger and coworkers. (Reproduced from [151] by permission of the PCCP Owner Societies), (c) 2D small flip angle C- H-HMQC pulse sequence by Ludwig et al. [66, 153]. (d) 2D-HMQC spectrum of aspirin recorded in 30 s with 64 increments after 90 min of polarization at 1.3 K (kindly contributed by C. Ludwig)...

More generally, note that the applieation of almost any multiple pulse sequenee, where at least two pulses are separated by a time eomparable to the reeiproeal of the eoupling eonstants present, will lead to exehanges of intensity between multiplets. These exehanges are the physieal method by whieh eoupled spins are eorrelated in 2D NMR methods sueh as eorrelation speetroseopy (COSY) [21]. 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Homonuclear shift-correlation spectroscopy (COSY) is a standard method for establishing proton coupling networks. Diagonal and off-diagonal peaks appear with respect to the two frequency dimensions. 

Now let us look at the NOESY spectrum (b) just as in COSY, we can identify a diagonal and a series of associated off-diagonal cross peaks. Thus the interpretation of the results is analogous to the method we have already learned for COSY. However, the cross peaks are not due to spin-spin coupling but to NOE effects between the protons concerned. However, if we look more closely we can see one big difference between the diagonal peaks, which look like irregular circles, and the cross peaks, which look just like all the peaks in the COSY spectrum. 

It is also important to note that UIC or GIC processing methods, when calculating hyphenated spectra such as HSQC-COSY/TOCSY83 or HSQC-... 

NMR methods of diastereomeric excess determination were used in the separation of 2-methyl-l-boraadamantane into optical antipodes (see Section 12.13.2.5.1) <2003MC121>. Total assignment of H (Figure 4) and 13C NMR spectra (Table 3) of THF-2-methyl-l-boraadamantane 15 was based upon COSY and HSQC experiments <2003MC121>. 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

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