COSY homonuclear correlated

The LR COSY-45° spectrum and H-NMR chemical shifts of an isoprenyl coumarin are given below. Determine the long-range H/ H homonuclear correlations based on the long-range COSV45° spectrum. Demonstrate with reference to problem -5.15 how they can be helpful in interconnecting different spin systems ... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

The HMQC spectrum, H-NMR chemical shift assignments, and C-NMR data of vasicinone are shown. Consider the homonuclear correlations obtained from the COSY spectrum in Problem 5.14, and then determine the carbon framework of the spin systems. 

Figures 13.7 and 13.8 are two examples of two-dimensional NMR spectroscopy applied to polymers. Figure 13.7 is the proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A poly(amide) 6 [29]. In this experiment, the normal NMR spectrum is along the diagonal. Whenever a cross peak occurs, it is indicative of protons that are three bonds apart. Consequently, the backbone methylenes of this particular polymer can be traced through their J-coupling. Figure 13.8 is the proton-carbon correlated (HETCOR) contour plot of Nylon 6 [29]. This experiment permits the mapping of the proton resonances into the carbon-13 resonances.

Fig. 13.7 Proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A, a poly(amide) 6.

Wemmer, D. E., Homonuclear Correlated Spectroscopy (COSY). Concepts in Magnetic Resonance An Educational Journal, 1989,1, (No 2)... 

Homonuclear correlation spectroscopy (COSY) experiments (see Chapter 9) substantiate the theoretical predictions, based on molecular orbital calculation, of the pattern of spin delocalization in the 3e orbitals of low-spin Fe(III) complexes of unsymmetrically substituted tetraphenylporphyrins [46]. Furthermore, the correlations observed show that this n electron spin density distribution is differently modified by the electronic properties of a mono-orf/io-substituted derivative, depending on the distribution of the electronic effect over both sets of pyrrole rings or only over the immediately adjacent pyrrole rings [46]. No NOESY cross peaks are detectable, consistently with expectations of small NOEs for relatively small molecules and effective paramagnetic relaxation [47]. 

Additional information on molecular conformation can be obtained by NOE, transfer NOE or 2-D homonuclear correlated NMR spectroscopy (COSY), which is used to measure the distance between nuclei. For detailed information on the various techniques see refs. 86 and 87. 

By way of example, useful 2-D techniques are homonuclear correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear multiple quantum coherence (HMQC) spectroscopy, and heteronuclear multiple quantum coherence/total... 

The number of sugar residues and constituent monosaccharides are determined by a combination of COSY (two-dimensional homonuclear correlation spectroscopy), HOHAHA (2D homonuclear Hartman-Hann spectroscopy) and HETCOR (direct heteronuclear correlation spectroscopy)... 

Two dimensional sequences are commonly used in solution state NMR spectroscopy to elucidate connectivity of atoms within molecular structures. Sequences are available for heteronuclear shift correlation, homonuclear correlation (COSY and INADEQUATE) and other sequences for longer-range connectivities. In general the basis of these... 

To keep experimental times and data storage requirements reasonable, far fewer data points are used in each dimension in 2D experiments than with typical ID experiments. Reasonable digital resolution can be obtained in the detected dimension (02) for H, homonuclear correlation (e.g., COSY) and H-detected, heteronuclear correlation (e.g., HMQC) spectra by using 1,024 or 2,048 data points. Due to their greater spectral widths, however, heteronuclear-detected correlation (e.g., HETCOR) spectra require 2,048, or even 4,096, data points in U2. 2D experiments are commonly performed with 2,048 data points in each dimension. 

Experiments that correlate the chemical shifts of two nuclei of the same type (homonuclear correlation) on the basis of the scalar (or spin-spin) coupling that exists between them are the most frequently performed of all the 2D experiments. Most of these experiments are adaptations of the basic COSY sequence. Another COSY-type experiment, TOCSY, is useful for observing correlations of individual protons with most, or all, of the other protons in an entire spin system. 

Traditionally, homonuclear 2D double quantum filtered correlation spectroscopy (DQF-COSY) and total correlated spectroscopy (TOCSY) spectra are valuable in the identification of resonances of individual monosaccharide units. In the presence of small couplings, through space connectivities detected by NOESY/ROESY (nuclear Overhauser effect spectroscopy/ rotational nuclear Overhauser effect spectroscopy) experiments are also useful in completing the resonance assignment. When the H NMR spectra of complex oligosaccharides are too crowded to fully elucidate the structure by homonuclear correlation methods, it is efficient to use 2D heteronuclear correlation methods, such as heteronuclear single quantum correlation... 

The methodologies based on H- H COSY and NOESY experiments are the most sensitive, as they rely on detection of high-sensitivity, abundant nuclei. COSY provides correlations among atoms that are /-coupled, while NOESY provides those among protons based on their separation in space. The drawback of using these homonuclear 2D methods is their complexity, as two coupled protons can produce many peaks in the 2D spectrum. 

Correlations anticipated in various homonuclear ( H- H) and heteronuclear ( C- H) 2D NMR experiments are conceptualized in Eigure 5.1. A hypothetical model compound (the chemical shifts are not accurate and are for illustrative purposes only) with three aromatic protons and four side-chain protons on its three side-chain carbons is used to illustrate the information available from each experiment. A set of five experiments, in addition to the standard ID proton and carbon spectra, are useful for characterizing any model compound or lignin. The correlation spectroscopy (COSY) experiment correlates directly coupled protons (Figure 5.1a). 

COSY-90 Correlating coupled homonuclear spins. Typically used for correlating protons coupled over 2- or 3-bonds, but may be... 

This then finishes this part of the chapter that has essentially laid the foundations for understanding 2D (or more generally nD) NMR. Throughout this the basic COSY sequence, either in its absolute-value or phase-sensitive forms, has been used as an illustrative 2D sequence. Not only is COSY a simple sequence, but the spectra are rather easy to interpret and require relatively little explanation. Beyond this basic sequence there are a variety of other COSY experiments which provide the chemist with new or modified information. These are, in fact, more widely used in the laboratory than the experiment described thus far, and it is these that are now addressed before progressing to consider other homonuclear correlation techniques. 

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