Shift correlation homonuclear couplings

The second way to improve the resolution of proton resonances is with homonuclear correlation spectroscopy, or COSY, experiment. In the basic form of the experiment the magnetization is excited with a 90° pulse. During the consequent fi period the chemical shift and homonuclear / couplings evolve, until a second 90° pulse is applied prior to acquisition. This pulse causes polarization transfer to take place between protons that are coupled to each other. If we consider a proton H coupled to another proton H, then the observable terms of the proton H magnetization for the COSY experiments are... 

The H- H COSY spectra correlate proton chemical shifts through homonuclear couplings. Starting from a separated, readily assigned signal the appearance of cross peaks allow identification and assignment of the complete spin system. For instance, H-5a shows correlation peaks with H-4a and H-4P, which correlate with H-3p (or H-3a), the latter showing further cor-... 

COSY Correlated spectroscopy, two-dimensional shift correlations via spin-spin coupling, homonuclear (e.g. HH) or heteronuclear (e.g. CH)... 

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

Another 2D homonuclear shift-correlation experiment that provides the coupling information in a different format is known as SECSY (spin-echo correlation spectroscopy). It is of particular use when the coupled nuclei lie in a narrow chemical shift range and nuclei with large chemical shift differences are not coupled to one another. The experiment differs... 

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. 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

Key experiments useful for substructure determination by NMR include the DEPT sequence (c.. Figs. 2.44-2.46) for analysis of CH multiplicities, as well as the two-dimensional CH correlation for identification of all CH bonds (e.g. Fig. 2.55 and Table 2.2) and localization of individual proton shifts. If, in addition, vicinal and longer-range proton-proton coupling relationships are known, all CH substructures of the sample molecule can be derived. Classical identification of homonuclear proton coupling relationships involves homonuclear proton decoupling. A two-dimensional proton-proton shift correlation would be an alternative and the complementary experiment to carbon-proton shift correlation. Several methods exist [68], Of those, the COSTsequence abbreviated from Correlation spectroscopy [69] is illustrated in Fig. 2.56. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

Although we might be able to get all the H-H coupling information through a long series of homonuclear decoupling experiments, there is a much simpler way the 2D NMR technique known as homonuclear shift correlation spectroscopy, or COSY. 

A special case of heteronuclear interations exists with coupling between Li and Li, which has most recently been detected by a 2D heteronuclear Li, Li shift correlation experiment [129], (see Section 3.2) after earlier attempts to find such couplings for partially labelled compounds [130] were unsuccessful. Such interactions are of interest because they can be observed also for chemically equivalent Li nuclei which are isochronous in the homonuclear case. From the known magnitude of a Li, Li coupling (see above) estimated values are ca 0.4 Hz. 

As in the case of homonuclear correlations the heteronuclear Li,X TOCSY experiment, pulse sequence (viii), forms an alternative to the HETCOR and HMQC experiments introduced above. It has been tested for H, Li as well as H, Li shift correlations [147]. The MLEV16 decoupling sequence [148] was used for magnetization transfer, and Li as well as H detection was successful. An advantage of these experiments compared with correlation experiments based on the sequences (iii), (iv) and (v) must be seen in the fact that pure absorption spectra are produced. Signal elimination as a consequence of small coupling constants, as observed for antiphase crosspeaks, is thus prevented. On the other hand, adjusting the two decoupler fields to match the Hartmann-Hahn condition Y/Bi(7) = y,Bi(S) is not trivial and needs considerable experimental experience. 

Section 7-7 Homonuclear Chemical-Shift Correlation Experiments Via Scalar Coupling 251... 

Section 7-10 Homonuclear Chemical-Shift Correlation Via Dipolar Coupling 267... 

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). 

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