NOESY interactions

The NOESY spectrum of 41 was also in agreement with the presence of both conformations. In the case of 41 A, the interactions H-5/H-9, and H-2/H-6 were the most relevant the latter was consistent with a pseudoequatorial (P) disposition of the hydroxyl group at C-2. For 4IB, the most important NOESY interaction was observed between H-7 and H-5 and between H-6 and H-4. 

Nuclear Overhauser effect spectroscopy NOESY Interactions through space/chemical exchange... 

Where Aij (x , x ,2) is the 3D NOESY-NOESY volume and r, is the interproton distance between spins a and b. This model provides a simple description of the 3D NOESY-NOESY interaction, however it does not include the effects of spin diffusion (multiple relaxation pathways). This oversimplification leads to dramatic systematic errors in the form of overestimation of all distances. 

There are basically three main types of 2D NMR experiments J-resolved, shift correlation through bonds (e.g., COSY), and shift correlations through space e.g., NOESY). These spectra may be of homonuclear or heteronuclear type involving interactions between similar nuclei (e.g., protons) or between different nuclear species (e.g., H with C). 

Figure 5.43 A typical NOESY spectrum the off-diagonal cross-peaks represent the nOe interactions between various nuclei.

The NMR techniques discussed so far provide information about proton-proton interactions (e.g., COSY, NOESY, SECSY, 2D y-resolved), or they allow the correlation of protons with carbons or other hetero atoms (e.g., hetero COSY, COLOC, hetero /resolved). The resulting information is very useful for structure elucidation, but it does not reveal the carbon framework of the organic molecule directly. One interesting 2D NMR experiment, INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment), allows the entire carbon skeleton to be deduced directly via the measurement of C- C couplings. 

The NOESY spectrum of 7-hydroxyfrullanolide reveals the spatial proximities between the various protons. Cross-peak D arises from the gemi-nal coupling between the exomethylenic geminal protons (8 5.71 and 6.06). Dipolar interaction between the 06 proton (8 4.97) and the allylic methyl protons (8 1.64) is inferred from cross-peak C. This interaction is possible only when the C-6 proton is a-oriented. The C-1/3 and 02/8 protons (8 1.31 and 1.45, respectively) exhibit cross-peaks... 

Moseley, H. N. B., Curto, E. V., Krishna, N. R. Complete Relaxation and Conformational Exchange Matrix (CORCEMA) analysis of NOESY spectra of interacting systems two-dimensional transferred NOESY. J. Magn. Reson. Sen B 1995, 108, 243-261. 

Identification of proton pairs interacting magnetically through space NOESY... 

The difference between ID NOESY and ID TOCSY is thus the different type of interaction in ID NOESY through space, and by ID TOCSY through-bond. The analogous 2D spectra are shown in Fig. 25. 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

A second class of multidimensional experiments uses through-space interactions perhaps the best-known example is the NOESY sequence, which is as follows ... 

One example of the use of 2D-NMR experiments in conformational analysis is the study of molecular interactions between cinchonidine and acetic acid [26]. These alkaloids are used as chiral auxiliaries in enantioselective hydrogenations, and the enantiomeric excess is dependent on solvent polarity, acetic acid being a good solvent This suggests that protonation and a preferred conformation play a role in achieving high enantioselectivities. With a combination of COSY-experiments, 3J coupling constants and NOESY experiments, it was shown that one conformer is preferred in acidic solutions. 

The intensity of an NOE, given by the volume V of the corresponding cross peak in a NOESY spectrum [11, 13, 14] is related to the distance r between the two interacting spins by... 

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