2D-NMR techniques

A number of 2D NMR techniques are available for a variety of purposes They are especially valuable when attempting to determine the structure of complicated natural products and the conformations of biomolecules... 

Section 13 19 2D NMR techniques are enhancements that are sometimes useful m gam mg additional structural information A H H COSY spectrum reveals which protons are spin coupled to other protons which helps m deter mining connectivity A HETCOR spectrum shows the C—H connections by correlating C and H chemical shifts... 

COSY (Section 13 19) A 2D NMR technique that correlates the chemical shifts of spin coupled nuclei COSY stands for... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Partial etherification of the beech wood MGX with p-carboxybenzyl bromide in aqueous alkali yielded fully water-soluble xylan ethers with DS up to 0.25 without significant depolymerization the Mw determined by sedimentation velocity was 27 000 g/mol [400,401]. By combination of endo- 6-xylanase digestion and various ID- and 2D-NMR techniques, the distribution of the substituents was suggested to be blockwise rather than uniform. The derivatives exhibited remarkable emulsifying and protein foam-stabilizing activi-... 

Applications Useful 2D NMR experiments for identification of surfactants are homonuclear proton correlation (COSY, TOCSY) and heteronuclear proton-carbon correlation (HETCOR, HMQC) spectroscopy [200,201]. 2D NMR experiments employing proton detection can be performed in 5 to 20 min for surfactant solutions of more than 50 mM. Van Gorkum and Jensen [238] have described several 2D NMR techniques that are often used for identification and quantification of anionic surfactants. The resonance frequencies of spin-coupled nuclei are correlated and hence give detailed information on the structure of organic molecules. 

The use of two-dimensional (2D) NMR techniques has become almost routine for detailed analysis of complex organic molecules containing carbon and hydrogen. In contrast, 2D 19F NMR methods are not nearly so commonly used in the analysis of fluorine-containing molecules. The reasons for this are generally a combination of instrumental requirements combined with intrinsic differences between fluorine and proton NMR, in particular the wide range of 19F chemical shifts, which to an extent negates the need for 2D, but also can create problems, for example with respect to uniform excitation of the entire 19F band width. 

However, as the interest in compounds that contain fluorine has grown, along with the quality of available instrumentation, so has the interest in 2D 19F NMR techniques. An excellent recent review by Bat-tiste and Newmark on the applications of 19F multidimensional NMR is available and should be consulted for detailed information about the hardware requirements and the application of the various available 19F 2D NMR techniques.23... 

All chair conformations depicted in the schemes throughout this chapter were elucidated using 2D NMR techniques and coupling constants. 

First, 91 was subjected to a Pt02-catalyzed iminium reduction to provide the amine as a single diastereomer (94, 83 %, Scheme 6.16). At this point 2D NMR techniques more clearly highlighted the correlations with the new me thine adjacent... 

Both 13C- and H NMR experiments using 2D NMR techniques established that 20 had a cis-eudesmane ring juncture. This was based on a 12% nOe effect between the C-10 methyl and H-5 proton. Based on this and other evidence, the relative stereochemistry as shown in 20 [6a-isocyano-5a-H, 7a-H, 10a-eudesm-4(14)ene] and its analogs (21-22) was proposed [40]. 

Stylotelline (23) is a constituent of a Stylotella sp. collected offshore in New Caledonia. 13C NMR spectra involving 2D NMR techniques provided the bulk of information leading to its structure. The absolute configuration was demonstrated after the tertiary isocyano group was removed to yield the known conjugated diene, ( + )-d-selinene (24). Optical rotation and spectral data of the transformation product were identical in all respects to those of the corresponding product obtained from eudesmols [42], Although neither the isothiocyanato nor the formamido compounds were isolated, the latter was prepared, which allowed an nOe observation between the axial C-3 and the amide protons. 

The kinetic information for NMR experiments is contained in the line broadening observed for a nucleus that resides in two different magnetic environments, and values for rate constants can be obtained using line-shape analysis.28,68,69 Line broadening experiments obtained using ID NMR is the method of choice when analyzing the kinetics of a molecule in two sites. 2D NMR techniques, such as 2D EXYS are employed when the kinetics are sequential, i.e. more than one step, or multiple sites are analyzed.69 For example in the case of supramolecular systems this technique was employed to measure the exchange kinetics in capsules.70... 

A second factor from which the solvent effect stems is associated with the insertion process. The reaction of species 6 with phenylacetylene revealed that the insertion took place into the H-Rh bond (Scheme 24). Although isolation of species 10 was not possible due to its high reactivity, 2D NMR techniques confirmed the structure. In CD2CI2, a polar solvent, the process took place smoothly even at room temperature to generate 10 (and 11 8 through reductive elimination from 10). However, the process was sluggish in toluene and more than 93% of 6 remained unchanged even after 24 h. 

Mono(hydride) zirconium species grafted on a silica surface show a high reactivity toward alkanes. This system has been studied using modern 2D NMR techniques, such as double quantum (DQ) rotor synchronized 2D H MAS and C- H HETCOR, to investigate the mechanism of formation of a zirconium bis(hydride) species and the simultaneous generation of silicon mono- and bis(hydride) species [111]. Figure 11.4 shows the H MAS NMR spectrum and DQ rotor-synchronized... 

The solution structure of the dilithium compound 237, generated by treating frawi-1,2-diphenyl-l,2-bis(trimethylsilyl)ethylene (236) with excess lithium metal in THF (Scheme 83), was studied by ID and 2D NMR techniques in the group of FUrstner". As a conclusion, the dilithium compound 237 adopts a C2-symmetric twisted frawi-arrangement with two bridging lithium centres in solution, underlining the various X-ray structural studies in the solid state by Sekiguchi and coworkers. 

In a move in the opposite direction, the overlaps resulting from concatenation of different polarization transfer mechanisms in combined 2D experiments can be eliminated by reducing the dimensionality of an experiment. Similarly to the successful transformation of basic 2D NMR techniques into their ID counterparts [32-34], a conversion of combined 2D NMR techniques into their ID analogs is feasible and has been explored by several groups [35-40]. From a different perspective this process can be seen as a twofold reduction of the dimensionality in a 3D experiment. Equally, concatenation of three polarization transfer steps in a single ID experiment represents transformation of a possible 4D homonuclear experiment into its ID analog. 

K.-H. Ruan, J. Wu, S.-P. So, L.A. Jenkins, Evidence of the residues involved in ligand recognition in the second extracellular loop of the prostacyclin receptor characterized by high resolution 2D NMR techniques. Arch. Biochem. Biophys. 418 (2003) 25. 

During the structural elucidation of the first flavone-xanthone dimer (swertifranche-side) through a series of ID or 2D NMR techniques including COSY, phase-sensitive ROESY, reversed-detected HMQC, HMBC, and selective INEPT experiments, H and C signals appeared as two peaks indicating two conformers and classically, the coalescence of the split signal was observed as the temperature was increased. 

The structures of the compounds were elucidated by a combination of NMR techniques (lH-, 13C-, and 13C-DEPT NMR) and chemical transformation, enzymatic degradation, and as well as mass spectrometry, which gives information on the saccharide sequence. A more recent approach consists of an extensive use of high-resolution 2D NMR techniques, such as homonuclear and heteronuclear correlated spectroscopy (DQF-COSY, HOHAHA, HSQC, HMBC) and NOE spectroscopy (NOESY, ROESY), which now play the most important role in the structural elucidation of intact glycosides. These techniques are very sensitive and non destructive and allow easy recovery of the intact compounds for subsequent biological testing. 

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