2D homonuclear and heteronuclear

In addition to well-resolved one-dimensional (ID) 1H and 13C spectra, which are usually sufficient for monitoring synthetic steps, HR-MAS techniques can be applied to two-dimensional (2D) homonuclear and heteronuclear experiments, which allow a wealth of structural information to be obtained. H,13C HMQC (heteronuclear multiple quantum coherence) spectra are particularly useful in the analysis of solid support-bound oligosaccharides, since the anomeric protons exhibit characteristic resonances. Such a spectrum of a polymer-bound trisaccharide glycal is shown in Figure 8.4. 

NM R experiments (ID and 2D, homonuclear and heteronuclear) are the preeminent techniques for the determination of molecular structures. However, careful application and analysis of mass spectral data can provide sufficient information to postulate tentative structures. In this respect, the application of tandem MS experiments, sometimes in conjunction with selective derivatization of the unknown compound, can be very informative about the stmcture. The high-resolution mass spectral data are critical to the support of NMR-deduced stmctures by providing molecular formulae for unknowns. 

In this section, 2D homonuclear and heteronuclear correlation experiments are reviewed. The longer experimental time for a 2D experiment is rewarded by enhancement of resolution achieved by adopting the second dimension. This gives us a chance to get information on both miscibility and polymer-polymer interactions. 

Akoka have also adopted this technique to produce an artificial reference cross peak in 2D homonuclear and heteronuclear correlation spectra. This approach shows potential as a universal reference in quantification. 

A novel alkaloid was isolated from the stem of E. arborescens Roxb. Its structure and stereochemistry were elucidated on the basis of the ID and 2D homonuclear and heteronuclear NMR spectroscopic data, and it was characterized as 10-hydroxy-ll-methoxy-benzo[g]hexahydroindolizin-3-one and named erythrinarbine [36],... 

The structures of two novel glycodienoid alkaloids isolated from E. latissima seeds have been established as (+)-16P-D-glucoerysopine (34) and (+)-15p-D-glucoerysopine (35). NOE experiments established that 1 and 2 are positional isomers. Their structures were elucidated from their ID and 2D homonuclear and heteronuclear NMR spectroscopic data. Another seven known alkaloids were purified and identified [51]. 

K. Wahala, S. Deb and T. Hase, The Stmcture of Isoflavones ly ID and 2D Homonuclear and Heteronuclear NMR Spectroscopy, n Isoflavones Chemistry,... 

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... 

Nolls, P., Parella, T. Spin-edited 2D HSQC-TOCSY experiments for the measurement of homonuclear and heteronuclear coupling constants application to carbohydrates and peptides. /. Magn. Reson. 2005, 176, 15-26. 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

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. 

In what, it is hoped, will be the final word on the NMR chemical shift assignments of 9-methoxyellipticine (2), Commenges and Rao (52) have revised several of the C-NMR assignments published earlier by Sainsbury and coworkers (55). In the present work, a combination of two-dimensional (2D) NMR techniques ( H- H homonuclear and heteronuclear chemical shift cor-... 

A simple one-dimensional spectrum is always recorded at the start of any structural analysis. Small modifications to the ID pulse sequence enables the selection of groups of signals or singlet signals, the separation of multiplets for determining coupling constants or multiplicity and the determination of molecular parameters such as relaxation times or diffusion constant. In addition ID pulse sequence can also be converted into 2D sequences (and vice versa) enabling homonuclear and heteronuclear correlation. Table... 

The 2D spectra in solid state direcdy probe intermolecular or intramolecular interactions through correlations between specific sites, mainly produced by homonuclear and heteronuclear dipolar coupfings. Then through these 2D spectra, it is possible to provide a better assignment in complex spectra, obtain high-resolution spectra and retrieve direct information on distance, proximity, bonding, n-n interactions, ring currents, etc. 

In the last decades a series of homonuclear and heteronuclear recoupling experiments have been developed that combine a 2D approach with fast MAS and dipolar multiple-quantum NMR spectroscopy. In this way, the chemical shift resolution of different chemical environments allows sites of interest to be distinguished and fully identified, and then it is possible to obtain structural information since dipole-dipole couplings are sensitive to the distances between the coupled nuclei as well as to the orientation of the internuclear vector. 

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). 

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. 

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. 

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