Nuclear Overhauser enhancement spectroscopy

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

NOESY/ROESY nuclear Overhauser enhancement spectroscopy... 

Nuclear Overhauser enhancement spectroscopy (NOESY) experiments play a very important role in structural studies in quinolizidine derivatives. For instance, the endo-type structure of compound 28 was proven by the steric proximity of the H-3a and H-12a protons according to the NOESY cross peak, while the spatial proximity of the H-6f3 and H-8/3 protons reveals that tha A/B ring junction has a /ra t-stereochemistry. Similarly, compound 28 could be distinguished from its regioisomer 29 on the basis of the NOESY behavior of its H-13 atom <1999JST153>. 

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>. 

Total assignment of the H and 13C NMR chemical shifts as well as the relative configuration of the Diels-Alder adducts 33-35 was accomplished with the help of 2D (111-111 COSY, H-111 NOESY (NOESY = nuclear Overhauser enhancement spectroscopy), H- C XHCORR (XHCORR = nucleus X-hydrogen correlation), H-13C COLOC) and NOE difference spectroscopy <1996JHC697>. 

Assignment of the isotropically shifted signals observed for the CuNiSOD example discussed in the previous paragraph has been achieved by means of anion titrations (not discussed here) and nuclear Overhauser enhancement spectroscopy (NOESY), to be discussed next. In Figure 3.24B the CuNiSOD active site is depicted with histidine nitrogens and protons identified for the discussion of the NOESY results. The copper(II) ion is coordinated to the N ligand atoms of his46... 

The stereochemistry of the obtained acetals was confirmed by nuclear overhauser enhancement spectroscopy (NOESY) measnrement in NMR spectra. 

Nuclear Overhauser enhancement spectroscopy ( H- H NOESY and NOE) experiments show only the TTC isomer in equilibrium with the TTT isomer for 6,8-dinitro-BIPS [36,55] and 6-nitro,8-bromo-BIPS. The TTC form dominates the equilibrium. Spectral broadening for several proton resonances in the spectra of 6,8-dinitro-BIPS and 6-nitro,8-bromo-BIPS indicate a rapid exchange between these two isomeric forms. The activation energy for this isomerisation is reported to be 43.6 kJ mol and the energy difference between the the TTC and TTT forms is 4.6 kJ mol [55]. 

The most important two-dimensional NMR experiments for solving stmctural problems are COSY (Correlation SpectroscopY), NOESY (Nuclear Overhauser Enhancement SpectroscopY), HSC (Heteronuclear Shift Correlation) and TOCSY (Total Correlation SpectroscopY). Most modem high-held NMR spectrometers have the capability to routinely and automatically acquire COSY, NOESY, HSC and TOCSY spectra. 

Aryl derivatives of thieno[3,2-f]pyridines, 36 and 37, have been the subject of two-dimensional (2-D) NMR studies. Phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY) and correlation spectroscopy (COSY) experiments confirm the nonplanar conformation of the two aromatic ring systems <1999SAA1035>. 

NMR nuclear magnetic resonance NO-heme nitrosylheme NOESY nuclear Overhauser enhancement spectroscopy... 

PTFE polytetrafluoroethylene PUFA polyunsaturated fatty acid PV peroxide value PVDF polyvinylidene difluoride PVP polyvinylpyrrolidone PVPP polyvinylpolypyrolidone RAS retronasal aroma stimulator RDA recommended dietary allowance RF radio frequency RFI relative fluorescence intensity RI retention index RNU relative nitrogen utilization ROESY rotational nuclear Overhauser enhancement spectroscopy RP-HPLC reversed-phase HPLC RPER relative protein efficiency ratio RS resistant starch RT retention time RVP relative vapor pressure S sieman (unit of conductance)... 

The spirobenzoxazepine 28 was studied by 2-D and nuclear Overhauser enhancement Spectroscopy (NOESY) NMR and X-ray crystallography, and in both cases the same chair conformation, 28, was formed (See Figure 5). The authors conclude that this compound 28 is a semirigid scaffold, able to present various substituents without undergoing hydrophobic collapse, and 28 behaves structurally as a privileged structure <2004TL1051>. 

Correlation spectroscopy (COSY), nuclear overhauser enhancement spectroscopy (NOESY), I-D NOE, and HPLC techniques were utilized in the identification of three trapped products formed from coupling of the generated C4-centered radical of artemisinin with manganese(n) tetraphenylporphorin <2001AGE1954>. 

H and 13C NMR techniques have widely been used to determine the configuration of new dioxepins and dithiepins and to elucidate the constitution and conformation of new naturally occurring substances. For example, the configuration of oximes 14 was determined by H and 13C correlated spectra, correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), heteronuclear correlation (HETCOR) spectroscopy, and hetero-nuclear multiple bond correlation (E1MBC) spectroscopy <1998CCA557>. 

A novel cysteine derivative, spongiacysteine, was isolated recently from marine sponge. It was converted to lactone 4 by reaction with 2,4,6-trichlorobenzoyl chloride, 4-dimethylaminopyridine (DMAP), and triethylamine. Selected nuclear Overhauser enhancement spectroscopy (NOESY) correlation and coupling constants are given <2004CL1262>. 

Notably, two isomeric products can be generated. The usual infrared (IR) and mass spectra as well as H and 13C NMR chemical shifts could not define which isomer was formed. The authors used different NMR techniques, such as 2-D heteronuclear multiple bond correlation (HMBC) experiments and phase-sensitive nuclear overhauser enhancement spectroscopy (NOESY) measurements to elucidate the product s structure. 

The bridged 15 exists as a 3 1 mixture of invertomers at 25 °C and the relative configuration of the stereocenters (C-5, C-6, and G-9) has been determined by one-dimensional nuclear Overhauser enhancement spectroscopy (1-D NOESY) experiments <2004TA3181>. 

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