Nuclear overhauser effect experiments

Detailed assignment of the protons in the cyclopentadienylidene-2//-thiopyran (76, X = H) was accomplished by both two-dimensional correlation spectroscopy and nuclear Overhauser effect experiments. Selective deuteration at C-3 (X = D) simplified the couplings with H-4 appearing as a dd with7 = 6.8, 1.2Hz <1991TL3499>. 

Chiral 2-nitroenamines 551-553 has been shown by Nuclear Overhauser Effect experiments to preserve in solution the EE conformation found in the solid state (Scheme 15)192. 

The a- and P- methyls were assigned from nuclear Overhauser effect experiments relative to 3-H. The HOD signal is due to exchange of the water, carboxy, amine, amide and phenol protons. 

In many locations it is advantageous to mount the whole of the magnet assembly on a vibration damping system as floor vibrations (which may arise from a whole host of sources including natural floor resonances, air conditioners, movement in the laboratory and so on) can have deleterious effects on spectra, notably around the base of resonances (Fig. 3.3). Whilst such artefacts have lesser significance to routine ID observations, they may severely interfere with the detection of signals present at low levels, for example those in heteronuclear correlation or nuclear Overhauser effect experiments. 

Once the structure ofevonine was established as (61 R = R = Ac,R = OH, R = O), related alkaloids isolated from Euonymus sieboldianus (see Table) were readily shown to be slight functional group modifications of this major alkaloid by spectral comparison and simple chemical transformation. Further confirmation of the manner in which evoninic acid (68 R = H) is attached to the sesquiterpenoid portion was also obtained in this study. The stereochemistry of evonine indicated in structure (61) and thus also of neoevonine, euonymine, and neoeuonymine (see Table) were advanced on the basis of n.m.r. and nuclear Overhauser effect experiments. ... 

P. D. Thomas, V. J. Basus, and T. L. James, Proc. Natl. Acad. Sci. U.S.A., 88, 1237 (1991). Protein Solution Structure Determination Using Distances from Two-Dimensional Nuclear Overhauser Effect Experiments Effect of Approximations on the Accuracy of Derived Structures. 

Based on the empirical results/ X-ray-diffraction data/ and solution-phase NMR experiments/ a transition state model (6) has been advanced to explain the observed enantioselectivity. The presence of an ortho substituent in the A arylmaleimide reactant directs aluminum coordination to occur with the lone pair of electrons anti to the nitrogen atom. A 3,5-dimethylphenyl moiety present on the ethylenediamine framework blocks one face of the dienophile, resulting in approach of the diene from the backside. A considerable amount of spectroscopic evidence, most notably that obtained fromNOE (nuclear Overhauser effect) experiments, has been accumulated to support this model. A -arylmaleimide derivatives that lack an ortho substituent and other dienophiles (e.g., maleic anhydride) can coordinate to the aluminum catalyst in alternative modes such that the reactive olefin is far removed from the chiral environment of the ligand scaffold, thereby resulting in cycloaddition reactions that exhibit little or no enantioselectivity. 

For large molecules, such as proteins, the main method in use is a 2D technique, called NOESY (nuclear Overhauser effect spectroscopy). The basic experiment [33, 34] consists of tluee 90° pulses. The first pulse converts die longitudinal magnetizations for all protons, present at equilibrium, into transverse magnetizations which evolve diirhig the subsequent evolution time In this way, the transverse magnetization components for different protons become labelled by their resonance frequencies. The second 90° pulse rotates the magnetizations to the -z-direction. 

The 2-D nuclear Overhauser effect spectroscopy (2-D-NOESY) experiment resembles the COSY however, the cross-peaks arise from... 

Another technique often used to examine the stmcture of double-heUcal oligonucleotides is two-dimensional nmr spectroscopy (see AfAGNETiC SPIN resonance). This method rehes on measurement of the nuclear Overhauser effects (NOEs) through space to determine the distances between protons (6). The stmcture of an oligonucleotide may be determined theoretically from a set of iaterproton distances. As a result of the complexities of the experiment and data analysis, the quality of the stmctural information obtained is debated. However, nmr spectroscopy does provide information pertaining to the stmcture of DNA ia solution and can serve as a complement to the stmctural information provided by crystallographic analysis. 

Although experimental studies of DNA and RNA structure have revealed the significant structural diversity of oligonucleotides, there are limitations to these approaches. X-ray crystallographic structures are limited to relatively small DNA duplexes, and the crystal lattice can impact the three-dimensional conformation [4]. NMR-based structural studies allow for the determination of structures in solution however, the limited amount of nuclear overhauser effect (NOE) data between nonadjacent stacked basepairs makes the determination of the overall structure of DNA difficult [5]. In addition, nanotechnology-based experiments, such as the use of optical tweezers and atomic force microscopy [6], have revealed that the forces required to distort DNA are relatively small, consistent with the structural heterogeneity observed in both DNA and RNA. 

NOE Nuclear Overhauser effect, change of signal intensities (integrals) dining decoupling experiments decreasing with spatial distance of nuclei... 

Figure 1. Pulse sequences of some typical 2D-NMR experiments. COSY = correlation SpectroscopY, DQFCOSY = Double Quantum Filtered COSY, RELAY = RELAYed Magnetization Spectroscopy, and NOESY = Nuclear Overhauser Effect SpectroscopY.

The nuclear Overhauser effect resulting from the broad-band decoupling during the decoupled INEPT experiment also contributes to the signal enhancement of the C lines. 

Gated decoupling The decoupler is gated during certain pulse NMR experiments, so spin decoupling occurs only when the decoupler is switched on and not when it is switched off used to eliminate either H- C spincoupling or nuclear Overhauser effect in a ID C spectrum, and employed as a standard technique in many other H-NMR experiments, such as APT and y-resolved. 

At the beginning of this section, we listed the various experiments that are available which make use of the Nuclear Overhauser Effect but as yet, we have made no attempt to indicate the pros and cons of each of these and under what circumstances one may be preferable over another. It is virtually impossible to give cast iron advice regarding the selection of one NOE experiment over another as the decision has to be based on a huge number of considerations, and on the instrumentation and software available to you. Having said that, we shall now attempt to establish some broad guidelines. 

Key Words Carbon-13 spin relaxation, T, Measurements, Nuclear Overhauser effect, Rotation-diffusion tensor, HOESY experiments... 

On irradiation at 365 nm, compounds 28 undergo ring closure to yield the corresponding triazasemibullvalenes 29, which were unambiguously identified by NMR and IR spectroscopy and conclusive support of the proposed structures comes from nuclear Overhauser effect (NOE) experiments <2002PPS38>. 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

As with the COSY experiment, the sequence starts with a pulse followed by an evolution period, but now the mechanism that couples the two spins (which must be in close proximity, typically <6 A) is the Nuclear Overhauser Effect (NOE). The second pulse converts magnetization into population disturbances, and cross-relaxation is allowed during the mixing time. Finally, the third pulse transfers the spins back to the x-y-plane, where detection takes place. The spectrum will resemble a COSY spectrum, but the off-diagonal peaks now indicate through-space rather than through-bond interactions. 

The observed polarization is primarily associated with the former parahydrogen protons. However, other protons may also experience a drastic signal enhancement due to nuclear spin polarization transferred to these nuclei via the nuclear Overhauser effect (NOE) or similar processes, both in the final reaction products as well as in their precursor intermediates. 

When one resonance in an NMR spectrum is perturbed by saturation or inversion, the net intensities of other resonances in the spectrum may change. This phenomenon is called the nuclear Overhauser effect (NOE). The change in resonance intensities is caused by spins close in space to those directly affected by the perturbation. In an ideal NOE experiment, the target resonance is completely saturated by selected irradiation, while all other signals are completely unaffected. An NOE study of a rigid molecule or molecular residue often gives both structural and conformational information, whereas for highly flexible molecules or residues NOE studies are less useful. 

The 1,3-dipolar cycloaddition of imidazolinone 123 with ethyl m-4,4,4-trifluorocrotonate 124 provided, after 36 h at reflux, the regio- and stereoisomer 125 (90%), accompanied with traces of three other unidentified cycloadducts (10%) <2001JFC275>. Compound 125 was isolated in 70% yield (Scheme 15). The structures of the final product were elucidated by nuclear Overhauser effect (NOE) experiments. This high selectivity is the result of a preferred ///////-orientation of both ester and CF3 groups in the transition state and of an impeded ////////-approach of the CFj-substituted terminus of the alkene to the sterically hindered ct-site of 123. 

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