Nuclear Overhauser Effect Difference Spectra

In many cases of interpretation of NMR spectra, it would be helpful to be able to distinguish protons by their spatial location within a molecule. For example, for alkenes it would be useful to determine whether two groups are cis to each other or whether they represent a trans isomer. In bicyclic molecules, the chemist may wish to know whether a substituent is in an exo or in an endo position. Many of these types of problems cannot be solved by an analysis of chemical shift or by examination of spin-spin splitting effects. 

We can take advantage of this dipolar interaction with an appropriately timed application of a low-power decoupling pulse. If we irradiate one group of protons, any nearby protons that interact with it by a dipolar mechanism will experience an enhancement in signal intensity. 

The typical NOE difference experiment consists of two separate spectra. In the first experiment, the decoupler frequency is tuned to match exactly the group of protons that we wish to irradiate. The second experiment is conducted under conditions identical to the first experiment, except that the frequency of the decoupler is adjusted to a value far away in the spectrum from any peaks. The two spectra are subtracted from each other (this is done by treating digitized data within the computer), and the difference spectrum is plotted. 

The NOE difference spectrum thus obtained would be expected to show a negative signal for the group of protons that had been irradiated. Positive signals should be observed only for those nuclei that interact with the irradiated protons by means of a dipolar mechanism. In other words, only those nuclei that are located within about 3 to 4 A of the irradiated protons will give rise to a positive signal. All other nuclei that are not affected by the irradiation will appear as very weak or absent signals. 

The spectra presented in Eigure 8.28 illustrate an NOE difference analysis of ethyl methacrylate. 

FIGURE 6.27 C NMR chemical shift correlations for 1,3-diol acetonides. (From Rychnovsky, S. D., B. N. Rogers, and T. I. Richardson, Accounts of Chemical Research 31 (1998) 9-17.) 

A handy method for solving these types of problems is nuclear Overhauser effect (NOE) difference spectroscopy. This technique is based on the same phenomenon that gives rise to the nuclear Overhauser effect (Section 4.5), except that it uses homonuclear, rather than a heteronuclear, decoupling. In the discussion of the nuclear Overhauser effect, attention was focused on the case in which a hydrogen atom was directly bonded to a atom, and the hydrogen nucleus was saturated by a broadband signal. In fact, however, for two nuclei to interact via the nuclear Overhauser effect. 

360 Nuclear Magnetic Resonance Spectroscopy Part Four 

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The use of reference deconvolution for the correction of artefacts in nuclear Overhauser effect difference spectroscopy [9] is illustrated by the spectra of fig. 3. The experimental technique used here differs slightly from that normally encountered in using a control spectrum in which the preirradiation is gated off rather than shifted in frequency, and in keeping the decoupler and transmitter at the same frequency. These modifications were... 

The NOESY (Nuclear Overhauser Effect SpectrocopY) spectrum is recorded using the same basic sequence. The only difference is that during the mixing time the cross-relaxation is responsible for the exchange of magnetization between different spins. Thus, a cross-peak indicates that two spins are experiencing mutual cross-relaxation and hence are close in space. 

Nuclear Overhauser effect (NOE) difference measurements were used to assign structure 79 for the product of reaction of diphenylnitrile imine with 5-ethylsulfonyl-2-methyl(27/)pyridazinone. Thus in the H NMR spectrum the ot/, o-protons of the arylhydrazino moiety (which were identified by two-dimensional heteronuclear multiple quantum correlation (2-D HMQC) spectroscopy) were shown in differential NOE (DNOE) experiment to be significantly enhanced on irradiation of pyridazine hydrogen H-7, proving their steric proximity <2000JST13>. 

We have investigated peptides whose structures were known beforehand from NMR or x-ray spectroscopy and related these structures to 2D-IR spectroscopy. Ultimately, one would like to deduce the structure of an unknown sample from a 2D-IR spectrum. In the case of 2D NMR spectroscopy, two different phenomena are actually needed to determine peptide structures. Essentially, correlation spectroscopy (COSY) is utilized in a first step to assign protons that are adjacent in the chemical structure of the peptide so that J coupling gives rise to cross peaks in these 2D spectra. However, this through-bond effect cannot be directly related to the three-dimensional structure of the sample, since that would require quantum chemistry calculations, which presently cannot be performed with sufficient accuracy. The nuclear Overhauser effect (NOE), which is an incoherent population transfer process and has a simple distance dependence, is used as an additional piece of information in order to measure the distance in... 

NMR other aspects Two other aspects of the H NMR spectrum of hydrogen bonded aggregates are useful in determining structure. Variable-temperature NMR can reveal dynamic processes that are fast (or slow) on the NMR time scale at room temperature [51]. Nuclear Overhauser effect (nOe) studies can be used to determine relative proximity of the hydrogen bonded protons and the diastereotopic methylene protons [40,43,46]. H NMR competition studies where two different hubs (for example, Hub(M)3 and Flex(M)3) compete for only three equivalents of CA allows direct assessment of the relative stability of the competing aggregates [45,55]. 

The optimized structure of [(Q )2—(Sc +(/ )-pybox)2] is calculated by using a DFT at the B3LYP/6-31G basis [Fig. 41(a) (top view) and A b) (front view)] (110). There are different sets of protons termed Ha and Hb in (Q )2, where Hb is shielded as compared with Ha by phenyl rings of (R)-pybox. This finding is consistent with two doublet peaks in the HNMR spectrum [Fig. A d)]. The nuclear Overhauser effects (NQE) are detected between Hb (or Ha) protons and phenyl protons of Sc +(R)-pybox (termed a) when irradiated at Hb [Fig. 41(/)... 

Brunner, Pines and coworkers reported on the enhancement of NMR signals in solid Cgo and C70 using a laser-polarized xenon. NMR signals emanating from surface nuclei of solids may be enhanced by the transfer of spin polarization from laser-polarized noble gases via SPINOE (spin polarization induced nuclear Overhauser effect). The paper describes experiments in which the spin polarization is transferred under MAS from laser-polarized - Xe to a nuclear spin with a low gyromagnetic ratio in the fullerenes Ceo and C70, which are polycrystalline materials with a low surface area. In C70, a different degree of enhancement of the NMR spectrum is observed for the different atomic sites in the molecule. 

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