Polarization Transfer and the Nuclear Overhauser Effect

Example 8.13 described the appearance of the l3C spectrum of 2-chlorobutane in the absence of 1H decoupling. 

In Section 11.8 we saw how polarization (departure from the equilibrium distribution) of nuclear spin state populations by a nearby unpaired electron can have a profound effect on NMR signal intensity. It turns out that in a l3C 1H experi- 

The NOE interaction between two proton magnetic dipoles takes place predominantly directly through space rather than through bonds (as is the case in normal spin coupling). This through-space interaction is very sensitive to the distance 

The 13C signal results from transitions between states 1 and 2 and between states 3 and 4. Its intensity is a function of the difference in populations between states 1 and 2 and between states 3 and 4. At equilibrium, this population difference is 2AC in both cases. Now, if the H states were saturated in a double-resonance experiment, AH would become zero. This would mean that states 1 and 3 would become equally populated, as would states 2 and 4. At first glance it would seem that the population difference between states 1 and 2 and between states 3 and 4 would still be 2AC, but another factor intervenes. Notice that the populations of states 1 and 4 are now polarized. At equilibrium before saturation, the difference between them was 2AH + 2AC, but at saturation it is only 2Ac- In an attempt to restore equilibrium, a number (Ar) of nuclei in state 4 relax to state 1, provided that we allow enough time for this relaxation to evolve. Thus, the population of state 1 will increase (by Ar), while that of state 4 will decrease (by A,), so the intensity of both 13C transitions (1 -a 2 and 3 - 4) will increase in proportion.1 

The NOE is an example of polarization transfer (or cross polarization), because polarization of one set of nuclear spin states (here, saturation of the H nuclei) results in the polarization of another set (here, the 13C nuclear spin states). The maximum Overhauser signal enhancement (q) is given by 

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The Overhauser effect has been widely employed as an NMR analysis method in many disciplines ranging from medical to chemical sciences, and broadly refers to the motion-mediated transfer of spin polarization from a species with a higher gyromagnetic ratio (y) to one with a lower gyromagnetic ratio. Because molecular motion is critical for efficient transfer, the Overhauser effect is most commonly observed in liquid samples. The Overhauser effect can be divided into two categories the nuclear Overhauser effect (NOE), where both species are nuclear spins, and Overhauser DNP, where the higher y spin is an unpaired electron. As Overhauser DNP is the focus of this review, some of the terminology and equations are specific to the Overhauser DNP effect. 

Discussion of the these three methods is outside the scope of this book, but in later chapters we consider other methods for producing much less dramatic non-Boltzmann distributions. By using rf irradiation to alter spin populations, the nuclear Overhauser effect results in signal enhancement (Chapters 8 and 10). Several techniques use pulse sequences to transfer polarization from nuclei with large y to nuclei with small y in solids (Chapter 7) and liquids (Chapters 9 and 12), hence to provide significant signal enhancement. 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

Heteronuclear incoherent magnetization transfer is the transfer of longitudinal magnetization. It can proceed in the laboratory frame and in the rotating frame. The nuclear Overhauser effect (NOE) [Nogl] is a manifestation of polarization tranter in the laboratory frame. In the extreme narrowing limit saturation of dipolar relaxation of the I doublet of a heteronuclear IS two-spin- system leads to an enhancement of the S-spin polarization by a factor... 

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

Hence, provided that I g is known and that R has been determined by means of an independent experiment, provides the cross-relaxation rate ct. This enhancement is called nuclear Overhauser effect (nOe) (17,19) from Overhauser (20) who was the first to recognize that, by a related method, electron spin polarization could be transferred to nuclear spins (such a method can be worked out whenever EPR lines are relatively sharp it is presently known as DNP for Dynamic Nuclear Polarization). This effect is usually quantified by the so-called nOe factor p... 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

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. 

Due to the great complexity of this class of molecules, nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are the tools most widely used to identify cucurbitacins. Both one- and two-dimensional NMR techniques have been employed for the structural elucidation of new compounds 2D NMR, 1H-NMR, 13C-NMR, correlated spectroscopy (COSY), heteronuclear chemical shift correlation (HETCOR), attached proton test (APT), distortionless enhancement by polarization transfer (DEPT), and nuclear Overhauser effect spectroscopy (NOESY) are common techniques for determining the proton and carbon chemical shifts, constants, connectivity, stereochemistry, and chirality of these compounds [1,38,45-47]. 

Nuclear Overhauser effect (NOE), the transfer of spin polarization from one spin population to another by cross-relaxation in nuclear magnetic resonance spectroscopy. The NOE in, e.g., NOESY spectra can be used to derive distance information between two nuclei in a molecule and, hence, serves as a tool for structure elucidation. 

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