Couplings of nuclear spins

Decoupling (Section 13 17) In NMR spectroscopy any process that destroys the coupling of nuclear spins between two nuclei Two types of decoupling are employed in NMR spectroscopy Broadband decoupling removes all the H- C couplings ojf resonance decoupling removes all H- C couplings except those between directly bonded atoms... 

Indirect or scalar coupling of nuclear spins through covalent bonds causes the splitting of NMR signals into multiplets in high-resolution NMR spectroscopy in the solution state. The direct or... 

Spin-spin splitting (Section 13.7) The splitting of NMR signals caused by the coupling of nuclear spins. Only nonequivalent nuclei (such as protons with different chemical shifts) can split one another s signals. 

Gutowsky, H. S., Jonas, J., Fu-Mimg, Chen, Meinzer, R. Intermolecular interactions and Electron Coupling of Nuclear Spins. J. Chem. Phys. 42, 2625 (1965). 

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

The coupling of nuclear-spin angular momenta with electron-orbital angular momenta. 

Scalar coupling—coupling of nuclear spins that is mediated by chemical bonds commonly utilized to transfer magnetization in triple resonance experiments. 

Coupling of nuclear spins requires that the nuclei split each other s signal equally. The separation between the two halves of the methyl doublet in 1,1-dichloroethane is equal to the separation between any two adjacent peaks of the methine quartet. The extent to which two nuclei are coupled is known as the coupling constant J and in simple cases is equal to the separation between adjacent lines of the signal of a particular proton. The three-bond coupling constant Jab in 1,1-dichloroethane has a value of 7 Hz. The size of the coupling constant is independent of the field strength the separation between adjacent peaks in 1,1-dichloroethane is 7 Hz, irrespective of whether the spec-tram is recorded at 200 MHz or 500 MHz. 

The situation is quite different with S-T -type CIDNP because nuclear spins are flipped in that case. Owing to the coupling of nuclear spin motion and electron spin motion, not only the electron spin state oscillates in such a system but also the nuclear spin state. Since, however, one-half of the pairs or biradicals cannot participate in this because their nuclear spin state does not allow an electron-nuclear flip-flop transition, the oscillation is not symmetrical. Its turning points are zero nuclear spin polarization and 100% nuclear spin polarization of one sign only. In contrast, the distribution of nuclear spin polarizations between singlet and triplet members of the ensemble is symmetrical. As an example, consider an ensemble of biradicals, where each biradical contains a single proton. Let the ensemble be created in the state T >, and without initial nuclear spin polarization. Half of the pairs, namely those that have nuclear spin /J>, cannot undergo flip-flop transitions. The others oscillate between T a> and S/3>. When all of those happen to be in S/ >, every nuclear spin of the triplet biradicals and every... 

In addition to the information available from g-values, we can obtain information about nuclei with nuclear spin quantum number / 0 which are close to the paramagnetic centre. The spins of such nuclei interact magnetically with the unpaired electron and give rise to a hyperfine interaction. There is a direct analogy here with coupling of nuclear spins in NMR spectroscopy. The hyperfine interaction is added to... 

A closely related 2D NMR technique, termed NOESY, permits the establishment of through-space coimectivities. This technique relies on the through-space dipolar coupling of nuclear spins and uses a 2D version of the nuclear... 

The heteronuclear coupling that is responsible for much of the broadening in the solid-state spectrum involves the coupling of nuclear spins to the detected nuclear spins, since the dipolar coupling is typically the dominant... 

The coupling of nuclear spin to electron spin by the Fermi contact interaction is most important for proton spins, but it is not necessarily the most important mechanism for other nuclei. These nuclei may also interact by a dipolar mechanism with the electron magnetic moments and with their orbital motion, and there is no simple way of specifying whether / will be positive or negative. 

Figure 1. Mechanisms for indirect coupling of nuclear spins arising from second-order perturbation theory according to the formulation of Ramsey/Contributions to the isotropic part Js and the anisotropic part /r of equation (1) are shown here. There is an additional term which arises from first-order perturbation, w ich is considered part of the orbital mechanism, given by equation (3).

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