Anisotropy shielding term

To a good approximation, three terms dominate the C-NMR chemical shifts diamagnetic, paramagnetic, and anisotropy shielding terms [57]. Lamb developed a theoretical expression for the diamagnetic term that focuses on the electron density at a specific nucleus [58]. Therefore, one would expect the carbon s hybridization to have a large chemical shift effect. Obviously, electron deficiency in a carbocation will profoundly effect the chemical shift [59-62]. 

The second factor that contributes to line broadening for solids is chemical shielding anisotropy (the term chemical shift anisotropy should be avoided in this context, since, strictly speaking, chemical shift is a scalar quantity and cannot be anisotropic). In solution, the observed chemical shift is the average of the shielding of a nucleus over all orientations in space, as the result of molecular tumbling. In a solid, shielding of a specific nucleus in a... 

Here, Hz is the Zeeman term, Hq is the quadrupolar interaction term for nuclei with 1 1, Hd is the dipolar interaction term for nuclei with 1 = 1/2, Hs is the electron shielding term and Hj is the J-coupling term. Spin relaxations will be induced by the time fluctuations of these interaction terms. For example, H spin-lattice relaxation behaviour is dominated by Hq, whereas Hq mainly determines the relaxation process of the H or magnetization in organic materials. In some cases without significant contributions from Hq and Hq, the time fluctuations of Hs and Hj also induce spin relaxation for example, the magnetization of a carbonyl carbon with a large chemical shift anisotropy relaxes due to the contribution from the time fluctuation of Hs. Nevertheless, since the main interest of polymer scientists is NMR, we focus on the description of the relaxation process in this chapter. 

The dominance of the paramagnetic shielding term in determining CO in the absence of large anisotropy effects is now generally accepted. Metal-carbonyl bonding, as monitored by NMR must therefore be explained in... 

All values are in ppm. The shielding terms follow the conventions of Chapter 2, namely, low frequency positive, with the isotropic shielding d = a a 22- )l > the shielding anisotropy for axial, symmetry, or more generally,... 

In order to discuss the origin of these terms we need to allow the spins to have anisotropic shielding tensors. Molecular tumbling in solution makes the chemical shielding in the direction of the external magnetic field a stochastic function of time and acts therefore as a relaxation mechanism, called the chemical shielding anisotropy (CSA) mechanism. The Hamiltonian for each of the two spins, analogous to Eq. (5), contains therefore two... 

The neighbor anisotropy term <7 of eq. (3.2) plays an important role in proton shielding, permitting, for example, a distinct differentiation between aromatic and olefinic protons due to the ring current effect. However, this contribution is small in 13C NMR (<2 ppm). A comparison of the methyl carbon shieldings in methylcyclohexene and toluene shows that the ring current effect often cannot be clearly separated from other shielding contributions ... 

The shielding factor is a property of the molecule, but as we see in later examples, the ability of the magnetic field to influence the motion of electrons depends on the orientation of the molecule relative to B0. Hence, O is a second-rank tensor, not a simple scalar quantity. It is always possible to define three mutually orthogonal axes within a molecule such that o may be expressed in terms of three principal components, on, molecular symmetry requires that two of the components of o be equal (and in other instances it is possible to assume approximate equality), so that the components may be expressed relative to the symmetry axis as chemical shielding anisotropy defined (ct — cr,). 

Another illustrative example is that of phenylacetylene. Table 6-7 summarizes the H NMR chemical shifts of its alkyne H-atom in a variety of solvents [273], Most solvents (except aromatic solvents) decrease the shielding of the acetylenic hydrogen nuclei. The corresponding low-field shifts have been interpreted in terms of weak specific association between the alkyne as hydrogen-bond-donor and electron pair-donor groupings of the solvent [273], The high-field shifts in aromatic solvents arise from the magnetic anisotropy of the solvent molecules (see below). The order of effectiveness of the solvent... 

The direction and magnitude of the shifts produced by lanthanide ions depends on their magnitude susceptibihty anisotropy terms. Because the unpaired electrons of the lanthanide ions are in 4f orbitals, and these are shielded by filled 5s and 5p orbitals, the magnetic terms are essentially independent of the ion s environment, and remain constant from complex to complex. In organic solvents such as chloroform, rare earths are found... 

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