Neighbor anisotropy term

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 contribution neighbor anisotropy effect. This term accounts for the fields arising from electronic circulations around the atoms surrounding the observed nucleus. It depends on the nature of the neighbor atoms and on molecular geometry. Sometimes, a medium term must be added in eq. (3.2), correcting for solvent and pH effects. 

The anisotropy term includes the contribution of electron currents on neighboring atoms or functional groups. 

Anisotropic Hyperfine Interaction. The anisotropic component of the hyperfine coupling has two contributions a local anisotropy owing to spin density in p- or type orbitals on the atom of observation, and nonlocal dipolar coupling with spin on other atoms. The first type of interaction is proportioned to the orbital coefficient (squared) of the pid orbiteds. To a first approximation the second term can be considered as a classic point dipolar interaction between the nucleus and the electron spin on a nearby atom. This depends on the total electron spin density at the neighbor (p ), the distance between the spins (r,2), and the orientation of the vector between them with respect to the external magnetic field (denoted by angle 0). In the point dipole approximation,... 

As noted earlier, Qa/9 may be equally well-defined in terms of other macroscopic properties such as the refractive index or dielectric tensor. However, the simple relation [Eq. (3.6)] cannot be expected to hold for the dielectric anisotropy Ae and electric polarizability aij. This is due to complicated depolarization effects caused by the relatively large near-neighbor electrostatic interaction. The internal field corrections [3.3] are necessary in the electric case. It has been shown that Qa can be used to describe orientational order both in uniaxial and biaxial phases. Furthermore, measurement of Qa/3 is particularly useful when description of flexible molecules using microscopic order parameters becomes problematic. Experimentally, both magnetic resonance and Raman scattering techniques [3.3] may be employed to monitor the orientational order of individual molecules and to determine microscopic order parameters. 

To include the effects of distant parts of the molecule, long-range terms may be added on. One contribution arises from the anisotropy of the magnetic susceptibility /lx of a neighboring group at distance R... 

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