Neighbor anisotropy effect

Nonlocal contributions neighbor-anisotropy effects ring-current effects electric-field effects steric effects... 

In early work, Spiesecke and Schneider (59) pointed out that inductive effects alone cannot account for a- and -signal shifts. They held diamagnetic neighbor-anisotropy effects (63) arising from anisotropic electron-charge distributions responsible for the deviations in the electronegativity correlations. For bonds with conical symmetry they applied McConnell s magnetic point-dipole approximation (64) for the estimation of this contribution, Act ... 

So it appears that, in general, neighbor-anisotropy effects are small and easily overshadowed by various other effects, and therefore are not of essential importance for >3C shielding interpretations. 

The contribution 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. 

Certain protons of fused troponoids show remarkable low-field shifts (Table XII, Scheme 75 for comparison, see Tables IX and X). These protons are abnormally deshielded by the paramagnetic anisotropy effect of peri-positioned or other neighboring groups, especially the tropone carbonyl groups. Similar low-field shifts are caused by adjacent nitro or halogen substituents [e.g., 74YZ1445 80JCS(P1)2081],... 

The mechanisms of substituent effects are numerous23 the most prominent being inductive, mesomeric, neighbor anisotropy, ring-current, electric-field and steric effects, as well as van der Waals compression. 

Influence of Induced Magnetic Moments (Neighbor Anisotropy) and Ring-Current Effects... 

One important difference between the solid and liquid phases is that the isotropy of the liquid is replaced by the anisotropy of the solid. The geometrical correlations that exist in a solid lattice may be expected to play an important part when reaction centers are localized and can react with their neighboring environment. In this section we point out some of these effects in the radiolysis of solid n-hexane the trans/cis hexene yields, the isomeric dimer pattern, and the fragmentation processes. 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

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 ... 

---