Ring anisotropy effects

A dramatic decrease in the magnitude of the magnetic susceptibility anisotropy is observed on going from thiirane to the open-chain analog, dimethyl sulfide, and has been attributed to non-local or ring-current effects (70JCP(52)5291). The decrease also is observed to a somewhat lesser extent in oxirane relative to dimethyl ether. 

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

Likewise, a systematic use of such anisotropy effects has been used to establish the conformation of small rings from the proton shielding in the corresponding allenic phosphine oxides,<1967,153tol56 1968,46 1969 164 to 166) for example in 170. 

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

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 ... 

Trifluoromethyl-substituted517 and perfluorophenyl-substitutedcarbocations518,519 have also been prepared and studied. Because of the relatively large fluorine chemical shifts, anisotropy and ring current effects play a relatively much smaller role than they... 

Table 1 also lists the <5(OHO) values of AA in the various solvents. There is little correlation with either e or the hydrogen bonding potential of the medium. The signal furthest downfield occurs in benzene solution and probably arises due to anisotropy effects of the benzene ring which is in close association with the enol tautomer14. ... 

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