Magnetic field effects three-bond

Both through-bond and pseudocontact contributions can be easily factorized into a series of products of two terms, each term depending either on the nucleus i (topologic and geometric location) or from the lanthanide j (electronic structure and crystal-field effects). For axial complexes, that is, possessing at least a three-fold axis as found in triple-stranded helicates, the molecular magnetic susceptibility tensor written in the principal magnetic axes system is symmetrical xx = mag-... 

To make the connection between polymer microstmctures and S Ci S, we need to know the dependence of the local magnetic field B on the local conformation. The y-substitu-ent effect, which shields an observed carbon nucleus, is the source of the dependence of the local magnetic field B on the local conformation. Because the observed carbon C° and its y-substituent C are separated by three intervening bonds (-C°-C-0 -C-C > -), their mutual distance and orientation are variable, depending on the conformation (f>) of the central bond. Note that the distance between C° and C is reduced from 4 to 3 A on changing their arrangement from trans 4> = 0°) to gauche - (4> = 120°). 

Conformation and Chemical Shifts.—Chemical shifts have been correlated with conformation for H shifts in polystyrene and poly(vinyl chloride) and C shifts in model compounds of polypropylene. In all these papers, the chemical shift is related empirically to the occurrence of three- and four-bond steric interactions, similar to those used in the rotational isomeric state treatment of polymer statistical mechanics, " and the shift is expressed as a sum of compositional and conformational increments. The origin of the shielding contributions (magnetic anisotropy, electric field effects, etc.) is not stated, except for polystyrene in which the magnetic anistropy of the aromatic rings are incorporated... 

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