Anisotropy effect

Anisotropy Effects. The deshielding influence of the phosphoryl group on the methylene protons of phosphorinanes has been discussed also the anisotropy of 

Phosphorus-31. A selection of reference standards for use with aqueous samples have been described.  

Electronic effects induced by varying the phenyl substituent Y in phosphonate (15) and phosphate (16) have opposite effects on Sp. Increased electron- 

Handler, Y. Aktalay, J. Hahn, and E. Daerr, Z. Anorg. Allg. Chem., 1981, 473, 20. M. Handler and J. Hellmann, Z. Anorg. Allg. Chem., 1982,490,11. 

8p of n compounds. Whilst di- and tri-chlorophosphoranes exhibit signals in the usual n phosphorane region, - the bromophosphoranes vary more widely. - Further work on the chemistry of Wittig oxyphosphetanes has utilized P chemical shift data. Several polycyclic hydridophosphoranes are reported which have 8p values varying from —45 to +46.  

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The SH, SCHs, and weakly directing halogens cause small shifts, which to a large extent are determined by magnetic anisotropy effects, especially in the case of the halogens. Attempts have been made to estimate these effects for the other thiophenes. Except for orthohydrogens, these effects are usually very small. 

Compound 68 can also be obtained by an acid-catalyzed cyclization of 42, which was prepared by the Michael addition reaction of 39 to mesityl oxide as shown in Section IV.A. As for the product 69, the presence of the tosyloxy group at the 5 position instead of the 6 position is determined, utilizing the anisotropy effect of the 1-acetyl group to the C-7 proton, by comparing its H NMR spectrum with that of 70, obtained in 69% yield by the treatment of 69 with NaH and AcCl. 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

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 correlation, especially when applied to a variety of parent molecules, is not as good as one might hope, however, indicating that other contributions must be operative. Spiesecke and Schneider (59) were the first to propose the existence of magnetic anisotropy effects (Section II-B-2) whereas Bucci (62) suggested an... 

Selected entries from Methods in Enzymology [vol, page(s)] Anisotropy effects, 261, 427-430 determination by dynamic laser light scattering (quasi-elastic light scattering), 261, 432-433 determination for nucleic acids by NMR [accuracy, 261, 432-433 algorithms, 261, 11-13, 425, 430 carbon-13 relaxation, 261, 11-12, 422-426, 431, 434-435 cross-relaxation rates, 261,419-422, 435 error sources, 261, 430-432 phosphorus-31 relaxation, 261, 426-427, 431 proton relaxation, 261,51,418-422 relaxation matrix calculations, 261,12] deuterium solvent viscosity effects, 261,433 effect... 

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. 

Similarly alkoxy and aryloxy substituents in the neighbouring phosphorus exert a downheld shift. It has been observed that certain substituents such as Br, Ph, etc. shield the phosphorus by a neighbouring group anisotropy effect. 

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