Non-equivalence, Configuration, and Medium Effects

Non-equivalence, Configuration, and Medium Effects.—Extensive studies of the n.m.r. spectra of epimers in solution have been published by the Moscow chemists. Following observations on compounds such as the valine derivative (69), that the signals 

A chiral solvent, (+)- or (—)-l-phenylethanolamine, was used in order to distinguish the dl- and wejo-forms of the diphosphine (77). The high-field signed was split into two resonances, showing it to be due to the dl-fotm The non-equivalence of the methoxy protons in chlorophos in various solvents and at different temperatures 

Phosphorus-31 chemical shift anisotropies for trimethylphosphine, its oxide, and its sulphide were 6, +210, and +127 p.p.m., respectively. jr-Bonding will provide a cylindrical mobile electron cloud which can circulate freely when the symmetry axis is parallel to the magnetic field, but which is hindered when it is perpendicular. Thus Cii — cTj should be positive, as is observed for the oxide and sulphide, and the smaller value for the sulphide compared to the oxide could reflect reduced n-bonding. 22 

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