Oxygen-17 quadrupolar effects

The configurational analysis of compounds 2 to 5 involves determining whether the labeled oxygen (170 in 2 and 4,180 in 3 and 5) occupies the pro-/ or pro-S position. In most cases, the compound is first stereoselectively deriva-tized at one of the two oxygens. The position of 170 or 180 is then determined by one of three methods (i) stereoselective derivatization and degradation followed by mass spectral analysis (ii) isotope shift, effect in 31P NMR or (iii) 170 quadrupolar effect in 31P NMR. Since these methods are straightforward, no further discussion is provided here. Specific examples will be presented in Section IV, E. 

Whereas the 0 effect on P NMR chemical shifts is sufficient for determining the configurations of chirally labeled samples of prochiral phosphorus atoms in a diastereomeric environment, additional use of the quadrupolar effect of O on P NMR resonances is required for configurational analyses of oxygen chiral phosphate monoesters. The basic strategy for the configurational analyses of phosphate monoesters is the same, i.e., the enantiomeric center in the monoester must be converted to a diastereomeric center in a cyclic phosphodiester so that... 

The paramagnetic effects of oxygen molecules on the sodium cations in zeolite LiNaX (approximate to 70% Li 30% Na) have been investigated using VT Na MAS NMR and Na MQ MAS NMR. 23 jyjg been used to calculate the isotropic chemical shifts, quadrupolar shifts, and the quadrupolar coupling constants. 

For sugars substituted by oxygen at C-1, delocalization is more difficult to prove because the quadrupolar resonance spectrum cannot be observed. The atomic polarizability of oxygen, 0.802, places it between fluorine and chlorine, so it seems very unlikely that the anomeric effect would comes from a radically... 

Methods based on the effects of the quadrupolar O nucleus (/ = f) and the 0 nucleus (7 = 0) on the P NMR spectral properties of the isotopically labeled P nucleus are now the only methods used to ascertain the configurations of tetrahedral phosphorus atoms in oxygen chiral phosphate esters. The chemical... 

The symmetry point group of the molecule in the quadrupolar field is thus C2v, which bears the same relation to Coov as D2/1 does to Doo/i- The reduction of symmetry from Doo/i to Coov or from D2/1 to C2V - is a result of a dipolar field along the z axis. The greater electronegativity of oxygen than carbon is the natural equivalent of the artificial dipolar field introduced in Section 2.2.6, the effect of which on an isolated atom was illustrated in Fig. 2.9. 

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