Axial chlorides, resonance frequencies

An unambiguous success of the n-a overlap model of the anomeric effect is its ability to rationalise 35C1 nuclear quadrupole resonance frequencies in axial and equatorial glycopyranosyl chlorides (David, 1979). The axial chlorides invariably resonate at lower frequency, in accord with the more ionic nature of the C—Cl bond and hence the more spherically symmetrical distribution of electrons around the chlorine nucleus. 

The resonance frequency of the axial chlorine is always lower than that of the equatorial chlorine (David 1979). Figure 2.6 suggests another point of view the dispersion of equatorial resonances, close to 0.5 Hz, has an order of magnitude, called the crystal effect by the specialists, of intermolecular origin. When first analysed, they should not be considered significant. On the other hand, the variation range of the axial resonances, 1.7 MHz, is quite superior to the crystal effects. This dispersion expresses the before-mentioned fact that the anomeric effect of chlorine (as with any other substituent) in a pyranose is not independent of the configuration of the rest of the molecule. Thus, the resonance of the axial D-manno chloride is by far the lowest and it is well known that the anomeric effect is intensified in a-D-manno derivatives. 

Let us now come back to equation (1). So far we have used it to calculate 35ci NQR frequencies from the computed 3pz orbital populations of two simplified models. Then we compared the calculated figures with the observed resonances of the more complicated pyranosyl chlorides. Conversely, we may start from these observed resonances, and use formula (1) to derive experimental values of the 3pz orbital populations of pyranosyl chlorides, so as to gain direct insight into their electronic structure. If no double bonding is involved, and if a constant value of 2 is adopted bor b9 we can accept as an experimental observation that on all pyranosyl chlorides examined, the 3pz orbital population on axial chlorine is higher than the 3pz orbital population on equatorial chlorine by an amount of about 5% (up to 7% in the case of mannose).Some people may consider that a statement in terms of ionicity i is more suggestive. Equation (1) may be written ... 

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