A point-charge representation of non-bonding electron pairs

6 A point-charge representation of non-bonding electron pairs 

In this section, we develop simple point-charge models for representing n-pairs on the acceptor atom in each of three molecules B = H2O, HjS and HjCO. We show that, when taken with the extended electric dipole model of HF, the 

This approach forms the basis of the Buckingham-Fowler method [12] discussed above. An alternative model [16], which is set out here, begins with the point charges on the atoms but accounts for the deviations from sphericity about atom centres by translating small fractions of the point charges from the acceptor atom centre of B along directions conventionally associated with non-bonding pairs. 

It is found [16] that physically reasonable solutions are generated for only a limited range of values of S and r in the region of 0.04 e and 1 A, respectively. Indeed, for r % 0.5 A, the double minimum apparent in the full calculation 

The main advantages of this model are its easy visualisation and its obvious affinity with the valence-shell-electron pair repulsion model. The difference of this approach from many earlier methods of representing non-bonding pairs by point charges is that it needs only relatively tiny charges (e.g. few hundredths of an electron) at reasonable distances (r 1 A). These small charges are generated naturally from the condition that the observed barrier 

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