PD Bond Dipole Models

Williams showed that an unrestricted bond dipole model gave erratic results for the bond dipole directions. To pursue the goals of chemical reasonableness and transferability, it seems appropriate to restrict the direction of the bond dipoles. The most natural direction is along the bond. Table 12 shows that restricted bond dipole models represent the electric potential about as well as monopole models. The two models also have nearly the same number of parameters. So the choice between these two models is a matter of convenience. If long distance interaction is considered, as in crystals, use may be made of the fact that dipole-dipole energy converges much faster than monopole-monopole energy. However, as mentioned above, if ions are considered, monopole interactions are still needed. Table 13 summarizes values of restricted bond dipole moments. 

The H —O dipole varies from 0.375 to 0.450 eA for this group of compounds, while the Me —O dipole varies from 0.242 to 0.311. The restricted bond dipoles of formyl groups attached to oxygen range from 0.189 to 0.231, while those for acetyl groups range from 0.265 to 0.308. 

The H —N dipole varies from 0.306 to 0.509, while the Me —N dipole varies from 0.249 to 0.508, except for trimethylamine and N-methylacetamide. The apparent reversal of the Me —N dipoles in trimethylamine may be an artifact caused by the nitrogen lone pair electrons. The acetylation of methyla-mine greatly reduces the Me —N dipole, whereas formylation does not. Formyl groups attached to nitrogen range from 0.217 to 0.306, while acetyl groups range from 0.100 to 0.363. 

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