Representation of the molecular electrostatic potential by atomic multipole and bond dipole models

Ab initio wave functions using the 6‐31G** basis set were calculated for a set of 24 organic molecules which included saturated and unsaturated hydrocarbons, fluoro compounds, hydroxyl and ether compounds, carboxylic acids, esters, amines, carbonyl compounds, amides, and N‐methyl amides. The electric potential on a grid within a van der Waals shell around each molecule was calculated directly from the wavefunctions. The electric potential values were modeled by placing multipoles up to quadrupoles at atomic sites. The electric potential was well fitted by a model which included atomic monopoles, dipoles, and quadrupoles. Also, the electric potential was modeled by placing dipoles at bond centers. This bond dipole was either allowed to point in any direction, or was restricted to the bond direction. In general the values shown for the monopoles and the values and directions for the bond dipoles were as expected from considerations of electronegativity and reactivity. For the general bond dipole model in a number of cases the direction of the bond dipole was not parallel to the bond direction. It is suggested that this is an artifact caused by the effects of lone pair electrons or electron delocalization on the model. The restricted bond dipole model fitted the electric potential about as well as the atomic monopole model. Atomic monopole values (net atomic charges) and bond dipole values for various atoms, functional groups, and bonds are discussed. Since bond‐dipole interaction energy has better long‐range convergence than monopole interaction energy, bond dipoles are a useful alternative to atomic monopoles.