Face to face repels, edge to face attracts

One of the figures on where the atoms go: Repulsion minimised on a sphere, the angles that fall out of it, and the arrangements that are not all alike.

One essay draws this figure, each at the values its own argument needs rather than at the setting shown above. What each one uses it to show is below, in the words of its own caption.

Zero dipole is not no interaction

Four molecules whose dipole moment is exactly zero, with the quadrupole moment that survives. Benzene’s is by far the largest. Acetylene’s has the other sign, because the ends of that molecule are hydrogens rather than lone pairs — the sign is a statement about which way the density is displaced, not a magnitude with a convention attached.

Two quadrupoles and two dipoles at separations from 8 to 34 Å, on logarithmic axes, with the slopes fitted rather than assumed. They come out at 5.00 and 3.00.

The interaction between two benzenes as one is tilted against the other, with only the quadrupole between them. Stacked, the two negative faces meet and the energy is +8.8 kJ/mol. Perpendicular, one molecule’s positive rim meets the other’s negative face and it is −4.6. The sign changes between 50° and 60°.