1912

Dipole Moments

Peter Debye (1884–1966)

A fundamental characteristic of atoms in a molecule is that they are all surrounded by clouds of electrons. And these electrons aren’t evenly distributed—quantum mechanics ensures that they’re more likely to be found in some places than others. Only if you have a perfectly symmetrical bond (such as hydrogen-hydrogen), will both atoms have the same electron density, with the bonding electrons spread out evenly between them.

But there are a lot of uneven chemical bonds in the world, as American chemist Linus Pauling would show systematically (see The Nature of the Chemical Bond). Before that, however, Dutch-American physicist and chemist Peter Debye gave science a way to think about them. An uneven distribution of charge is called a dipole. One end ends up with more negative charge, and the other with more positive. The “dipole moment,” defined by Debye in a 1912 paper, is the strength of the charges times the distance between them, and it can be added up and measured for whole molecules as well. Solvents with a strong dipole moment have higher boiling points (attraction between charges on adjacent molecules makes them stick together more) and are much better at dissolving other polar species. Dimethyl sulfoxide (commonly referred to as DMSO) is an example of a high-dipole organic solvent—it will even dissolve many ionic compounds (salts), which makes it very useful for a range of chemical reactions. Dipolar interactions are very important in dissolving compounds and when compounds interact with each other (as when a drug molecule binds to its biological target in the body).

Debye himself emigrated from Germany to America as World War II was starting. In recent years, accusations surfaced that he might have been a spy for the Germans, which was surprising considering his efforts to help his Jewish colleagues. These charges do not appear to have been substantiated, and some new evidence suggests that he might have been a spy for the British instead. Debye made a number of contributions to physics and chemistry, but it now seems that he was carrying around even more in his head than anyone realized.

Peter Debye, posing for a bronze bust commissioned by the Netherlands’s Ministry of Education in 1937. A Nobel Prize increases the chances of this sort of thing happening.