The Forces between Molecules - Molecular Modelling - page 21

2

The Forces between Molecules

When molecules are near enough to influence one another, we need to concern ourselves
with the balance between the forces of attraction and repulsion.We know that such attractive
forces exist, for otherwise there would be nothing to bring molecules together into the solid
and liquid states, and all matter would be gaseous. A study of the forces between atomic or
molecular species constitutes the subject of intermolecular forces .
People have speculated about the nature of intermolecular forces ever since the ideas of
atoms and molecules first existed. Our present ideas, that molecules attract at long range
but repel strongly at short range, began to emerge in the nineteenth century due to the
experimental work of Rudolf J.E. Clausius and Thomas Andrews. It was not until the early
twentieth century, when the principles of quantum mechanics became established, that we
could truly say that we understood the detailed mechanism of intermolecular forces.
Although we talk about intermolecular forces , it is more usual and convenient to focus
on the mutual potential energy , discussed in Chapter 1. If we start with two argon atoms
at infinite separation, then their mutual potential energy at separation R tells us the energy
change on bringing the atoms together to that distance from infinity.
Even for the simplest pair of molecules, the intermolecular mutual potential energy will
depend on their relative orientations in addition to their separation. Perhaps you can now
see why the study of intermolecular forces has taken so much effort by so many brilliant
scientists, over very many years.

2.1 Pair Potential

So, to start with, we concern ourselves with two atomic or molecular species A and B, and
ask how they interact. No chemical reaction is implied, and I should say straightaway that
I am not going to be concerned with bond making and bond breaking in this chapter. That is