# Bond Angles and intermolecular interactions

It's fun to consider bond angles of molecules in isolation. For example, what is the number of different bond angles exhibited by the most polar form of $\ce{SIBrClF2}$?

However, can bond angles really be considered in isolation? The method I would employ to determine the number of different bond angles only considers the molecule in isolation; I just consider van der Waals repulsions between the component atoms. Molecules generally do not exist in isolation. We can study molecules in the gas phase as to minimize intermolecular forces, but still, intermolecular forces exist, as it is hard to isolate exactly one molecule.

In the case of $\ce{SIBrClF2}$, I would expect a number of rather strong dipole-dipole interactions among the $\ce{SIBrClF2}$ molecules.

Therefore my question is: are there any cases in which considering solely intramolecular forces leads one astray when determining the bond angles of a molecule? I.e. one might look at a molecule and expect a certain bond angle to be < 180 degrees but find out the angle is in fact > 180 degrees due to intermolecular forces prying the bond angle open?

• You are asking so many questions on this site, that by now you should know that meaningful titles are very important. It would be a charm if there actually wasn't only a very general and broad topic in the title field. – Martin - マーチン Sep 3 '14 at 8:41
• For every bond angle larger than $\alpha > 180^\circ$, there is a bond angle $\alpha' < 180^\circ$, because three atoms always have to be in a plane, hence $\alpha' = 360^\circ -\alpha$. – Martin - マーチン Sep 3 '14 at 8:46

## 1 Answer

When people talk about bond angles in molecules, they are usually talking about the equilibrium bond angle in isolation. This can be derived from the energy minimized structure of the molecule in isolation.

As you point out, the angles in a real, non-isolated molecule will fluctuate due to interactions with neighboring molecules. However, the actual angle formed by three successive atoms in a molecule will naturally fluctuate even if the molecule is isolated and has some kinetic energy. Everything's always jiggling around.