The Molecular Potential Energy Surface - Molecular Modelling - page 65

5

The Molecular Potential Energy
Surface

For one-dimensional problems, we speak about a molecular potential energy curve .
The simple potential energy curves we have met so far have all shown a single min-
imum. From now on, life gets more complicated (or interesting, depending on your
viewpoint).

5.1 Multiple Minima

The plot in Figure 5.1 shows how the ethane molecular potential varies with dihedral
angle. The figure shows a full rotation of 360 ◦ ; all the remaining geometrical variables
were kept constant. Note that there are three identical minima (and of course three identical
maxima), and the differences between maxima and minima are all the same. The chemical
interpretation is that these three minima correspond to conformers where the hydrogens are
as far apart as possible (i.e. in the trans position). The maxima correspond to conformers
where the C-H bonds eclipse each other. Multiple minima are common in potential energy
surface studies, as we will see.
Consider now the substituted ethane CH 2 Cl-CH 2 Cl. Aplot of the potential energy versus
the ClC-CCl dihedral angle is shown in Figure 5.2. There are three minima, one lower than
the other two. The three minima are referred to as local minima and the minimum at 180 ◦
is called the global minimum . The global minimum corresponds to a conformation with
the two chlorines as far apart as possible. The two other minima correspond to conformers
where each chlorine is trans to hydrogen.