Biomedical Engineering Reference
In-Depth Information
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
100
200
300
400
Dihedral angle/degrees
Figure 5.1
Ethane dihedral motion
8
6
4
2
0
0
100
200
300
400
Dihedral angle/degrees
Figure 5.2
Plot of potential energy versus dihedral angle for CH
2
Cl-CH
2
Cl
5.2 Saddle Points
One-dimensional potential energy curves show minima, maxima and points of inflexion.
Two- and higher-dimensional surfaces often show a new feature called a
saddle point
.
Saddle points are a maximum in some variables and a minimum in the remainder. The one
type of saddle point that interests chemists is where we have a minimum in all variables but
one, and a maximum in this remaining one. This corresponds to a transition state in theories
of reaction mechanisms. I just mention it here, despite the fact that molecular mechanics
cannot properly treat bond breaking or bond making. I am not going to deal in detail with
saddle points until I have given you a firm grounding in quantum chemistry, but you should
be aware of their existence.
