Computational Chemistry

minimum through a transition state to another minimum. There are four known
forces in nature: the gravitational force, the strong and the weak nuclear forces, and
the electromagnetic force. Celestial mechanics studies the motion of stars and
planets under the influence of the gravitational force and nuclear physics studies
the behaviour of subatomic particles subject to the nuclear forces. Chemistry is
concerned with aggregates of nuclei and electrons (with molecules) held together
by the electromagnetic force, and with the shuffling of nuclei, followed by their

C1

C2

C3

.

CH 3

C1

C2

C3

C1

C2

C3

C1

C2

C3

.

C3

C2

C1

.

C1

C2

C3

.

C3

C2

C1

.

CH 3

C2

C3

C1

.

C2

C3

C1

total of 6 eclipsing interactions

total of 3 eclipsing interactions no eclipsing interactions 3 eclipsing interactions

(CH / CC, CH / CH, CH / CH)

no eclipsing interactions no eclipsing interactions no eclipsing interactions

sawhorse drawings Newman projections

hiltop

transition state

minimum

a

c

b

CH 3

3 eclipsing interactions

(CH / CC, CH / CH, CH / CH)

3 eclipsing interactions

(CH / CC, CH / CH, CH / CH)

CH 3

CH 3 CH 3

Fig. 2.10 The stationary points on the propane potential energy surface. Hydrogens at the end of
CH bonds are omitted for clarity

20 2 The Concept of the Potential Energy Surface