Computational Chemistry

RX, and a ketone, thesp^2 carbon of which corresponds to the incipient cationic
carbon of the transition state.
One may wish a more precise approximation to the transition state geometry
than is represented by an intermediate or a compound somewhat resembling the
transition state. This can sometimes be achieved by optimizing to a minimum,
subject to the constraint that the bonds being made and broken have lengths

D3h

Input structure with this symmetry will be

optimized to the transition state

Input structures not of D3h symmetry will

be optimized to the minimum-energy

conformation

energy

dihedral angle

Fig. 3.9 Optimizing ethane within D3hsymmetry (i.e. by constraining the HCCH dihedral to
be 0, or by inputting a structure with exact D3hsymmetry) will give the transition state, while
optimization without requiring D3dsymmetry gives the ground-state conformer

reaction coordinate

energy

D3d

chair

C2v

boat

C 2

twist, or twist-boat

A C2v input structure will be optimized to

the transition state linking the C 2 conformers

Cs

halfchair

Fig. 3.10Optimizing cyclohexane within C2vsymmetry gives a transition state, not one of the
minima

62 3 Molecular Mechanics