Computational Chemistry

4 and so was able to undergo Wolff rearrangement ring contraction to the ketene
precursor of 4.
A remarkable (and apparently still unconfirmed) prediction of MM is the claim
that the perhydrofullerene C 60 H 60 should be stabler with some hydrogensinsidethe
cage [ 29 ].

3.4 Geometries Calculated by MM

Figure3.13compares geometries calculated with the Merck Molecular Force Field
(MMFF) with those from a reasonably high-level ab initio calculation (MP2(fc)/
6–31G*; Chapter 5 ) and from experiment. The MMFF is a popular forcefield,
applicable to a wide variety of molecules. Popular prejudice holds that the ab initio
method is “higher” than molecular mechanics and so should give superior geome-
tries. The set of 20 molecules in Fig.3.13is also used in Chapters 5 , 6 , and 7 ,
to illustrate the accuracy of ab initio, semiempirical, and density functional calcula-
tions in obtaining molecular geometries. The data in Fig.3.13are analyzed in
Table3.3. Table3.4compares dihedral angles for eight molecules, which are also
used in Chapters 5 , 6 , and 7.

(CH 2 )n

(CH 2 )n

2

.

.

3

O

O H H

H

O

H

1

O H

5

O H

N 2

6

H

4

Fig. 3.12 Some molecules ( 1 , 2 , 4 ) which have been synthesized with the aid of molecular
mechanics

3.4 Geometries Calculated by MM 67