Computational Chemistry

as being more reliable than thein vacuoones. Solvent effects are discussed in
Chapter 8 Section 8.1.
6.Lack of caution about comparing energies calculated with MM.The method
calculates the energy of a molecule relative to a hypothetical strainless idealiza-
tion of the molecule. Using MM to calculate the relative energy of two isomers
by comparing their strain energies (the normal MM energies) is dangerous
because the two strain energies are not necessarily relative to the same hypo-
thetical unstrained species (strain energies are not an unambiguous observable
[ 34 ]). This is particularly true for functional group isomers, like (CH 3 ) 2 O/
CH 3 CH 2 OH and CH 3 COCH 3 /H 2 C¼C(OH)CH 3 , which have quite different
atom types. For isomers consisting of the same kinds of atoms (alkanes cf.
alkanes, say), and especially for conformational isomers andE/Z isomers
(geometric isomers), a good MM forcefield should give strain energies which
reasonably represent relative enthalpies. For example, the MMFF gives for
CH 3 COCH 3 /H 2 C¼C(OH)CH 3 strain energies of 6.9/–6.6 kJ mol–1, i.e. relative

0

04000 3500

MMFF^3732

3000 2500 2000

FREQ_VAL

FREQ_VAL

1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

4e+002

47

93

1.4e+002

8e+002

1.2e+003

IR_INTENS

IR_INTENS

4000 3000

Methanol

Experimental

2000 1000

20

40

60

80

MP2/6-31G*

(^37953223)
3144
3076
1418
1294
10691043
1083
(^370036732826)
2865, 2844
2981
(^10561033)
1016
Fig. 3.17 Experimental (gas phase), and MM (MMFF) and ab initio (MP2(fc)/6–31G*) calcu-
lated IR spectra of methanol
3.6 Strengths and Weaknesses of Molecular Mechanics 77