Computational Chemistry

magnitude of energy differences. Further information on the reliability of these
methods is provided by the calculations for the four reactions summarized in
Fig.6.3, which were discussed in Section6.3.1in connection with geometries.
Figure6.4, based on the energies in Fig.6.3, makes these results clear. In all four
cases the semiempirical methods give the relative energies of the products semi-
quantitatively; the worst deviation from experiment [ 96 ] is for the PM3 relative
energy of HCN, which is 40 kJ mol"^1 ("99 cf."59 kJ mol"^1 ) too low. In fact, in
two of the four cases (H 2 C¼CHOH and HNC reactions) the AM1 product relative
energies are the best (and in the other two cases, the MP2 energies are the best);

exp 282

AM1 308

PM3 261

AM1 –34

exp –42

PM3 –53

MP2 –72

Relative E, kJ

0

200

400

–200

• 400

300

reaction

100

–100

–300

..

reaction

AM1 291

PM3 215

MP2 140

exp 129

AM1 –75

PM3 –99

MP2 –87

exp –58

reaction

AM1 348

PM3 243

MP2 173

exp 161

AM1 –130

PM3 –131

MP2 –120

exp –98

reaction

AM1 101

PM3 106

MP2 26

exp 13-20

MP2 –287

exp –293

AM1 –313

PM3 –282

O

H

H NC N C C

C

C

transition

states

products

MP2 233

Fig. 6.4 Relative energies (kJ mol"^1 ) for the four reactions of Fig. 6.3. Compared to the reactants
(the four species shown), the transition state energies are all positive and the product energies all
negative

422 6 Semiempirical Calculations