Computational Chemistry

are essentially improved versions of MNDO. The general impression is that the
semiempirical and ab initio transition states are qualitatively similar in most cases,
with MNDOC geometries being sometimes a bit better. The semiempirical and ab
initio geometries were in most cases fairly similar, so that as far as geometry goes
one would draw the same qualitative conclusions.
Semiempirical and ab initio geometries are compared further in Fig.6.3, which
presents results for four reactions, the same as for the ab initio calculations sum-
marized in Fig. 5.21. As expected from the results of Fig.6.2, the semiempirical

C

C C

..

C2v

1.502

1.4821.496

1.486

1.473

1.496

60.7

61.0

59.1 58.6

59.560.4

C H O

1

1.335

1.3561.293

1.296

1.286

1.295

1.4241.406

1.406

1.5721.571

1.521

113.8112.0

114.0

121.2120.3

118.4

308

261

233

282

Cs

348

243

173

161

C

N C

1.803

1.831

1.897

1.700

1.7781.753

1.226

1.2171.206

1.121

1.0951.088

1.119

1.0951.085

65.0

68.065.9

74.172.7

78.5

291

215

140

129

1.217

1.212

1.196

1.3991.384

1.455

1.298

1.388

1.177

59.064.3

65.9

67.5

63.9

72.8

H

N C

Cs

O

HNC

H

O

N C H

CNC

N C C

C C C

Cs

Cs

Civ Civ

C3v

C3v

D2d

1.1251.097

1.091

110.1

109.7109.4

1.3951.433

1.426

1.181

1.1811.154

1.1631.159

1.180

1.4391.441

1.462

1.120

1.0981.093

–130

–131

–120

–98

0

0

0

0

1.298

1.297

1.313

115.4

115.4117. 2

–313

–282

• 287
  –293
  101
  106
  26
  13-20

0

0

0

0

0.968

0.9510.974

1.372

1.3571.369

1.336

1.3351.337

115.7

117.7114.4

109.1108.1

109.1

AM1

PM3

MP2

0 AM1

0 PM3

0 MP2

0 exp

1.490

1.4991.503

1.231

1.210

1.223

1.117

1.0981.091

108.5

107.5

107.2

121.2

120.0

120.3

• 34


• 53
  –72


• 42

0

0

0

0

–75

–99

• 87
  –58

0.967

0.976

1.003

1.178

1.1781.187 1.1601.156

1.177

1.0691.071

1.069

Cs

1.413

1.408

1.389

1.920

1.9202.014

85.6

86.0

92.2

47.2

47.0

47.247.0 44.2

43.6

1.413

1.4081.406 C

C C

Fig. 6.3 Geometries (A ̊, degrees) and relative energies (kJ mol"^1 ) for four reactions, the same as
for the ab initio calculations of Fig.5.21; most Hs are omitted, for clarity. In Fig. 5.21, raw
energies in hartrees and ZPEs were given. The experimental relative energies [ 96 ] are somewhat
approximate for the ethenol (vinyl alcohol) and cyclopropylidene reactions

418 6 Semiempirical Calculations