Computational Chemistry

Equation (5.198) was used to calculate rate constants for the three unimolecular
reactions in Fig.5.30(cf. Fig. 5.21). Reactants, products and transition state
structures were created with Spartan [ 37 , 215 ] at the AM1 (a kind of semiempirical
method;Chapter 6) level. Transition states were calculated with Spartan’s transition
state routine starting from a guess based on the reactant and product structures and
the experience that bonds being broken or made in a transition state tend to be
roughly 50% longer than in a reactant or product. The AM1 structures were used as
inputs for MP2/6–31G (Section 5.4.2), B3LYP/6–31G (a kind of DFT calcula-
tion;Chapter 7), and G3(MP2) and CBS-QB3 calculations (Section 5.5.2.2b) with
Gaussian 03 [ 62 ]. A few remarks are appropriate on why I chose these particular
four computational levels, and on possible pitfalls associated with them. The first
two methods/basis are the most popular for routine calculations at correlated levels,
while G3(MP2) and CBS-QB3 are, where applicable (see above), reasonable
choices for high-accuracy multistep calculations. The Hartree–Fock level does
not, as a rule, give reasonably accurate reaction barriers [ 216 ], although the rule is
not unbreakable; for example, simple HF/6–31G* calculations give fairly good
torsional barriers for hindered methylbenzenes [ 217 ]. Hartree–Fockrelativebarriers
in a series of related reactions can be useful [ 218 ]. Note that the high-accuracy
Gaussian and CBS methods were developed for thermodynamics, not kinetics.
Nevertheless, they have been applied to the calculation of reaction barriers, and
CBS-QB3 in particular has been implied to be suitable for this purpose [ 186 ].
However, this and other standard Gaussian and CBS high-accuracy methods were

H 2 C

O

H

H

CH 3 C

H

O

C

O

H

H

C

H

H

ethenol

(vinyl alcohol)

ethanal

(acetaldehyde)

C

C

C

H

H H

H

..

cyclopropylidene allene

..

H

H

C C C

H

H

H

isocyanomethane

(methyl isocyanide)

propanenitrile

(acetonitrile)

CH 3 N C N C

C

H H

N C CH 3

C

Fig. 5.30 Reactions used to illustrate the calculation of rate constants and halflives with
Eq. (5.198). Cf. Fig.5.21

326 5 Ab initio Calculations