Computational Chemistry

DH¼Hðpdts=TSÞ#Hðreactants)

’Sbond energies(reactants)#Sbond energiesðpdts=TSÞ

$ð 5 : 180 Þ

(pdt or TS depending on whether we are considering reaction enthalpy or activation
enthalpy; we can ignore bonds that are neither broken nor made). Thus an exother-
mic process, which from the definition hasDH<0, has stronger bonds in the
products than in the reactants; in some sense the bonds lose heat energy, becoming
tighter and stabler. The bond energy tables given in most organic chemistry text-
books can be used to calculate rough values ofDH(reaction), andaccuratereaction
enthalpies can sometimes be obtained from the more sophisticated use of bond
energies and similar quantities [ 142 ]. To see an application of simple bond energy
tables [ 143 ], consider the keto/enol reaction:
Using Eq.5.180:

CH 3 C

O

H

H 2 C C

O

H

H

reactants

products

transition state

activation energy (> 0) ===> krate

reaction energy

(< 0 in his case) ===> Keq

energy

geometry

Fig. 5.25The reaction energy, the energy difference of products and reactants, determines the
extent of a reaction, i.e. its equilibrium constant. The activation energy (the simple ab initio energy
difference shown here is not exactly the conventional Arrhenius activation energy), the energy
difference of transition state and reactants, partially determines the rate constant. Unfortunately,
“energy” is ambiguous, since chemists use the terms potential energy, enthalpy (heat energy), and
free energy: seeSection 5.5.2.1

5.5 Applications of the Ab initio Method 299