A Toy Model of Chemical Reaction Networks - TBI - Universität Wien

3.5. PERFORMANCE 31
3.5 Performance
In order to validate the energy calculation, predicted total atomization energies
(TAE) have been compared to experimental ones, see figs. 3.9 and 3.10.
Experimental TAE values are taken from [18].
Fig. 3.9 shows well how every −CH 2 − unit corresponds to an energy increment
[11]. The distance between two successive points is almost constant.
Indeed, this can be rationalized by the fact that S and H can be brought
into a quasi-block-diagonal form, every block corresponding to one −CH 2 −
unit. It follows that the total energy is proportional to the number of these
units.
Calculated TAE (kcal/mol)
1500
1000
500
1500
1000
500
500 1000 1500
500 1000 1500
Experimental TAE (in kcal/mol)
Figure 3.9: Comparison of calculated and experimental Total Atomization
Energy (TAE) for homologous series of alkanes (methane to hexane, left) and
cycloalkanes (cyclopropane to cyclohexane, right).
Fig. 3.10 shows a reasonable correlation, given the rough approximations
of the Toy Model. The same TAE are calculated for cis/trans isomers because
their are topologically equivalent (Z,Z- and E,Z- and E,E-2,4-hexadiene, for
example ). Moreover, a big part of the energy differences between the C 6 H 10
stems from electrostatic repulsion, which depends on the steric configuration
ignored by the Toy Model (see ch. 7 for improvements).
The Toy Model does obviously not include the calculation of vibrational
and rotational energy and of entropy. [18, 67] discuss methods for their
approximation. However, it seems that their contribution is small.
Highest Occupied MOs (HOMOs) of type σ instead of π are predicted for
some olefinic systems. However, this is in agreement with the results of [57,
vol. I, fig. 10.33].