Computational Chemistry

4.3.5.3 Resonance Energies

The SHM permits the calculation of a kind of stabilizing energy, or, more accu-
rately, an energy that reflects the stability of molecules. This energy is calculated by
comparing the total electronic energy of the molecule in question with that of a
reference compound, as shown below for the propenyl systems, cyclobutadiene,
and the cyclobutadiene dication.

cyclic species

nonbonding level

C 3 C 4 C 5 C 6 C 7

0

acyclic species

nonbonding level

C 2 C 3 C 4 C 5 C 6

a- 2 b

a-b

a+b

a+2b

0

a- 2 b

a-b

a+b

a+2b

Fig. 4.21 The MO pattern for acyclic and cyclicpsystems, as predicted by the simple H€uckel
method

138 4 Introduction to Quantum Mechanics in Computational Chemistry