Computational Chemistry

charges, have been excluded from consideration here). A high-level calculation
places a total charge (carbon plus hydrogen) – albeit defined in a different way – of
"0.37 on the CH 2 group andþ0.37 on the ring (cf."0.487 and +0.487 for the
exocyclic carbon and the ring carbons in the SHM calculation).
There are many other applications of the SHM [ 21 c, e] including fairly recent
and perhaps unexpected ones such as correlation with UV solvent shifts [ 51 ] and
even physicochemical properties [ 52 ].

4.4.4 Strengths and Weaknesses of the Extended Hu ̈ckel Method....

Huckel Method€

4.3.6.1 Strengths

The SHM has been extensively used to correlate, rationalize, and predict many
chemical phenomena, having been applied with surprising success to dipole
moments, ESR spectra, bond lengths, redox potentials, ionization energies, UV
and IR spectra, aromaticity, acidity/basicity, and reactivity, and specialized books
on the SHM should be consulted for details [ 21 , 22 ]. The method will give probably
give some insight into any phenomenon that involves predominantly thepelectron
systems of conjugated molecules. The SHM may have been underrated [ 53 ] and
reports of its death are probably exaggerated. However, the SHM is not used
very much in research nowadays, partly because more sophisticatedpelectron

+

c = – 0.368 in

c = 0.254 in

0.612 in

c = 0.815 in

0.282 in

0.523 in

q 1 = – 0.487

q 2 = 0.122

q 3 = 0.182

+2.170 0

a - 2 b

a + 2 b

a - b

a

a + b

a + 0.311 b

y 2

y 2

y 2

y 2

y 1

y 1

y 1

y 1

Fig. 4.24 The SHM charges on the atoms of a molecule can be calculated from the number of
electrons in each occupied MO and the coefficients of these MOs. The predicted dipolar nature of
methylenecyclopropene has been ascribed to a cyclopropenyl-cation-like resonance contributor

144 4 Introduction to Quantum Mechanics in Computational Chemistry