Chapter 25 Some Additional Topics
319
With  = (RC) + (PC), there are seven variational parameters when
0 < || < . A variationally-better  = (RC) + (PC) is of course obtained
when the atomic orbitals for the (RC) and (PC) differ.
3.
The Electronic Structure of C 6 H 6
In Ref. 6c, the six -electrons of C 6 H 6 are partitioned to form two sets of
3-electron 3-centre bonding units. Two increased-valence structures of the type
Y A B have been incorporated
5 into the valence-bond structures, to give six
Kekulé- and six Dewar increased-valence structures of types III and IV (cf.
Figure 4 of Ref. 5).
(-q)
(+q)
(+q)
(-q) (-q)
(+q)
(+q)
(-q)
III
IV
The results of STO-6G VB calculations
6c for the 2p-electrons of C 6 H 6 and the
six 1s-electrons of H 6 with D 6h symmetry show that resonance between the
Dewar-type increased-valence structure generates a lower energy than does
resonance between the Kekulé-type increased-valence structures. This result has
been associated
5 with the more favourable distribution of the atomic formal
charges in structure IV relative to that for structure III.