Page 62 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

ABCD and EFGH we can order A < B < C < D and E < F < G < H 41
on the basis that C–H bonding interactions outweigh C−C antibonding inter-
actions arising from weaker p-p overlaps. Placement of the set ABCD in SECTION 1.2
relation to EFGH is not qualitatively obvious. Calculations give the results Molecular Orbital
Theory and Methods
shown in Figure 1.17. 54 Pictorial representations of the orbitals are given in
Figure 1.18.
The kinds of qualitative considerations that we used to construct the ethene MO
diagram do not give any indication of how much each atomic orbital contributes to
the individual MOs. This information is obtained from the coefficients provided by
the MO calculation. Without these coefficients we cannot specify the shapes of the
MOs very precisely. However, the qualitative ideas do permit conclusions about the
symmetry of the orbitals. As will be seen in Chapter 10, just knowing the symmetry
of the MOs provides very useful insight into many chemical reactions.

1.2.5. Qualitative Application of MO Theory to Reactivity: Perturbational
MO Theory and Frontier Orbitals
The construction of MO diagrams under the guidance of the general principles and
symmetry restrictions that we have outlined can lead to useful insights into molecular
structure. Now we want to consider how these concepts can be related to reactivity. In
valence bond terminology, structure is related to reactivity in terms of the electronic
nature of the substituents. The impact of polar and resonance effects on the electron

D 0.892
σ ∗ 0.845
H 0.640
G 0.621
C 0.587
π ∗ 0.243
π – 0.371
B – 0.506
F – 0.562
A – 0.644
E – 0.782
σ – 1.01
Fig. 1.17. Ethene mole-
cular orbital energy
levels. Energies are in
atomic units. From W.
L. Jorgensen and L.
Salem, The Organic
Chemists Book of
Orbitals, Academic
Press, New York, 1973.

54
W. L. Jorgensen and L. Salem, The Organic Chemist’s Book of Orbitals, Academic Press, New York,
1973.