Chemistry Reference
In-Depth Information
Table 2.9
Symmetries of atomic bases involving sz-mixing in first-row hydrides
Molecule
AO basis
Symmetry
HF
ð
szh
Þ
S
1
szh
z
¼
p ð
h
1
þ
h
2
Þ
H
2
O
A
1
1
NH
3
szh
z
¼
p ð
h
1
þ
h
2
þ
h
3
Þ
A
1
1
2
ð
CH
4
sh
s
¼
h
1
þ
h
2
þ
h
3
þ
h
4
Þ
A
1
1
2
ð
h
1
h
2
h
3
þ
h
4
Þ
zh
z
¼
T
2z
(Magnasco, 2009b) and NH
3
(Magnasco, 2009c). For all polyatomic
molecules, we shall show that sp hybridization opens the interbond angle
beyond 90
and that minimization of the valence H
uckel energy against
the hybridization parameter yields straight bonds satisfying the principle
of maximum overlap.
€
2.7.1 The Methane Molecule
We now apply our model to investigate the formation of four C-H bonds
in the methane molecule CH
4
of symmetry T
d
(Magnasco, 2004a). For
this molecule, tetrahedral sp
3
hybridization is completely determined by
molecular symmetry.We use the usual notation for the eight valence AOs,
calling s, x, y, z the 2s and 2p orbitals onC, and h
1
h
4
the 1s orbitals
on the H atoms at the vertices of the tetrahedron (Figure 2.14). Molecular
;
h
2
;
h
3
;
Figure 2.14
The cube circumscribing the tetrahedral CH
4
molecule
