Chemistry Reference
In-Depth Information
Figure 1.1. (a) Energy levels of N
2
. The low-lying 1
σ
g
and 1
σ
u
orbitals are not
shown. (b) Spectrum of N
2
. Adapted from Eland, 1984; p. 7.
from its conceptual simplicity but at the same time warning us of the complexity
in the formation of 2D and 3D networks when even simple molecules are involved.
Many of the phenomena to be discussed in the topic will be introduced in this
section with simple molecules.
Figure 1.1(a) shows the energy levels of N
2
. The core N1
s
orbitals are com-
bined to produce 1
u
molecular orbitals (MOs) (the number 1 indicates
that these are the lowest energy orbitals of their respective symmetries). Subscript
g (from German
gerade
, meaning even) refers to symmetry with respect to inver-
sion (the centre of the molecule acting as the inversion centre), while subscript u
(from German
ungerade
, meaning uneven) is used for antisymmetry with respect
to inversion.
The N2
s
and N2
p
orbitals of the two nitrogen atoms combine into pairs of
sp
hybrids and of
p
π
atomic orbitals on each nitrogen atom. When the larger lobes
of the
sp
hybrids are directed towards each other along the molecular axis they
give rise to bonding
σ
g
and 1
σ
σ
∗
(3
σ
u
) orbitals, while the face-
to-face combination of the smaller lobes gives rise to non-bonding 2
σ
(2
σ
g
) and antibonding
σ
g
(lone-pair combinations). The
p
π
orbitals, which consist of N2
p
orbitals directed
perpendicularly to the molecular axis, produce bonding
σ
u
and 3
π
∗
π
(
π
u
) and antibonding
(
π
g
) orbitals.
The orbital energies can be experimentally determined e.g., through ionization
mediated by light excitation (photoelectron emission). According to Koopmans'
