Chemistry Reference
In-Depth Information
Fig. 10.2
The orbitals of
formaldehyde (from Jaffe
and Orchin)
give rise to characteristic absorption bands. Such groups of atoms, usually containing
p
bonds, are
referred to as
bonds are compounds that contain
carbonyl or nitro groups and aromatic rings. A molecule that serves as an example of carbonyl
arrangement, one that is often referred to, is a molecule of the formaldehyde. In this molecule, the
carbon atom is linked to two hydrogens and to one oxygen by
chromophores
. Examples of such molecules with
p
bonds. The hybrid sp
2
orbitals bond
one electron of carbon with one of oxygen in an sp orbital. In addition, there are two unbonded n
electrons on oxygen that point away from the carbon atom. The orbitals of formaldehyde, the simplest
of the carbonyl compounds, were illustrated by Orchin and Jaffe [
88
], as shown in Fig.
10.2
.
As described above, the molecule has
s
bonded skeleton, shown above. The carbon atom is
attached to two hydrogen atoms by a single bond and to the oxygen atom by a double bond. This bonding
of the carbon to the two hydrogens and one oxygen atoms is by means of sp
2
hybrid orbitals. The orbitals
are approximately at 120
angles from each other. In the ground state of the molecules, the pair of
electrons that form a bond is paired and has opposite or antiparallel spin. In this state, the formaldehyde
molecule is planar. The Pauli exclusion principle [
83
] states that no two electrons can have identical
quantum numbers. That means that if two electrons are in the same orbital and three of their quantum
numbers are the same, the fourth quantumnumber, the spin quantumnumber, must be different. The total
spin quantum number of a molecule is designated by a letter
s
and
p
J
and the sum of the spins of the individual
electrons by a letter
S
. The spin quantum number of a molecule
J
is equal to [2
S
]+1.
* antibonding orbitals.
Absorption of light energy by a chromophore molecule results in formation of an excited state and an
electronic transition from the ground state to an excited state. Such light may be in the ultraviolet or in
the visible region of the electromagnetic spectrum, in the range of 200
This arrangement of electrons in the p orbital can generate
p
bonding and
p
m
m to approximately 780
m
m.
Promotion of electrons out of the
bonding orbitals to the excited states requires a large amount of
energy and rupture of bonds in the process. On the other hand, the electronic transition to promote one
of the n electrons on the oxygen atom in formaldehyde to the
s
antibonding
or the
nonbonding
orbital,
n
, as one might deduce, is a
type of orbital where the electrons make no contribution to the binding energy of the molecule. In
formaldehyde, this n
! p
* level requires the least amount of energy. The name,
antibonding
! p
* transition to the excited state gives rise to an absorption band (at about
270
m). This is a relatively weak band and it suggests that the transition is a forbidden one
(forbidden does not mean that it never occurs, rather than it is highly improbable). It is referred to
as a symmetry forbidden transition. The reason for it being forbidden is crudely justified by the fact
that the
m
p
* is in the
xz
plane (see Fig.
10.2
). The n electrons in the p orbital are in the
xz
plane and
p
perpendicular to the
* orbital. Because the spaces of the two orbitals overlap so poorly, the
likelihood of an electronic transition from one to the other is quite low. As stated above, in the
ground (normal) state of the molecules, two electrons are paired. The pairing means that these
electrons have opposite or anti-parallel spins. After absorbing the light energy, in the singlet excited
