Chemistry Reference
In-Depth Information
An immediate consequence is that the vibration of a homonuclear diatomic will not be
IR active.
6.2.2
Infrared Absorption and the Greenhouse Gases
IR absorption by gases in the atmosphere is thought to be responsible for climatic warm-
ing effects. Sunlight in the visible and ultraviolet (UV) parts of the spectrum that arrives at
the surface of the Earth is absorbed and partially re-emitted in the IR region. The Earth's
atmosphere is roughly 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide
(CO
2
), along with small amounts of methane (CH
4
) and water vapour. Nitrogen and oxy-
gen are present as homonuclear diatomic molecules and so, as we have seen, their bond
vibrations will not absorb IR radiation. The atomic argon component is similarly benign
and so if only these three gases were present the heat radiated by the Earth would escape
back into space.
However, CO
2
,CH
4
and water all have IR-active vibrations. Hence, these relatively
minor constituents of the atmosphere will absorb IR radiation, trapping some of the radi-
ated energy and warming the planet. With these gases present the atmosphere acts in a
similar way to the glass of a greenhouse: visible wavelength radiation can pass through
the glass into the greenhouse where it is absorbed by the plants and soil and is partially
re-emitted at IR wavelengths. Glass is practically opaque in the IR and so the re-emitted
energy is trapped, warming the interior of the greenhouse. The absorption of energy by
IR-active vibrations of atmospheric components causes heating in a similar way.
To some extent the Earth's greenhouse effect is a good thing; the surface temperature of
the moon varies between
123
◦
C. The moon's lack of an atmosphere means
that all the radiation from the sun reaches the surface during the daytime and heat emitted
by the surface is rapidly lost to space during the night. On the Earth these extremes are
tempered by the atmosphere with the 'greenhouse' gases helping to hold the heat radi-
ated by the planet's surface through the night-time. However, since the amount of heat
retained by the atmosphere is dependent on the concentration of relatively minor compo-
nents, increases in the average amounts of CO
2
,CH
4
and other IR-active molecular species
will lead to additional warming of the planet.
−
233 and
+
H
2
O,
C
2
v
We can now show that the three main greenhouse gases are IR active as a result of their
molecular symmetry. For the case of water, we have already found in Section 5.6 that the
irreducible representations are
(
ν
)
=
2
A
1
+
B
2
(6.10)
The two
A
1
modes are the symmetric stretch and bending motions and the
B
2
vibration is
the asymmetric stretch. By reference to the standard character table in Appendix 12 for
C
2v
,
we find that all three modes are IR active, because
Z
belongs to
A
1
and
y
to
B
2
.
CO
2
,D
∞
h
CO
2
is a linear triatomic molecule belonging to the point group
D
∞
h
. A basis which gives
three degrees of freedom to each atom is shown in Figure 6.5a. This basis can be split into
