Page 186 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

Infrared Spectroscopy 797

causes some of the molecules to vibrate with increased oscillatingdipolemomentinthemolecule.Iftheradiation-
amplitude, which increases the vibrational energy. The induced dipole moment oscillation has the same frequency
increase in vibrational energy is at the expense of radi- as the dipole moment oscillation resulting from a molecu-
ation energy, resulting in the absorption of IR radiaition lar vibration, then the radiation may induce the molecule
at certain frequencies. The absorption frequency of a fun- to vibrate with increased amplitude. If a vibration causes
damental absorption band in the IR is the same as the no change in dipole moment, then there is no way the radi-
frequency of the molecular vibration that caused the ab- ation can excite that vibration. In a homonuclear diatomic
sorption. This provides a means of observing molecular molecule such as H 2 or Br 2 , the two atoms have identical
vibrational frequencies that can provide a molecular “fin- excesscharges(namely,zero),andthedipolemomentdoes
gerprint” for identifying molecules. The frequencies can not change during the vibration as it is always zero. There
also be used to characterize internal features within the is no way the electric field of the radiation can induce the
molecule that can provide information about the molecu- two similar atoms to move in opposite directions as re-
lar structure. quired in a vibration. The selection rule for IR absorption
requires that in order to be IR-active, a molecular vibration
must cause a change in dipole moment. The IR absorp-
F. Dipole Moment Change
tion intensity is proportional to the square of the change
There must be some means by which the radiation energy in dipole moment with respect to the change in the normal
can be transferred to the molecule when the molecule ab- coordinate. This reflects the fact that the more the dipole
sorbs radiation energy. This involves the dipole moment moment changes during a vibration, the greater the proba-
of the molecule. A dipole consists of a positive and a neg- bility that the radiation of the proper frequency can excite
ative charge of equal magnitude separated by a distance. that vibration.
The dipole moment is the magnitude of either charge mul-
tiplied by the spacing. Within a molecule, we can picture G. Symmetry and Infrared Activity
atoms as particles with small excess negative or positive
charges, since chemical forces act to make some atoms If a molecule has some symmetry, a particular vibration
have a slight excess or deficiency of electrons. We can of that molecule may be IR-inactive; that is, the vibration
picture the negative charge of the dipole as the total ex- will not give rise to any IR absorption. This is because the
cess negative charge of the negative atoms concentrated dipole moment change can be zero as a direct consequence
at the center of the excess negative charge and can pic- of the symmetry.
ture the positive charge of the dipole as the total excess One symmetry element is the center of symmetry. If a
positive charge on the positive atoms concentrated at the molecule in the equilibrium configuration has a center of
center of the excess positive charge. In CO 2 , for exam- symmetry,onecanstartatanyatomandgoinastraightline
ple, the center of excess negative charge is between the through the center and an equal distance beyond, where
two electronegative oxygens and the center of excess pos- one will find another atom of identical type. An example
itive charge is at the relatively electropositive carbon. At is carbon dioxide O C O (see Fig. 7). Such a molecule at
equilibrium, these two charge centers coincide with zero equilibrium would have a dipole moment of zero. The vi-
spacing, so the dipole moment is zero. brationallydistortedmoleculewhereeachatomhasmoved
Many molecular vibrations cause the dipole moment to to the end of its displacement vector may have less sym-
change. For example, in HBr, the bromine is more elec- metry than the molecule at equilibrium. One can repeat
tronegative than the hydrogen, so the bromine has a slight this procedure for the vibrationally distorted molecule.
excess negative charge and the hydrogen a slight excess If it still has a center of symmetry, the vibration is said
positive charge. During the vibration, the H–Br spacing to be symmetric with respect to the center of symmetry.
changes and also the amount of excess charge on each An example in Fig. 7 is the in-phase stretch of O C O,
atom changes, causing the dipole moment to change. where both CO bond lengths are always equal. Such a
The electromagnetic radiation imposes an electric field vibration is IR-inactive since the dipole moment (which
on the molecule. This electric field exerts forces on is always zero) does not change. If the vibrationally dis-
charges, and by definition the forces on positive and neg- torted molecule no longer has a center of symmetry, then
ative charges are oppositely directed. The atoms with ex- the vibration is said to be antisymmetric with respect to
cess negative charge are pulled in one direction, while the the center of symmetry. An example in Fig. 7 is the out-
atoms with excess positive charge are pulled in the op- of-phase stretch vibration of O C O, where the positive
posite direction. These forces tend to induce a change in carbon is not a longer midway between the two negative
dipole moment. The electric field of the radiation oscil- oxygens. The dipole moment changes during this vibra-
lates at the radiation frequency, and this tends to induce an tion, which is IR active.