Page 201 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GRB/MBQ P2: GLM Final Pages
Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

812 Infrared Spectroscopy

are those whose vibrational forms are nearly the same in or C S bond. This means that the rest of the molecule
a series of related molecules. does not affect the OH, C N, or SH vibrations mechani-
cally very much since the nearly stationary attached car-
B. Vibrational Interaction bon atom localizes the vibration. In this manner, one can
see that such group vibrations as OH, C N, and SH are
If all the bonds in a molecule vibrated separately, the di-
group frequencies; that is, their presence in a molecule
atomic vibrational frequency formula given earlier could
be used to predict the whole spectrum. The fact is that gives rise to absorption in a predictable frequency range.
bonds do not usually vibrate separately and much interac- While the OH vibration is mechanically unaffected by
tion occurs. However, interactions occur only if vibrations the rest of the molecule, the OH group has a force constant
have the same type of symmetry. For example, in a pla- that can be changed by hydrogen-bonding effects. An al-
nar molecule such as vinyl fluoride, planar vibrations do cohol in dilute CCI 4 solution has a free OH band near
−1
not interact with non-planar vibrations since these have 3640 cm . Hydrogen bonding lowers the OH frequency
different symmetries with respect to the plane. and increases the bandwidth and intensity. A pure alcohol
One type of interaction occurs when two identical bonds is hydrogen bonded (OH ··· O) and absorbs broadly and
−1
share a common atom. An example is the H 2 O molecule, strongly near 3300 cm .
which has two OH bonds with a common oxygen atom. In In a ketone, the high-frequency C O bond is connected
the H 2 O molecule, one OH bond cannot be vibrationally to the rest of the molecule by two low-frequency C C
excited without also exciting the other identical OH bond bonds. In the carbonyl vibration the two attached car-
at the same time. The second bond vibrates either in phase bons hardly move, making the carbonyl a good group
oroutofphasewiththefirst.Inonecase,bothbondsstretch frequency, mechanically nearly independent of the rest
at the same time and contract at the same time, and in the of the molecule. The carbonyl frequency can be varied by
other, one bond stretches while the other bond contracts. electron donation or withdrawal effects from the attached
groups, and these can shift the frequency. These effects
In the H 2 O gas-phase spectra, these are observed at 3652
−1
and 3756 cm , respectively. Thus, the in-phase and out- are fairly well understood, which means that the shifts are
predictable.
of-phase vibrations do not have the same frequencies. The
The CH 2 ,NH 2 , and SO 2 groups discussed earlier are
main reason for this is that, when the two vibrating bonds
attached to the molecule by low-frequency C C, C N, or
are not at the equilibrium length, both bonds exert restor-
C S bonds, so the attached atoms hardly move, isolating
ing forces on the common oxygen atom. The force resul-
tant is different for the in-phase and out-of-phase stretch the CH 2 ,NH 2 , and SO 2 stretching vibrations. This means
vibrations, and this affects the frequencies. This is called that both the in-phase and out-of-phase stretching vibra-
vibrational interaction. tions for these groups are good group frequencies, nearly
There are several XY 2 groups that have correlatable independent mechanically of the rest of the molecule.
stretching frequencies. For example, alkane CH 2 groups In a group such as the C S group, the vibration is not
have stretching vibrations that absorb near 2930 and isolated like a C O vibration. The C S is attached to the
−1
2850 cm , amino NH 2 groups have stretching vibrations rest of the molecule by C CorC N bonds, which have
−1 nearly the same frequency as the C S bond. As a result,
that absorb near 3370 and 3300 cm , and sulfone SO 2
groups have stretching vibrations that absorb near 1300 interaction will take place and more than one vibration
−1
and 1130 cm . In each case the higher wave number in- will involve C S stretching.
volves the out-of-phase stretch and the lower wave number In such groups as CH 3 and SO 3 , there are three identical
involves the inphase stretch. bonds. These interact, so the group vibrates as a whole in
Consider the XYZ group where the X Y bond and the threedifferentmodes: anin-phasestretchandtwodifferent
Y Z bond have quite different frequencies when uncon- out-of-phase stretch vibrations. These are good group fre-
nected. Examples include the C O H group in alcohols, quencies because the group is connected to the molecule
the C C N group in nitriles, and the C S H group in with a low-frequency C CorC S bond. In a group such
as a benzene ring there are six identical or nearly identical
mercaptans. Here the other groups on the carbon are ig-
C C bonds, which interact to give six different stretching
nored. The XYZ group as a whole has two stretching fre-
modes. Some of these are group frequencies.
quencies, but the interaction is different from that for the
XY 2 case. In the high-frequency vibration of the XYZ C. Survey of Group Frequencies
group, only the atoms of the high-frequency bond move
appreciably. In the examples given, this means that when Table IV contains some selected group frequencies used in
the high-frequency OH, C N, or SH bonds vibrate, the qualitative analysis. Some discussion of the data follows.
−1
attached carbon hardly moves since it is attached to the In the region from 4000 to 2000 cm , various XH groups
−1
high-frequency bond with a low-frequency C O, C C, absorb. In the region from 3700 to 3100 cm , OH groups