isotopic substitution (for example, CD 3 CCH and CH 3 CCD for methyl-
acetylene) is sometimes crucial in determining what normal vibrations con-
tribute to combination bands.14.16 Fingerprint Regions
Having spent the chapter discussing how vibrations of molecules absorb light
of specific frequencies, we now introduce a useful generalization. Although a
normal vibration involves all atoms in a molecule, in many instances a normal
vibration is mostly due to a simple motion between two or three atoms in one
part of the molecule. One consequence is that it is easy to describe normal mo-
tions by their majority component, like C–H stretch, O–H stretch, CH 2 wag,
CH 3 deformation, or a similar description.
Another consequence is that all normal motions that can be given the
same general description have similar vibrational energies. An equivalent way
of stating this idea is that similar normal modes absorb infrared light from
similar regions of the spectrum. That is, certain regions of the infrared spec-
trum correspond to characteristic types of vibrational motions of molecules.
Such regions are called group frequency regionsor fingerprint regions,and they
typically refer to the fundamental vibration itself, not the overtones or com-
binations.
For example, the fingerprint region for an O–H stretch (say, for a series of
different alcohol molecules) is about 3100–3800 cm^1 , depending on the spe-
cific molecule the O–H group is bonded to. Granted, this seems like a large
range. However, it can be virtually guaranteed that such a motion will not be
observed in the 100–500 cm^1 region. The masses of O and H are the same
for all OH groups, and the force constant of the O–H bond does not change
much with a change in the rest of the molecule. C–H stretches show up in the
region 2800–3300 cm^1. C–H bending motions appear in the 1300–1500 cm^1
and 500–900 cm^1 regions of the infrared spectrum. Other fingerprint regions
can be identified. Table 14.7 lists several fingerprint regions that are useful in
vibrational spectroscopy.
A more compact way than Table 14.6 to illustrate the fingerprint regions of
various atomic combinations in molecules is the correlation tables. Correlation
tables, like the one shown in Figure 14.34, can illustrate where certain groups
of atoms will absorb in the vibrational spectrum. Additional correlation tables
are shown in Appendix 4. These tables usually contain qualitative intensity in-
formation, allowing one to make judgments on the strength of an absorption
in a vibrational spectrum. [In correlation tables, usually VS very strong,
S strong, M medium, W weak, VW very weak, SP or SH sharp
(that is, narrow), and BR broad (that is, wide).] Correlation tables are use-
ful for identifying compounds, because the right set of absorptions in the right
fingerprint region(s) almost guarantees the presence of a certain grouping of
atoms in a molecule. The following example illustrates.Example 14.20
An unknown compound shows vibrational absorbances occurring at 3287,
2215, and 729 cm^1. Keeping in mind that these are not all of the vibrational
frequencies of the molecule, use the concepts of fingerprint regions and cor-
relation tables to determine whether or not such absorptions are likely for the
following molecules.504 CHAPTER 14 Rotational and Vibrational Spectroscopy
Table 14.7 Various infrared fingerprint
regionsa
Motion type IR region
C–H stretch 2800–3300
O–H stretch 3100–3800
CC, CN stretch 2100–2500
CO stretch 1600–1800
CC stretch 1600–1700
C–H bend 1300–1500, 500–900
O–H bend 1200–1600
C–O stretch 900–1300
C–C stretch 800–1150aAll units are cm (^1). Limits are approximate, since there are
usually examples of molecules whose motions are outside
the specified range.