a.Wa t e r, H 2 O
b.Methane, CH 4
c.Acetylene, C 2 H 2
d.Ethane, CH 3 CH 3
e.Ethynol, CHCOH (that is, the alcohol made from acetylene, or ethyne)Solution
a.Although the 3287 cm^1 absorption might be due to an O–H motion, nei-
ther of the other two absorptions correspond to any type of motion due to
oxygen and hydrogen bonded together. Therefore, water is probably not a
likely candidate to have this spectrum.
b.The correlation tables show an absorption in the range where 729 cm^1
would fall, but neither of the other two absorptions agree with the tables.
(Although the 3287 cm^1 absorption might seem close, it seems high for an
alkane. Too, the peak at 2215 cm^1 is not near any alkane fingerprint region.)
Therefore, methane is not a likely candidate for such absorptions.
c.The 3287 cm^1 absorption is in the range of an alkyne C–H absorption.
Further, the absorption at 729 cm^1 is close to the region where alkyne C–H
bonds absorb (this is the C–H bending motion). Finally, the absorption at
2215 cm^1 falls in the range of CC vibrations. The conclusion is that this
could represent a possible (partial) spectrum of acetylene.
d.As for methane, although the C–H fingerprint regions may be represented,
there is no acceptable fingerprint region that could explain the absorption at
2215 cm^1. Therefore, ethane is not a likely candidate for having these ab-
sorptions.
e.Although C–H vibrations and CC vibrations are represented (as with
acetylene, above), there is no mention of an absorption for an O–H motion.
Therefore, we make a qualified statement that these absorptions might de-
scribe this molecule, but we would need to check for the presence or absence
of absorptions belonging to an O–H group. If such normally medium or
strong absorptions are absent, then identification of the compound as ethynol
might not be the best identification.The last part of Example 14.20 illustrates a major pitfall in using correlation
tables and fingerprint regions for understanding vibrational spectra. They
help, certainly, but they do not guaranteeidentification. All substances have
their own characteristic spectra, and a positive identification of a molecule
rests on being able to match a vibrational spectrum exactly, or as closely as pos-
sible. Fingerprint regions and correlation tables provide clues and hints. But in
almost all cases, that is all they provide. Nonetheless, they are useful in mak-
ing general interpretations about the structure of a molecule on the basis of its
vibrational spectrum. For small molecules, where group-theoretical analyses
can also be applied, such tools are indispensable for identification of unknown
molecules.14.17 Rotational-Vibrational Spectroscopy
Although it is easier to discuss rotations and vibrations of molecules separately,
in reality such motions of molecules occur simultaneously. (Translations are
also occurring, and translational motion accounts for a large part of the kinetic
energy of a molecule in the gas phase. However, translations do not contribute
directly to the topic at hand.) When a sample is in the gas phase, molecules are506 CHAPTER 14 Rotational and Vibrational Spectroscopy