By comparing the solutions of Examples 14.19 and 14.22, we see that vi-
brational absorption spectroscopy and Raman spectroscopy provide different
information. Absorption and Raman spectroscopies are complementary tech-
niques. In fact, by comparing the results of absorption and Raman spectra and
doing a group-theoretical analysis, one can provide evidence for or against a
particular symmetry for a molecule. (See the end-of-chapter exercises for an
example of this sort of analysis.) Raman spectra can provide additional infor-
mation by analyzing the polarization differences between the incoming and
outgoing light (another advantage of using lasers as a light source). Such analy-
ses are beyond our scope; additional information can be found in more de-
tailed texts about Raman spectroscopy.
Raman spectroscopy can be used to probe other energy levels besides vi-
brational. For example, rotational and electronic energy levels can also be in-
vestigated using Raman scattering. Raman spectroscopy is just one more tool
in the modern chemist’s arsenal for the study of atoms and molecules.14.19 Summary
This chapter gives a brief introduction of rotational and vibrational spec-
troscopy. It is an enormously powerful subject when applied to the study of
molecules. Performed and analyzed properly, a rotational or vibrational spec-
trum can yield direct information about the structure of a molecule. Pure ro-
tational spectroscopy, which utilizes the microwave portion of the electromag-
netic spectrum, allows us to calculate sizes of molecules that agree with the
atomic scale assumed by Dalton’s atomic theory. Microwave spectroscopy has
also been useful in observing the universe around us. Astronomical observato-
ries have identified specific molecules existing in interstellar space, like H 2 O,
HCN, H 2 S, C 2 H 2 ,even C 2 H 5 OH—all from microwave (that is, rotational)
spectra observed by microwave “telescopes.” Such knowledge supports the idea
that the chemicals in our world exist not just here but throughout the universe.
Vibrational spectroscopy also provides valuable information about the
structures of molecules, especially when one combines the complementary
tools of infrared absorption and Raman spectroscopies. Vibrational spectra,
coupled with group theory and symmetry, are indispensable tools for inferring
the structure of a molecule, for determining how atoms in a molecule are
bonded together. Fingerprint regions are also a quick method of establishing
what groups of atoms exist in a molecule. Together, rotational and vibrational
spectroscopy deal with the relative nuclear motions of molecules.514 CHAPTER 14 Rotational and Vibrational Spectroscopy