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Chapter 13 Infrared Spectroscopy Tutorial
Resonance
Every object has a natural frequency at which it vibrates. As a sound wave travels
through the air, it carries energy with it. This energy is what causes eardrums to vibrate
when struck by the wave. In the same manner, a larger, less sensitive object also bends
slightly with each compression or rarefaction which hits it. Normally this deformation is
so slight that it is not noticeable, but if the frequency of the sound wave just matches the
natural frequency of the object struck, the object absorbs part of the sound wave's energy
and resonates.
Many people have heard the story of an opera singer's voice shattering a crystal glass.
Like molecular bonds, the crystal has tones or harmonics at which it naturally vibrates. At
other frequencies it does not vibrate. When an opera singer's voice rises, it eventually hits
one of the harmonics of the crystal glass. When it does so, it begins to vibrate (resonate)
with the note. It is only at that fundamental tone that it can fully absorb the sound energy.
At lower notes the sound energy just passed through the glass without much effect.
Similarly, each molecular bond has a characteristic harmonic infrared frequency at which
it vibrates. It is only at that particular frequency that the molecular bond can absorb IR
energy.
Thus resonance is the key to infrared analysis: If a molecule's bonds vibrate at a
particular frequency, wavelengths of infrared radiation are absorbed by those bonds if
they are at the same frequency.
For example, if carbon is double bonded to oxygen, it vibrates at a frequency that
matches the frequency of infrared energy at a wavelength of 5.8 μm. Whenever molecular
frequency matches infrared energy's frequency, that infrared energy is absorbed by the
molecules. Molecules of different substances vibrate at different frequencies, and
therefore, are responsive to different wavelengths. Because some molecules can be
composed of several different atoms, it is also possible that one part of a molecule would
absorb at one wavelength, and another part of that molecule at another wavelength. So
the total molecule might absorb at multiple wavelengths, because of the many different
bond groups on that molecule. One way to tell one molecule from another is by analyzing
at different wavelengths of infrared energy.
Chemical bonds can be thought of as tuning forks, each bond naturally tuned to a
different pitch or vibrational frequency. If the tuning fork is struck softly, it sounds a
particular note. Striking the fork harder does not change the fundamental pitch of the
fork. It only vibrates more forcefully and consequently sounds louder. It does not change
to a different note. Chemical bonds are like nature's molecular tuning forks.
13-6
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