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Summary
At the atomic and molecular level, all matter, regardless of size or composition, is
in constant motion.
The bonds holding atoms together in molecules can be thought to behave much
like a "Slinky" spring. The spring-like wagging, stretching, and bending motions
of the bonds are forms of molecular vibration.
Each molecular bond has a natural frequency associated with each type of
vibration.
Energy can be absorbed by the molecule if the frequency or wavelength of the
incident energy matches (resonates with) the natural frequency of one or more of
the molecule's bonds.
The whole classification of normal vibrations is quite complex and would require a
detailed mathematical treatment that is beyond this document. The main point is that
vibrational characteristics of molecules allow us to determine what kind and how much
of a molecule is present.
QUALITATIVE ANALYSIS: IDENTIFYING COMPOUNDS
Spectroscopy is the study of how energy (typical electromagnetic energy) interacts with
matter. We use selected areas between 1.8 to 14.1 μm band of the electromagnetic
spectrum because higher energy levels, such as those used in mass spectrometry, break
the molecules down to the atomic or subatomic particle level. While nuclear magnetic
resonance and other techniques are useful for determining the atomic properties of
molecules, they are not designed for "fingerprinting" molecules and identifying them.
A spectrophotometer is an instrument for recording variations in intensity of
electromagnetic radiation at various frequencies. A simple diagram of the functional parts
of an infrared spectrophotometer, such as the SapphIRe Analyzer, is shown in Figure
13-7.
Optical
Filter
Source
Chapter 13 Infrared Spectroscopy Tutorial
Sample
Fixed
Cell
Slit
Detector
Figure 13-7. Diagram of a Spectrophotometer
13-7
Output Device
(Display or Analog)
Intensity
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