CH ENGR 104C Lecture Notes - Lecture 4: Infrared Spectroscopy, Electromagnetic Spectrum, Absorption Band

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Infrared Spectroscopy
BACKGROUND INFORMATION
Before introducing the subject of IR spectroscopy, we must first review some
aspects of the electromagnetic spectrum. The electromagnetic spectrum is composed of
energy that may behave both as a particle and as a wave. When we describe this energy
as a particle, we use the word photon. When we describe this energy as a wave, we use
the terms frequency (v) and wavelength (). Frequency is the number of wave troughs
that pass a given point in a second and wavelength is the distance from one crest of a
wave to an adjacent crest. Frequency and wavelength are inversely related, according to
the equation E = hv = hc / . Therefore, as frequency increases, wavelength decreases.
When we discuss IR spectroscopy, we introduce a new unit of measurement called the
wavenumber (v). The wavenumber is the number of waves in one centimeter and has
the units of reciprocal centimeters (cm-1). Since the wavenumber is inversely
proportional to wavelength, it is directly proportional to frequency and energy which
makes it more convenient to use.
I. Bond Vibrations: The Basis of IR Spectroscopy
Spectroscopy is the study of matter and its interaction with electromagnetic
radiation. All matter contains molecules; these molecules have bonds that are continually
vibrating and moving around. These bonds can vibrate with stretch motions or bend
motions. Imagine two balls attached by a spring, representing a diatomic molecule. The
movement of each ball toward or away from the other ball along the line of the spring
represents a stretching vibration (fig. 1). Stretching can either be symmetric or
asymmetric. A molecule with three or more atoms can experience a bending vibration,
a vibrational mode where the angle between atoms changes (fig. 2). In the following
examples, imagine a triatomic molecule ABC.
Fig. 1: Stretching Vibrations
Symmetric Stretch: allows molecule to move through space
A B C
OR
A B C
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Asymmetric Stretch: leads to an increase or decrease in bond length
A B C
OR
A B C
Fig. 2: Bending Vibrations
A A
B OR B
C C
Each excited vibrational state is reached when a molecule is exposed to a specific
frequency. In order for a bond to be promoted to the excited state, it must be exposed to
radiation of the exact same frequency as the energy difference between ground and
excited states (E). Determining these frequencies and representing them allows us to
determine the bonds that exist in a molecule. These frequencies all lie within the
infrared region of the electromagnetic region, a region of lower wavelength than visible
light. A machine called an IR Spectrometer passes infrared radiation through a sample
of an unknown compound and uses a detector to plot percent transmission of the radiation
through the molecule versus the wavenumber of the radiation. A downward peak on the
plot represents absorption at a specific wavenumber. In sum, IR spectroscopy is useful in
determining chemical structure because energy that corresponds to specific values allows
us to identify various functional groups within a molecule. An IR spectrum usually
extends from radiation around 4000 cm-1 to 600 cm-1 and can be split into the functional
group region and the fingerprint region. The fingerprint region is different for each
molecule just like a fingerprint is different for each person. Two different molecules may
have similar functional group regions because they have similar functional groups, but
they will always have a different fingerprint region. In this course, we will only focus on
the functional group region when identifying the structures of molecules. This is because
the fingerprint region, which extends from about 1450 cm-1 to 400 cm-1, is very complex.
It has many absorptions and makes it quite difficult for students and scientists to make
precise bond assignments. Additionally, since stretching vibrations are typically found in
the functional group region, they are the most convenient vibrations to analyze when
determining the types of bonds a molecule has. Thus, we tend to ignore bending
vibrations because they are usually found in the fingerprint region.
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