Molecular Motion
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Molecular Motion and Infrared Spectra

Molecules and everything in them can move in many ways, and energy is associated with all of those different kinds of motion.

There is translational movement. Molecules can move from one place to another.
There is also rotational movement. That is, they can turn around and around.
The third way shown here is vibrational movement.
In addition, electrons can move from place to place and orbital to orbital.

These types of motion and the energy associated with them influence the physical properties of chemicals. The color of chemicals is often associated with changes in electron energy levels.

The phase of chemicals is associated with translational and rotational motion of molecules. In the gas phase molecules are free to move from one place to another and also have considerable translational motion.

Rotational motion can involve the entire molecule rotating or the internal parts of the molecule rotating with respect to one another. These types of rotation are found in molecules in both the gas phase and the liquid phase.

Vibrational motion is found in molecules in all three of the phases. It is the vibrational motion and the energy needed to change it that we will focus on now.

This diagram shows you some of the different kinds of vibrations that can occur.  Vibrations can be classified as asymmetric stretching, symmetric stretching, and bending. These vibrations can be fast or slow, or take place at any of several different speeds or intensities. They can also change from one speed to another. When they change from one speed to another, the atoms absorb or release very specific amounts of energy--just enough to get from one speed to another.

61molmo3.JPG (8234 bytes)

The light energy needed for vibrational changes is generally that of infrared light. If the light has just the right amount of energy, it can be absorbed by the molecule and cause a change in the way the atoms of that molecule vibrate.

From our perspective outside the molecules we cannot detect the actual vibration of the atoms in the molecules, but we can measure the energy going in or coming out. The energy needed to change the vibration in the molecules corresponds to the energy contained in infrared light. A different amount of energy is needed to change the vibrations for each different combination of atoms and thus requires a different frequency of infrared light.

The amount of energy that it takes to change the vibration of a particular type of bond is very characteristic of that bonding arrangement. It is so characteristic that if you use the right kind of equipment, you can measure the frequencies of infrared light that are absorbed by molecules and correlate those frequencies with some of the bonding arrangements that are present in the molecules.

An example of this is shown here. I know you cannot see the details but I don't want you to focus on the details just yet.

Infrared spectrum of unspecified compound.[61ir02.JPG (8495 bytes)]

Look at the line going across the graph. Going from left to right represents different frequencies of infrared light. The trace of this line going up and down across the graph is called an infrared spectrum. When the line goes down, that means the molecule absorbed that particular frequency of infrared light. That absorption of energy results in a change in the vibration of some part of that molecule. So every absorption in the spectrum corresponds to a particular part of the molecule.

Notice that absorptions occur at different places. A carbon-hydrogen bond has a different set of vibrations than an oxygen-hydrogen bond, and it takes a different amount of energy to change vibrations. We could go all the way across each spectrum and note the different kinds of vibrations that correspond to each one of those absorptions.

You can see an animated example of the kinds of vibrations and how they relate to the peaks on an IR spectrum at this website  (You will need the Chime plug-in.)

IR Spectra Interpretation

It is not important for our purposes in this course that you learn how to give a complete interpretation of these spectra. However, I do want you to be able to interpret some of the simpler and more dependable portions of these spectra. We will do that as we talk about each of the different classes of organic compounds.

As you look at the different spectra in this lesson, you will notice that the spectrum for each one is different but not completely different. Some of the absorption positions repeat because some of the bonding arrangements are the same from one sample to another. But you should note that the total spectrum of each one is different. A professional would be able to identify the particular bonding arrangements and functional groups that are present. You will learn to identify some of the bonding arrangements and functional groups that are present.

In the following pages, we will look at how we will apply this to the types of compounds we have studied.


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E-mail instructor: Eden Francis

Clackamas Community College
2001, 2003 Clackamas Community College, Hal Bender