CHEMISTRY 321 – MOLECULAR MODELING EXPERIMENT 2
INFRARED SPECTRA OF ORGANIC COMPOUNDS
The purpose of this experiment is to use molecular modeling to predict the infrared spectrum of a
common organic compound.
Background
When molecules absorb infrared (IR) radiation, vibrational motions of bonds are
enhanced (excited). Since functional groups are specific collections of different types of bonds
(i.e., C-H, C=O, N-H, etc.), the infrared spectrum of an organic compound gives information
about the types of functional groups contained in the molecule. Different bonds require different
energies (hence different frequencies) of IR radiation to become vibrationally excited. Thus, an
infrared spectrum consists of a plot of signal (absorption or transmission of IR radiation) vs.
frequency (given as cm-1). Molecular modeling software uses strengths (actually force
constants) of the different bonds in a molecule to predict the frequencies at which absorption of
IR radiation will occur.
In this experiment, you will construct a simple compound and use CAChe to predict its
IR spectrum. Then, you will compare the two predicted spectra with an actual spectrum obtained
from our Fourier Transform Infrared spectrometer.
Procedure:
1. Open a new workspace in CAChe.
2. Construct a model of cyclohexanone. The structure of this compound is shown
below.
O
Cyclohexanone
3. Optimize the geometry of this molecule using Beautify | Comprehensive.
4. Save this molecule in your folder.
5. Change the background color to white, if it is not already. . Select View | Color
palette from the top menu bar. The Define Colors window will appear. Scroll through the
colors until you can highlight Background Color by clicking on it. Select Set, and then select
the white square under Basic colors. Click on OK, and the colors window will close. Click on
OK again, in the Define Colors window. That window will close, and the background will
change to white.
6. Select Experiment | New.
7. In the Experiment window, select Property of: Chemical Sample; Property:
Optimized Geometry; Using: Standard procedure.
8. Select Start.
9. When the calculation is done, close the Experiment Status window and the
Experiment window.
10. Select Experiment | New.
11. In the Experiment window, select Property of: Chemical Sample; Property: IR
Transitions; Using: MOPAC PM 3 Force.
12. Select Start.
13. When the calculation is done, close the Experiment Status window and the
Experiment window.
14. Select View | IR transitions from the top menu bar.
