Current Topics in Chemistry - Bruice

Dr. Glaser's "Chemistry is in the News"
To Accompany Bruice, Organic Chemistry, 3/e.
Chapter 12. Mass Spectroscopy, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy.

For each of the following questions, please refer to the following article:

INFRARED SPECTROSCOPY
by C. Claiborne Ray (New York Times, December 2, 1997)

Editorial Comments

"You can never believe what you read in the papers!" - You have heard it many times, right? Well, it depends on where you read it. Papers like the The New York Times and The Washington Post usually are quite accurate. And that is even true when it comes to science; the Science pages in The New York Times are quite outstanding. Althought there are times when one wonders. I remember well reading for the first time about "cold fusion". I was living in Connecticut back then and I was reading this article in The New York Times about the promise of "Cold Fusion". It was April 1, 1988. When I realized what date it was, I was quite amused and mentioned to my (future) wife that she should not miss reading this wonderful April hoax piece in the Times. Well, little did I know that the Times was actually serious at that time and it took a few more years until the "Cold Fusion" idea was put on ice. (For a really entertaining hoax, do take a look at the Great Moon Hoax.)

Now that you have studied Infrared Spectroscopy you are in a perfect position to judge just how accurate the science piece by Ray is. What do you think?

Finally, note that an address is given to which you can submit questions about science. By all means, try it out.

Pertinent Text References
Chapter 12. Mass Spectroscopy, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy.

Questions

Question 1: What is the range of reciprocal wavenumbers (1/cm) for a typical infrared spectrum? Which sub-range is referred to as the fingerprint region?

Answer 1: Typical range for entire spectrum 400 - 4000 1/cm. Range for fingerprint region 400 - 1,400 1/cm.

Question 2: What types of vibrations are distinguished? Order these types of vibrations according to decreasing energy.

Answer 2: Stretching vibrations > bending vibrations > twisting vibrations.

Question 3: Which one of the following molecules cannot be detected by Infrared spectroscopy? CO₂, CO, or O₂?

Answer 3: Vibrations are IR-active only if the dipole moment of the molecule changes during the vibration. The oxygen molecule is symmetric and its only vibration, the strechting mode, does not lead to a change in the electrical dipole moment of the molecule. The dipole moment always is zero. In contrast, the CO molecule is highly polar and, what matters, the dipole changes during the vibration. For carbon dioxide, there are some vibrations that are IR active (asymm. stretching) and some that are not (symm. stretching).

Question 4: The article talks about the Cassini mission to Saturn carrying infrared spectroscopy equipment. Indeed, infrared spectroscopy is widely used in astronomy. Take a look at this spectacular false-color image of The Galactic Center Across the Infrared. This pictures uses blues and reds for the "infrared" and the "near-infrared" ranges, respectively. Is the near-infrared region below 400 1/cm or above 4000 1/cm?

Answer 4: Near-IR is between the IR- and UV-regions. Higher energy than IR. Above 4000 1/cm.

Chemistry Online Exploratory.
Take a look at the visualization center of Chapter 12 on infrared spectroscopy and note the links "Freqs". Follow these links to find the results of theoretical calculations of the vibrational properties. You will find computed vibrational frequencies as well as computed intensities of these various vibrational modes. Read the frequency files of carbon dioxide, carbon disulfide and of carbon oxysulfide and find the vibrational frequencies of the symmetric anf asymmetric stretching vibrations. Compare & contrast.