Vibrational Spectrum of Acetone

Molecules vibrate in response to their absorbing infrared light. Absorption occurs only at specific wavelengths, which gives rise to the use of vibrational (infrared/Raman) spectroscopy as a tool for identifying chemical structures. The energy or frequency of a particular vibration is proportional to the square root of a quantity called a "force constant" divided by a quantity called the "reduced mass".



The force constant depends on the "flatness" or "steepness" of the energy surface and reflects the difficulty of moving the atoms involved in the vibration from their equilibrium positions. The steeper the energy surface, the larger will be the force constant and the larger the frequency. The reduced mass reflects the total (weighted) mass of the atoms involved in the vibration. The smaller the mass, the larger will be the frequency.

Acetone may be used to explore the relationship between frequency and force constant, and further to examine why certain frequencies are of particular value in the use of vibrational spectroscopy as an analytical tool.

Build acetone. Click on . Start with sp2 carbon (), add sp2 oxygen () to make the carbonyl group and then add two sp3 carbons (). Click on and then on .

 





Enter the Calculations dialog (Setup menu) and request calculation of an equilibrium geometry using the Hartree-Fock 3-21G model. Check Freq. to the right of "Compute" to specify calculation of vibrational frequencies, and finally click on the Submit button at the bottom of the dialog. Name the job "acetone".

 



After a calculation has completed (1-2 minutes), select Vibrations from the Display menu. The dialog which results contains a list of vibrational frequencies for acetone from the lowest frequency (at the top) to the highest frequency. First, click on the top entry (the smallest frequency) and, when you are done examining the vibrational motion, click on the bottom entry (the largest frequency).
 
The smallest frequency is associated with torsional motion of the methyl rotors. The largest frequency is associated with stretching motion of CH bonds. Methyl torsion is characterized by a very flat potential energy surface (small force constant), while CH stretching is characterized by a very steep potential energy surface (large force constant).
 




Locate the frequency corresponding to the CO stretch and animate the motion. The experimental frequency is around 1740cm-1, but the Hartree-Fock calculations will yield a higher value (around 1940 cm-1).

This frequency is a useful "chemical identifier" (for carbonyl functionality) because it "stands alone" in the infrared spectrum.