Infrared spectroscopy is a commonly used analytical technique. The most common use is in the qualitative identification of chemical compounds present in a sample. The infrared portion of the electromagnetic spectrum (what we perceive as heat) contains photons of the correct energy to excite the normal vibrational modes of a molecule. The precise infrared frequencies absorbed by a molecule will depend on the types of chemical bonds present. This allows the chemist to determine what types of bonds are present in the sample. Based on other information, individual chemical compounds can often be identified in a sample.

In this section we will calculate the infrared spectrum for the formaldehyde molecule, pictured below. The software will allow us to animate the various infrared absorptions to see the corresponding molecular motions.

1. Use the guest account (username: guest, password: guest) to log in the WebMO demo site.
Click New Job -> Open Editor. A small window opens where you build molecules.

2. Build a molecule of formaldehyde. Use CleanUp -> Comprehensive- Mechanics.

3. Choose Gaussian as the computational engine. Type in/Choose the following:
Job Name: CH2OPM3, Calculation: Geometry Optimization, Theory: PM3, Basis Set: Basic: 3-21G(or accept default), Charge: 0, Multiplicity: Singlet.

4. Click on the blue continue arrow. You should now see your job listed. When the calculation is finished, click on New Job Using This Geometry, and Type in/Choose the following:
Job Name: CH2OPM3Vib, Calculation:Vibrational Frequencies, Theory: PM3, Basis Set: Basic: 3-21G(or accept default), Charge: 0, Multiplicity: Singlet.

5. Click on the blue continue arrow. You should now see your job listed. When the calculation is finished, open the file and scroll down to the Vibrational Modes window.
How many transitions are shown (count the number of frequencies)? _________

For linear molecules, the number of normal modes is given by 3N - 5, where N is the number of atoms. For nonlinear molecules, the corresponding equation is 3N - 6.
Is the number of transitions equal to the expected number? _________

6. Click on the filmstrip next to one of the frequencies and observe the corresponding vibrational motion. Try to identify the type of motion for each transition. (The molecule can be rotated if needed). In the table on the next page, record the frequencies and type of motion for each. The type of motion can be described as a C-H or C-O stretch (symmetric or
asymmetric), etc.

Due to the approximations implicit in the PM3 method, calculated vibrational frequencies are often higher than the experimental values. For better comparison with experimental results, the calculated frequencies are often multiplied by a scaling factor (fudge factor!). The scaling factor is listed below. Perform the corrections and list the new results in the appropriate column. Do the scaled values give a better comparison with the experimental values?