change the dipole moment (the product of charge separation and bond length), and so a heteronuclear diatomic molecule does exhibit an infrared absorption spectrum.
CHE151Lab10_GreenhouseEffect.doc Dr . Roderick M. Macrae CHE 151 For a polyatomic molecule, the total number of vibrational modes is 3N-6, where N is the number of atoms (3N-5 if the molecule is linear). Not all of these modes are necessarily infrared-active ; some, such as the symmetric stretch of CO 2 , do not lead to a change in dipole moment. The strength with which a molecular vibration absorbs energy is related to the extent to which the dipole moment changes during the vibration. The frequency of the vibration is proportional to (k/ μ ) 1/2 , where k is the force constant of the bond (related to bond strength), and μ is the reduced mass corresponding to the vibration (a function of the masses of the atoms involved in the vibration). Note: Instructions on use of the WebMO interface are included in the Appendix . Exercises For each of the following molecules, using the program Gaussian 09, carry out a calculation of the type B3LYP/6-31G(d) geometry optimization + vibrational frequencies . List the frequency (in cm -1 units, typically used for infrared spectra) and intensity of all peaks in the IR spectrum. If there are no infrared peaks, write “no peaks”. Save the spectrum as a .png graphics file. 1. Nitrogen, N 2 , and oxygen, O 2 . (Note that O 2 has two unpaired electrons, and hence has triplet multiplicity.) 2. Water, H 2 O. (NB One of the frequencies has low but nonzero IR intensity.) 3. Carbon dioxide, CO 2 . (Note the degeneracy of the two bending modes.) 4. Methane, CH 4 . (Note degeneracy among modes. How many modes have zero IR intensity?) Extension (if time permits): Rerun CO 2 calculations using different methods (HF, B3LYP) and different basis sets (STO-3G, 3-21G, 6-31G(d)). Which method/basis combination gives results closest to experiment (667 cm -1 , 1288 cm -1 , and 2349 cm -1 respectively for the bend, symmetric stretch, and asymmetric stretch.) Lab Report Include all relevant infrared frequencies and intensities, and figures of all IR spectra. Why are oxygen and nitrogen not greenhouse gases? Which gases are greenhouse gases?
CHE151Lab10_GreenhouseEffect.doc Dr . Roderick M. Macrae CHE 151 Appendix: Using the WebMO Interface URL for WebMO: and follow the link. To login to WebMO , use the username student and the password student . The interface consists of a job manager , which allows you to inspect calculations which are either currently running or have completed, and a molecule editor , which allows you to construct a molecule and submit it for computation. As everyone will use the same login, you will need to include your initials in the job title to identify it as yours. (By default the program just uses the chemical formula of the molecule on which you are doing calculations.) To create a new job, click New Job then Open Editor from within the Job Manager .
- Fall '15