exp38_04_1_07 - Experiment 38 Chemistry 541/2 Physical...

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1 Experiment 38 Physical Chemistry Laboratory Rotation-Vibration Spectra of Small Gas Phase Molecules: H 2 O, CO 2 , and HCl (Revised 4/1/07) Introduction You will learn some basic concepts of infrared absorption spectroscopy by recording, analyzing, and simulating the rotation-vibration spectra of H 2 O(g), CO 2 (g), and HCl(g). Analysis of the spectra allows you to obtain basic structural information about the molecules such as the H- Cl bond distance in several vibrational states. Before you start. .. Read Chapter 13 (pp. 495-537) entitled "Molecular Spectroscopy" in McQuarrie and Simon, that gives detailed information about the general quantum mechanical phenomena involved. You may read the discussion of a similar experiment in Shoemaker, Garland & Nibler (p. 397 in 6 th Edition). Chapter III of Herzberg's classic text [1] is the source of all of the Chemistry 541/2 Chemistry 541/2
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2 Fig. 1. The sample region of the FT-IR. Fig. 2. Background scan window. information in this handout. Refer to the original text if you need more details. You should also bring a memory stick (RAMdisk) or writable CD disc to class in order to store your data. Procedure 1) This may be done either before or after collecting the spectral data. Use the program Spartan to optimize the H 2 O, CO 2 , and HCl molecules at the MP2/6-31G* level of theory. If you do not remember how to do this, refer to the handout for Experiment 50 (procedure #3). Save the output files and record the rotational constants (in cm -1 ), vibrational frequencies (in cm -1 ) and corresponding intensities and degeneracies, and optimized geometry for comparison to experimental results. 2) Data collection is performed using a Perkin-Elmer (Spectrum GX) Fourier transform infrared (FTIR) spectrometer with a DTGS detector. The program that runs the spectrometer is called “Spectrum.” Double-click the “Spectrum” icon and logon to “chem. 541”(no password is needed, choose “OK”). It is important to save your results as (ASCII) text files, so check the path for saving spectra. From the “Setup” menu, choose “Options” and you will see a list of paths. To the right of the “Spectra” listing either type in the path to your directory or choose “browse” to look for it. It is useful to simultaneously have the Windows Explorer open to define your directory and verify that files are being stored there. The spectrometer is supposed to be purged by N 2 (g) in order to reduce background water and carbon dioxide. Check that the purge gas is flowing (you can hear it flow when the sample region door is open). First, record a detector curve (also called a background scan) without any sample in the sample region (see Fig. 1). From the instrument menu, choose “scan background”. You will see the background detector curve on the screen after each scan. The window for recording background scans is shown in Fig. 2.
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exp38_04_1_07 - Experiment 38 Chemistry 541/2 Physical...

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