Chemistry 2302 IR Anions Winter 2012 The Vibrational Spectra of Polyatomic Anions Recommended Preparatory Reading: •
The subheading Infrared Spectra of Complex Molecules in section 13.4 of Laidler Introduction: In this experiment vibrational spectra of polyatomic anions in the infrared region of the electromagnetic spectrum are measured, and differences in the spectra of similar species are attributed to the effect of the crystalline environment in which the small ions exist. When transitions occur between vibrational energy levels in molecules, infrared radiation is emitted or absorbed. In a diatomic molecule there is only one bond and, therefore, only one way for the bond to vibrate. However, in polyatomic molecules there are several bonds that can vibrate. These bonds do not vibrate independently of one another. There are collective motions of some (or all) of the atoms called normal modes of vibration. Mortimer’s (your textbook) Figure 22.6 shows some normal vibrational modes for three atoms in a linear and non-­‐linear arrangement. IR-­‐radiation of a particular wavelength will be emitted or absorbed for each vibrational mode that is IR-­‐active. These modes give rise to the peaks that we will observe in the spectra obtained during this experiment. Briefly explain the differences expected between infrared spectra of gas phase diatomic molecules (i.e. HCl and DCl molecules from the vibrational-­‐rotational experiment) and those of small polyatomic species which are part of a crystalline lattice (i.e. the anions in this experiment). Experimental Infrared spectra will be obtained on the Bruker Alpha IR spectrometer located in C-­‐3041. PLEASE treat the instrument with care (it is new and very expensive!). Spectra will be measured for one of the following pairs of solids: a) K2SO4 and KClO4 b) KClO4 and KIO4, KClO4 and KBrO4, or KBrO4 and KIO4 c) KNO3 and K2CO3 d) KClO3 and KBrO3, KClO3 and Na2SO3, or Na2SO3 and KBrO3 e) K2CrO4 and KMnO4 f) NaNO2 and NaClO2 g) NaNO3 and KNO3 h) NaClO3 and KClO3, or NaBrO3 and KBrO3 One pair of anions will be chosen for you to work with by your lab instructor. Look up their structure, vibrational frequencies and band assignments in Nakamoto (5) (preferably before your perform the experiment). This book is on reserve at the QEII Library. An electronic copy is also available (it can be accessed by performing a catalogue search on the QEII Library’s website. You will need this information to complete your report. Page 1 of 4 Chemistry 2302 IR Anions Winter 2012 The KBr that you will use has been dried and is stored in a desiccator. It is made especially for infrared work. Follow your instructor’s directions to first prepare an approximately 200 mg KBr disc. It will be used as a reference. Also prepare separate KBr disks containing each of the anions to be studied. For these disks, again use 200 mg of KBr mixed with only 2 mg of the salt containing the anion. Bring your three disks to the Bruker Alpha FTIR Spectrometer. If the instrument’s green light is flashing it is on standby mode. Turn the instrument on by pressing the small green button located next to the power cord on the back of the instrument. If the instrument’s green light is solidly lit there is no need to press this button. Login to Windows XP on the computer attached to the Bruker Alpha FTIR spectrometer (username: Nick password: bruker). Open the OPUS software by double-­‐clicking located on the top of the desktop (usename: chem2302 password: physchem). Click the OK button on the Windows that appears after OPUS starts. Notice any messages that appear in a yellow balloon in the lower right corner of the screen. The instrument might have to warm up or run a short test. Please wait for these processes to finish before continuing. A window with check marks might appear after a test completes. If so click on the continue button. Ensure that the standard sample holder is installed in the instrument (it has a blue cover on top). If it is not installed inform an instructor. Place your pure KBr reference disk in the metal disk holder and place the holder in the instrument’s sample compartment. Perform the following steps: 1. In the OPUS software, click the small black arrow next to the measurement button and select “setup measurement” 2. Click the load button. Pick the file named anion.xps and click OK. 3. Select the advanced tab and notice the parameters that are set 4. Click on Save and Exit 5. Click the measurement button (not the small black arrow) 6. In the measurement window that appears click Start Background Measurement. A green bar on the bottom of the screen displays the number of scans completed. 7. When “No Active Task” appears remove the metal disc holder from the instrument place the first sample disk in the holder and replace the holder in the instruments sample compartment. 8. Type the sample name and form (solid in KBr disk) in the measurement window 9. Click Start Sample Measurement. When the scans complete the spectrum will be displayed on the screen. 10. Click the small black arrow next to the Peak Picking button. Select interactive. 11. A small square and line will be displayed in the middle of the spectrum. Click and drag the square to near the bottom of the spectrum. The wavenumbers of any peaks that lie above the line will be labeled above the peak. 12. When you are satisfied with the number of peaks that are labeled click on the save button located near the top left corner of the spectrum. 13. In the sidebar, make sure the report tab is selected. Place checkmarks in the four boxes labeled print, save, unload and measure next sample. Click the GO button. 14. The spectrum is automatically saved, printed and closed. The measurement window appears again. Follow steps 7 – 14 to obtain your second spectrum. Page 2 of 4 Chemistry 2302 IR Anions Winter 2012 When you are finished using the instrument, please leave it on. You may close the OPUS software, however. Disposed of the three discs in the regular garbage unless one or two of them contain toxic compounds. In this case, follow an instructor’s directions for proper disposal. Please ensure that the area where you prepared the disks is left clean and tidy. Clean and thoroughly dry all of the equipment (die, mortar, pestle) you used when preparing the cell. Rinse the used weighing bottles with distilled water and place them in the oven to dry. Results Using the spectra obtained from samples in KBr disks, make band assignments for your IR spectra by indicating what vibrational mode is responsible for the presence of each sample band. Draw the best Lewis electron-­‐dot structure and the 3D molecular geometry for each of your anions. Also give the name of the molecular geometry. Describe, using diagrams, each vibrational mode. Discussion Compare experimental vibrational frequencies with those in the literature. Nakamoto (5) is an extremely accessible reference for IR and Raman characterization of small inorganic entities. Why might the experimental frequencies differ slightly from the literature values? Besides Herzberg, V. II (6), Nakamoto (5), and Gadsden (7) on reserve, you will find in the lab copies of some of the original publications on the vibrational-­‐rotational spectra of some of the small molecules we study. These are something of a curiosity. If you find any original publications that are not in the collection, please make a copy for the lab. Explain why the polyatomic atom anions measured as solids fail to show rotational fine structure as do HCl and DCl gases. Keeping in mind that your two polyatomic anions are quite similar, account for differences in the spectra of the two species. Questions 1) a) Explain how polyatomic molecules can have some vibrational modes that do not give rise to vibrational IR bands (IR-­‐inactive). b) Predict the shape of a CS2 molecule. List all of the vibrational modes for this molecule and label each as IR-­‐active or IR-­‐inactive. 2) Give the number of normal vibrational modes for each of the following molecules: C6H12 H2CO CH3COOH Page 3 of 4 Chemistry 2302 IR Anions Winter 2012 References
Available in the lab: 1. Shoemaker et al. "Experiments in Physical Chemistry", McGraw-­‐Hill, Toronto, 1989. 2. Rodney J. Sime, "Physical Chemistry", Holt, Rinehart, and Winston, Orlando, 1990. 3. Ira N. Levine, Quantum Mechanics, 5th Edition, Prentice Hall, New Jersey, 2000. Available on reserve in the library: 4.
Gordon M. Barrow, “Molecular Spectroscopy”, McGraw-­‐Hill, Toronto, 1962. 5. Kazuo Nakamoto, "Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed. Part A, Theory and Applications in Inorganic Chemistry", 1997. 6. Gerhard Herzberg, "Molecular Spectra and Molecular Structure II. Spectra of Polyatomic Molecules", van Nostrand, 1953. 7. J.A. Gadsden, "Infrared Spectra of Minerals and Related Inorganic Compounds", 1975. The following are available in the lab and in some cases in the periodicals section of the library (as a consequence of exhuming original references, some of the copies provided (*) are from microcard, microfilm, or inter-­‐library loan....please do not misplace them...they are hard to come by): K2SO4 and KClO4: 9. Henry Cohn, J. Chem. Soc. (1952) Part IV, 4282. 10. Hans Siebert, Z. Anorg. Allg. Chem (1954), 225.* 11. S.D. Ross, Spectrochemica Acta 18 (1962), 225. NaNO3 and KNO3: 12. B. Sundara Rama Rao, Proc. Indian Acad. Sci. 19A (1944), 93.* 13. B. Dundara Rama Rao, Proc. Indian Acad. Sci. 10A (1939), 167.* 14. M. Anbar, M. Halmann, and S. Pinchas, J. Chem. Soc. (1960) Part I, 1242. 15. K. Bulis and C.J.H. Schutte, Spectrochemica Acta 18 (1962), 307. NaClO3 and KClO3: 16. A.K. Ramdas, Proc. Indian Acad. Sci. 36A (1952), 55.* 17. A.K. Ramdas, Proc. Indian Acad. Sci. 36A (1953) 451.* 18. A.K. Ramdas, Proc. Indian Acad. Sci. 37A (1953) 451.* 19. J.L. Hollenberg and D.A. Dows, Spectrochemica Acta 16 (1960), 1155. Calcite and aragonite: 20. S. Bhagarantum and T. Venkatarayuda, Proc. Indian Acad. Sci. 9A (1939), 224.* Page 4 of 4