In the Laboratory
An Interactive Dry Lab Introduction to Vibrational Raman Spectroscopy Using Carbon Tetrachloride
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Monty L. Fetterolf* and Jack G. Goldsmith Department of Chemistry, University of South Carolina Aiken, Aiken, SC 29801; *
[email protected] Although the Raman effect was observed almost 70 years ago (1) and the first compilation of spectra appeared in 1939 (2), Raman spectroscopy is a technique often missing from the undergraduate chemistry laboratory curriculum. This is despite the availability of theoretical descriptions from textbooks, reference books, and this Journal (3–8). However, with more than 6,000 Raman-related publications in a recent year (1996) spanning all areas of chemistry, its importance as an instrumental method cannot continue to be overlooked. Adding research-grade Raman instrumentation to a program can easily cost $100,000 and is an unlikely option for most institutions. Portable instruments are now available in the $20,000–$30,000 range, but are also likely to be passed over in favor of more traditional instrumentation—especially if the department faculty does not include a Raman expert. Homebuilt alternatives (9, 10) are available at a lower hardware expense, but these still require assembly time and expertise, and they may not provide the performance of a commercial instrument. A preferable alternative would be one where students receive the benefit of an introduction to Raman spectroscopy but without the cost of instrumentation. Up till now, dry labs for Raman spectroscopy have dealt with a particular aspect of the technique and have been limited by the need to copy spectra from textbooks or journal articles. Examples of this approach can be found in the two Raman dry labs published in this Journal: one interpreting the Raman spectra of ZXY3 compounds (11) and the other using pure rotational Raman spectroscopy for determining bond distances in CO2 (12). While these provide excellent paper-based physical chemistry labs, there is no interactivity, and calculations involving the data are based on approximations of intensities and peak positions. However, with the Galactic Industries Corp. (Salem, NH) DataViewer (13, 14 ) software and appropriate information on the instrument used to obtain spectra, students can perform a dry lab that illustrates several fundamental characteristics of Raman spectroscopy and lack only the hands-on component of the experiment. Many very informative and educational comparisons needed for a deeper understanding of the Raman technique are possible and can be achieved without expensive equipment or the lengthy data acquisitions that do not fit into a teaching lab schedule. Here we present an introduction to Raman spectroscopy that includes a description of the instrumentation, with illustrations, and the information and spectra necessary for a dry lab of the type just discussed. The hope is to create a dry lab that has some relevance, contains enough realism to be interesting, and is more than a paper exercise. This article also clearly demonstrates the flexibility and interactivity present in DataViewer-based labs centered around other phenomena or techniques for use in either a physical chemistry or an instrumental analysis course. 1276
Lab Overview To provide a framework for students as they read the background material, the familiar infrared absorption spectroscopy is discussed and contrasted to the techniques of Raman spectroscopy. Carbon tetrachloride is an excellent molecule on which to base this discussion and contrast. It exhibits only one infrared-active transition but four Raman-active transitions, all at less than 1000 cm{1. Because the spectra from both techniques display sharp, well-defined, and distinct peaks, undue complications are avoided during analysis. Carbon tetrachloride is a fairly common organic solvent and students will probably have had some direct lab experience with it. Most students also have solid experience with Internetbased programs and this lab allows them to draw on that experience. Students are encouraged to view figures and analyze data using several freeware Internet browsers. This keeps them actively involved with the lab and prevents it from becoming a paper exercise. A complete experimental description of the instrumentation and data acquisition procedures begins the background section. Equipment figures are supplemented with an extensive discussion of purpose and relative placement of accessories. This should give students a reasonable mental image of the equipment and techniques involved and leave them less in the dark as they work through the rest of lab. The remainder of the background introduces the classical theory of infrared absorption and Raman scattering by molecules. These discussions include the gross requirements for IR and Raman activity, the geometry of photon–molecule interaction giving rise to oscillating dipole moments for IR and polarizability tensors for Raman, the Stokes and antiStokes Raman scattering events, and the room temperature populations of molecular vibrations. Finally, a general introduction to the use of symmetry as a predictor of IR or Raman activity but limited to carbon tetrachloride is developed. Once the background information has been studied, a full analysis of the IR and Raman data is shown. Peaks are assigned on the basis of the reasoning and arguments developed in the background section. By drawing on information obtained from both the IR and Raman spectra, the student gains an appreciation for the complementary nature of the two vibrational techniques. A full assignment of all peaks would be more difficult without both spectra. Again, carbon tetrachloride is an excellent molecule for this lab. The polarization of Raman peaks is dramatically demonstrated using the polarized spectra in which the peak due to the totally symmetric vibration disappears as predicted. The relative intensities of the Stokes and anti-Stokes Raman peaks leads to an experimental measurement of room temperature based on the Boltzmann population distribution. A calculation worksheet written using Mathcad and available for download or use over the Internet using MathBrowser is provided. Finally, a high resolution Raman spectrum is pro-
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Laboratory
vided of the 459 cm{1 peak, to demonstrate the ability to resolve the three most abundant chlorine isotope combinations in CCl4. This initiates a discussion on reduced mass and its relationship to force constants and fundamental vibrational frequencies. A worksheet is provided to guide students through the analysis of the spectra, and instructions are supplied for obtaining MathSoft’s MathBrowser and Galactic’s DataViewer freeware from the Internet. Although the lab can be set up to run entirely from a floppy disk, the highlight for students is the real interactivity with computers and locations some distance away. As a package, this lab introduces students to the techniques, subtleties, and sophistication of vibrational Raman spectroscopy through comparison with the familiar IR absorption technique and through the use of straightforward classical physics descriptions. The complete vibrational analysis of carbon tetrachloride is shown and discussed thoroughly. The topics of Boltzmann statistics and isotope abundance are presented as needed for spectrum analysis, showing the importance of these topics to spectroscopy in general. The use of Internet technology enhances the excitement associated with this dry lab, builds confidence in computer skills, and opens up the possibilities of the Internet to students.
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Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Sep/ abs1276.html.
Literature Cited 1. Raman, C. V.; Krishnan, K. S. Nature 1928, 121, 501. 2. Hibben, J. H. The Raman Effect and Its Chemical Applications; Reinhold: New York, 1939. 3. Strommen, D. P. J. Chem. Educ. 1992, 69, 803. 4. Strommen, D. P.; Nakamoto, K. J. Chem. Educ. 1977, 54, 474. 5. Hoskins, C. L. J. Chem. Educ. 1975, 52, 568. 6. Bulkin, B. J. J. Chem. Educ. 1969, 46, A781. 7. Bulkin, B. J. J. Chem. Educ. 1969, 46, A859. 8. Tobias, R. S. J. Chem. Educ. 1967, 44, 70. 9. Galloway, D. B.; Cioklowski, E. L.; Dallinger, R. F. J. Chem. Educ. 1992, 69, 79. 10. Fitzwater, D. A.; Thomasson, K. A.; Glinski, R. J. J. Chem. Educ. 1995, 72, 187. 11. DeHann, F. P.; Thibeault, J. C.; Ottesen, D. K. J. Chem. Educ. 1974, 51, 263. 12. Hoskins, C. L. J. Chem. Educ. 1977, 54, 642. 13. Galactic Industries Corp. Home Page; http://www. galactic.com/ (accessed Jul 1999). 14. Goldsmith, J. G. J. Chem. Educ. 1998, 75, 1091.
JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education
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