The Generation of Heteronuclear Diatomic Gases for Determination of Rotational Spectra A Variation of a Classic Physical Chemistry Experiment Davkl F. Rieck, Frederick A. Kundell, and Paul J. Clements Richard A. Henson School of Science and Technology, Salisbury State University, Salisbury. MD 21801 Obtaining and analyzing the rotational spectra of diatomic molecules has long been a standard laboratory exercise in physical chemistry courses.' The use of gaseous HCI as the diatomic molecule is common because of the excellent results that can be achieved. Typically, a lecture bottle of HC1 is used to purge the infrared gas cell, the data are collected, and the experimental portion of the exercise is completed very quickly. We have found that requiring the students to synthesize HBr, DBr, and HCI enhances this already worthwhile experiment. The HC1 is easily prepared by the reaction of sulfuric acid on NaCI? and the HBr and DBr can both be synthesized via electrophilic aromatic substitution.3The use of a simple organic reaction serves to reinforce the students' organic chemistry background and at the same time demonstrates an interdependence of seemingly (to many students) unrelated branches of chemistry. Procedure HBr and DBr. The preparation of HBr and DBr can be achieved simultaneously from the reaction of bromine on dstoluene (ds-benzene works extremely well also, but poses a greater health risk). In a typical experiment, 2.00 mL dstoluene (299%) and an iron catalyst were placed in a 25.00mL round-bottom flask that was outfitted with a cold-water condenser. The reaction began immediately upon the addition of 1.00 mL of bromine. The HBr and DBr gasses that were evolved were passed through a loosely packed glasswool plug and an oil bubbler in order to trap out Brz before being introduced into the gas cell. The gas cell was vented through a Tygon tube to a beaker of water. By positioning the vent tube just above the water, one could determine when acid was beine vented due to the characteristic white fumes. The cell wadhowed to purge until the reaction was nearly complete. The residual protons in the 2998deuterated toiuenekere sufficient to provide an excellent spectrum of HBr ( v r; 2557 cm-1) as well as DBr ( u 1842 cm-'1. In
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fact the intensity of the HBr spectrum was slightly greater than that of the DBr spectrum. HC1. For the synthesis of HCI, an excess amount (several grams) of NaCl was placed in a 250-mL two-neck flask that was fitted with a cold-water condenser (vented at the top) and an addition funnel containing concentrated sulfuric acid. The HpSOa was added slowly to the NaCI, and the HCI that was generated was vented through an oil bubbler (the role of the bubbler is to monitor the rate of gas evolutionClz is not generated) and into the gas cell. Comments In addition to the usual parameters reported (e.g., moment of inertia, interatomic distance, vibrational contribution to the heat caoacitv, . . etc.), students are ex~ectedto discuss briefly the reactions used. The discussion of electrophilicaromaticsubstitut~onshouldaualitativelvaddress the observed isotope effect (i.e., why s99% ds-toluene affords nearly equal amounts of HBr and DBr). Student response to this exercise has been quite positive. The opportunity to perform simple organic and inorganic preparations in a physical chemistry laboratory effectively demonstrates the interdependence of the different areas of specialization. Acknowledgment DFR and FAK would like to thank D. Leister, L. A. Mills, M. Palenchar, and S. Silbert.
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Shoemaker, D. P.; Garland, C. W.; Steinfeld,J. I.; Nibler, J. W. Experiments in Physical Chemistry, 4th €4.McGraw-Hill: ; New York, 1981: aa 439-448. dlton, F. A,; Wilkinson. G. Advanced Inorganic Chemistry:Wiley: New York, 1988;Chapter 3. Brewster, R. 0.; Vandemerf, C. A,; McEwen. W. E. Unitized Experiments In Organic Chemisify, 4th ed.;Van Nostrand: New York. 1977: pp 320-324.
