Electronic Spectra of Conjugated Systems: A Modern Update for a

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Molecular Modeling Exercises and Experiments

edited by

Ronald Starkey University of Wisconsin–Green Bay Green Bay, WI 54311-7001

Electronic Spectra of Conjugated Systems: A Modern Update for a Classic Experiment

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Leonard J. Soltzberg Department of Chemistry, Simmons College, Boston, MA 02115-5868; [email protected]

This junior/senior physical chemistry experiment improves upon a classic physical chemistry laboratory (1–5) by incorporating multielectron configuration interaction (MECI), as implemented in desktop molecular modeling software. MECI permits much-improved calculation of excited-state energies, compared with the Hückel method usually employed in the traditional experiment, so that the calculated wavelengths for UV–vis absorption in conjugated systems can be compared satisfactorily with laboratory data. In addition to integrating computational chemistry with laboratory work, this experiment affords practical significance to several concepts in quantum chemistry and molecular modeling; these include the Franck–Condon principle, symmetry-allowed transitions, molecular orbital delocalization, and choice of semiempirical parameter sets. We use ChemDraw 5.0 and MOPAC 97 as implemented in Chem3D Pro 5.0 (CambridgeSoft Corporation, Cambridge, MA, http://www.camsoft.com), running on PowerMac G3s (300 MHz) and iMacs (350 MHz) under MacOS 9.0. Other popular desktop modeling packages, such

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as HyperChem (Hypercube, Inc., Gainesville, FL, http:// www.hyper.com) and CAChe (Fujitsu Systems Business of America, Inc., Beaverton, OR, http://www.CAChe.fujitsu.com) could be used as well. W

Supplemental Material

The student handout, notes for the instructor, and a comparison of experimental and computed λmax values are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4.

Shalhoub, G. M. J. Chem. Educ. 1997, 74, 1317–1319. Anderson, B. D. J. Chem. Educ. 1997, 74, 985. Bahnick, D. A. J. Chem. Educ. 1994, 71, 171–173. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1989; pp 440–445. 5. Moog, R. S. J. Chem. Educ. 1991, 68, 506–508.

Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu