Chemical Education Today edited by
NSF Highlights
Susan H. Hixson
Projects Supported by the NSF Division of Undergraduate Education
Molecular Modeling in the Undergraduate Chemistry Curriculum
National Science Foundation Arlington, VA 22230
Richard F. Jones Sinclair Community College Dayton, OH 45402-1460
by Martin B. Jones
In 1996 the chemistry department at Adams State College (ASC) prepared a five-year plan that included a specific objective of exposing all students at all levels to the use of computer-based molecular modeling. The burgeoning use of molecular modeling in educational, research, and industrial laboratories, the benefits to student understanding of rather abstract concepts, and the availability of appropriate software designed for personal computers were major factors in our decision to address this deficiency in our curriculum. In the spring of 1997, the department purchased one computer with software dedicated to molecular modeling for carrying out preliminary investigations. In the summer of 1997, I attended an NSF-sponsored workshop on Molecular Modeling held at Lebanon Valley College (LVC), run by Richard Cornelius and Carl Wigal (1). In this workshop, the attendees were introduced to the use of molecular modeling experiments to support laboratory experiments as well as to provide clarification (visual and otherwise) of lecture concepts. Subsequent to the workshop, I submitted a proposal to the National Science Foundation to outfit a molecular modeling computer laboratory with six stand-alone computers equipped with molecular modeling software. While the proposal was being reviewed, I taught a special topics course to six seniors using the computer and software purchased previously. This allowed me to test various modeling experiments before introducing the technique on a larger scale. The proposal was funded, and in the summer of 1998 the computers and software were purchased and installed in a small study room on the chemistry floor of our science and mathematics building. The general strategy of our department has been similar to that espoused at the LVC workshop, that is, to use modeling experiments in support of laboratory experiments in an attempt to meld theoretical and practical aspects of chemistry. The molecular modeling software is also installed in the lecture classrooms, so instructors can illustrate appropriate topics with calculations or graphical representations. Introduction of molecular modeling in the first-year courses permits subsequent courses to investigate more sophisticated modeling exercises. Use in Introductory and General Chemistry To date the major use of molecular modeling in these two classes has been to compare structural features of simple molecules built with the modeling software with predicted features from VSEPR and hybridization theories and periodic trends. For example, measurement of bond angles for the molecules CH4, NH3, and H2O clearly reinforces a central tenet of VSEPR theory that bond angles are governed primarily by the number of electron sets, rather than the num-
ber of atoms surrounding the central atom. Construction of the molecules BF3, BCl3, BBr3, and BI3 permits determination of bond length and provides both a visual and a quantitative measure of the difference in atomic size of the halogens. Measurements of bond angle and bond length of simple organic molecules support the predictions of hybridization theory. Additional experiments planned for these classes involve use of electron density calculations to illustrate differences in types of bonding (covalent, polar covalent, ionic) and the difference between contributing structures and resonance hybrids (e.g., of nitrate or carbonate ion) (2). Use in Organic Chemistry The predominant use of molecular modeling has been in the organic chemistry course. The software is used for support of several laboratory experiments as well as for homework assignments and lecture. Dehydration of 2methylcyclohexanol affords a mixture of alkenes that clearly follows Zaitsev’s rule (3). Students use the modeling software prior to the laboratory work to calculate heats of formation of the two possible alkenes, then predict which product should be obtained in greater yield (3). Analysis of the product mixture by GC confirms their prediction. Nitration of methyl benzoate is a classic experiment that illustrates the directing influence of deactivating substituents (4). Students calculate heats of formation of intermediate arenium ions from ortho-, meta-, and para-substitution of the starting material, and predict which ion is more stable, leading to the major product (5). In conjunction with an experiment involving competing nucleophiles, students calculate an approximate energy of activation for carbocation formation from differing substrates and use the data to rank substrate reactivity in SN1 reactions (4). Calculated heats of formation and connectivity indices are compared to experimental boiling points of an isomeric set of hydrocarbons to determine whether structural framework or thermodynamic stability contributes more to a liquid’s boiling point (6). Students learn some of the limitations of simple, PC-based molecular modeling by comparing IR spectra predicted with the software with spectra they obtain with an FTIR spectrometer (7). Use in Other Chemistry Classes Students in analytical and environmental chemistry classes calculate predicted UV–Vis and IR spectra of CFC and potential CFC replacements (8), then discuss the relative atmospheric stability of these compounds. In physical chemistry, students conduct an experiment involving measurement of absorption maxima for a series of conjugated dyes
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Chemical Education Today
NSF Highlights to calculate energies and oscillator strength (9). A modeling component based on particle-in-a-box computations has been added to this experiment (10). Use in Undergraduate Research Students have used molecular modeling to determine relative stabilities of complexes of metal ions with polymersupported oligoethers (11) and for simple MO calculations of substituted aromatic molecules. In the former case, the results were then compared with data obtained from analysis of metal ion concentration after mixing of the polymer with aqueous solutions containing known amounts of metal ions. Concluding Remarks Each year, more molecular modeling experiments and homework assignments are added to courses in an attempt to facilitate student understanding of chemistry principles. The next major task is to begin incorporating molecular modeling into the biochemistry courses. Several experiments with a biochemistry emphasis have been published (12). Representative student comments from written evaluations suggest that the addition of molecular modeling is beneficial to their learning: The introduction of the computer into the classroom & assignments is a big help. Computer graphics help to get the point across.
Such comments and student response during the modeling exercises have convinced the chemistry department at ASC that the addition of molecular modeling has enhanced student learning and understanding of chemistry. Acknowledgments Partial funding of this project was obtained from the National Science Foundation’s Division of Undergraduate Education, Instrumentation and Laboratory Improvement Program, Grant No. DUE-9850497. Matching funds from Adams State College are greatly appreciated. Thanks also go to Mel Armold, Frank Novotny, Neil Rudolph, and Larry Sveum of the ASC Chemistry Department for enthusiastic participation in this curriculum enhancement. Literature Cited 1. Cornelius, R. D. Molecules—Molecular Modeling for Chemical Education; http://www.molecules.org (accessed May 2001).
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2. Shusterman, G. P.; Shusterman, A. J. J. Chem. Educ. 1997, 74, 771–776. 3. Jones, M. B. Dehydration of 2-Methylcyclohexanol; in-house publication, Adams State College: Alamosa, CO, 1990 (revised 1998) (adapted from Taber, R. L.; Champion, W. C. J. Chem. Educ. 1967, 44, 620). This experiment includes the molecular modeling component as well as the laboratory work. An alternative molecular modeling experiment is available: Moores, B. W. Dehydration of 2-Methylcyclohexanol; http:// www.molecules.org/experiments/Moores/Moores.html (accessed May 2001). 4. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Organic Laboratory Techniques: A Microscale Approach, 3rd ed.; Saunders: Fort Worth, TX, 1999. 5. Jones, M. B. Regiochemistry of Electrophilic Aromatic Substitution: Stabilities of Arenium Ions; in-house publication, Adams State College: Alamosa, CO, 1999 (adapted from Wigal, C. T. Predicting Intermediate Stability Using Molecular Modeling, in-house publication, Lebanon Valley College: Annville, PA, 1996). 6. Jones, M. B. Boiling Point. A Laboratory and Molecular Modeling Experiment, in house-publication, Adams State College: Alamosa, CO, 1999 (revised 2000); also available at http:// www.molecules.org/experiments/jones/jonesbp.html (accessed May 2001). 7. Jones, M. B. Infrared Spectra of Organic Compounds, in-house experiment, Adams State College: Alamosa, CO, 1998. 8. Novotny, F. J. CFCs and Potential Replacements, in-house experiment, Adams State College: Alamosa, CO, 1998. 9. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; McGraw-Hill: Boston, 1996, pp 378–383. 10. Nieman, G. Spectra of Dyes and the Particle-in-the-Box; http:// www.molecules.org/experiments/Nieman/Nieman.html (accessed May 2001). 11. Jones, M. B.; Thompson, J.; Novotny, F. J. “Synthesis and Testing of Polymer Resins for Metal Ion Removal from Aqueous Solutions”, presented at the Joint 55th Southwest/15th Rocky Mountain Regional Meeting of the ACS, El Paso, TX, October 21, 1999; Abstract 10. 12. Dabrowiak, J. C.; Hatala, P. J.; McPike, M. J. Chem. Educ. 2000, 77, 397–400. Wolfson, A. J.; Hall, M. L.; Branham, T. R. J. Chem. Educ. 1996, 73, 1026–1029. Lee, M. J. Chem. Educ. 1996, 73, 184–187. Jones, C. M. J. Chem. Educ. 1997, 74, 1306–1310.
Martin B. Jones is in the Department of Chemistry, Adams State College, Alamosa, CO 81102;
[email protected].
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
