Exploring the C4H7+ Energy Surface: A Computational Discovery Process for Students   

Roger S. Macomber

Natural Science Division, Pepperdine University, Malibu, CA 90263    

Abstract

This article poses a seven part exercise to students interested in honing their skills with molecular modeling and computational chemistry. All the exercises explore different aspects of the C4H7+ carbocation manifold: cyclopropylcarbinyl, homoallyl, cyclobutyl, and bicyclobutonium ions. The student is first asked to generate structures for all possible isomers of C4H6 and all isomeric C4H7+ ions where the charge resides (formally) on an sp2 carbon. Next, the conformational mobility of each ion is explored qualitatively. Then the student carries out semi-empirical MO calculations (e.g., AM1 or PM3) to provide the gas phase heat of formation of each C4H6 and C4H7+ isomer. With this information, the student predicts the most stable carbocation accessible from each hydrocarbon, and determines which carbocation rearrangements are likely to occur. Finally, the student is asked to explain why solvolyses of homoallyl, cyclobutyl, or cyclopropylcarbinyl substrates often yield comparable mixtures of products. An appendix with a complete analysis of each exercise is included.

Journal of Chemical Education, 1998, 75, 1346            Full text (PDF)

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