title: The barrier to the methyl rotation in Cis-2-butene and its isomerization energy to Trans-2-butene, revisited.
abstract: We respond to the two questions posed by Weinhold, Schleyer, and McKee (WSM) in their study of cis-2-butene (Weinhold et al., J Comput Chem 2014, 35, 1499), in which they solicit explanations for the relative conformational energies of this molecule in terms of the Quantum Theory of Atoms in Molecules (QTAIM). WSM requested answers to the questions: (1) why is cis-2-butene less stable than trans-2-butene despite the presence of a hydrogen-hydrogen (H⋯H) bond path in the former but not in the latter if the H⋯H bond path is stabilizing? (2) Why is the potential well of the conformational global minimum of cis-2-butene only 0.8 kcal/mol deep when the H⋯H bonding is stabilizing by 5 kcal/mol? Both questions raised by WSM are answered by considering the changes in the energies of all atoms as a function of the rotation of one of the two methyl groups from the minimum-energy structure, which exhibits the H⋯H bond path, to the transition state, which is devoid of this bond path. It is found that the stability gained by the H⋯H bonding interaction is cancelled by the destabilization of one of the ethylenic carbon atoms which, alone, destabilizes the system by as much as 5 kcal/mol in the global minimum conformation. Further, it is found that the 1.1 kcal/mol stability of trans-2-butene with respect to the cis-isomer is driven by the considerable destabilization of the ethylenic carbons by 11 kcal/mol, while the changes in the atomic energies of the other corresponding atoms in the two isomers account for the observed different stabilities. The error introduced into QTAIM atomic energies by neglecting the virials of the forces on the nuclei for partially optimized structures is discussed.
authors.affiliation: Department of Chemistry and Physics, Mount Saint Vincent University, Halifax, Nova Scotia, B3M 2J6, Canada.|Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3, Canada.|Department of Chemistry, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada.
authors.forename: Chérif F
authors.initials: CF
authors.lastname: Matta
authors.affiliation: Department of Physics, Faculty of Science, Space Astronomy Laboratory, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China.
authors.forename: SeyedAbdolreza
authors.initials: S
authors.lastname: Sadjadi
authors.affiliation: 19460 Wilderness Dr., West Linn, Oregon.
authors.forename: Dale A
authors.initials: DA
authors.lastname: Braden
authors.affiliation: Philipps-Universität Marburg Hans-Meerweinstr., Marburg, 4 D-35032, Germany.|Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain.
authors.forename: Gernot
authors.initials: G
authors.lastname: Frenking
keywords: H-H bonding; QTAIM; bond path; bonding analysis