Allinger's gauche hydrogen hypothesis as tested by an alternative force field model |
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Authors: | Eiji Ösawa John B Collins PvR Schleyer |
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Institution: | Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan;Institut für Organische Chemie Universität Erlangen-Nurnberg, 8520 Erlangen, Henkestr 42, West Germany |
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Abstract: | Allinger's hypothesis that gauche hydrogen, rather than gauche methyl interactions are mainly responsible for conformational equilibria was tested by molecular mechanics calculations using the Engler force field model. For the gauche-anti conformational energy of n-butane, effective cancellation among non-bonded in teractions involving the methyl groups results in the gauche hydrogen interactions across the central C-C bond as the single largest contribution to the energy difference. However, the contribution of this interaction is only one third of the 0.9 kcal difference. For 2,3-dimethylbutane, strain analysis of the gauche-anti conformational energy contributions reveals that geminal and gauche CH3/CH3 interactions dominate over the gauche hydrogen interaction Similar analyses for several monosubstituted cyclohexanes confirm that “across-the-ring” interactions. between axial substituents and syn -axial hydrogen atoms are still largely responsible for the instability of the axial relative to the equatorial conformer. In disagreement with Allinger's proposal, the “equatorial hydrogen effect” is found to contribute only a minor amount to the conformational energy difference. Allinger's hypothesis is concluded to be force field dependent, and not to have general validity. |
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Keywords: | Current address: Dept of Chemistry Univ of Calif Berkeley Calif |
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