Elucidation of the origin of substituent effects. Theoretical investigation of field effect in aliphatic systems by ab initio molecular orbital calculation using simple models.

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The electronic substituent effect in the aliphatic systems was investigated using ab initio calculation at STO-3G and 4-31G levels. The relative energy changes, δΔE, in isodesmic reaction of the 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids were compared with those for 1-substituted bicyclo[2.2.2]octanes(XC8H13), CH3X, and H–X molecules perturbed with isolated pure point charges. The interaction energies between isolated dipole and point charge system, F–H···(±), were proportional to the reciprocal square of the distance between the center of dipole and the charge. Isodesmic reaction energies for the present systems at the distance longer than 5 Å can be practically explained by charge-dipole interaction. From the comparison of δΔE in XCH3···(±), XC8H13···(±), and XC6H5···(±) systems with those for X–H···(±), it was concluded that the pure field parameters for various substituents should be determined on the basis of the isodesmic reaction energy for X–H···(±) system. Correlations between δΔE in X–H···(±) system and the field parameters recently proposed, σF and σI(gas), were generally better in the cases calculated at 4-31G level than those at STO-3G level.
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