Elucidation of the Origin of Substituent Effects. Theoretical Investigation of Field Effect in Aliphatic Systems by ab initio Molecular Orbital Calculation Using Simple Models

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.