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Through-Space Charge and Dipolar Effects in Substituted Ethanes and 1,1,1-Trifluoroethanes

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journal contribution
posted on 19.12.2001, 00:00 by Elizabeth M. Nolan, R. G. Linck
The results of ab initio calculations with the 6-31+G* basis set are reported on over sixty compounds of the type CZ3CH2X (Z = H, F) and on a few longer chain-substituted alkanes. Data for the eclipsed conformations with Z = H and both the staggered and eclipsed conformations with Z = F are presented. The charges on the atoms in these molecules are determined with the Atoms in Molecules (AIM) and Natural Population Analysis (NPA) methods of analysis. The charges on the fluorine atoms in the trifluoroethyl derivatives parallel those of the hydrogen atoms of the ethyl derivatives. A potential gradient model for charge variations is presented. In this model, the magnitude of the charge is dictated by a through-space electrostatic interaction that arises from the charges on the atoms of the CH2X group. The length of the vectors from the atoms in the CH2X group to the bond critical point of the C−Z (Z = H, F) bond and the angle this vector makes with the C−Z bond are critical parameters. The model is tested in systems in which the charge of the methylene carbon atom can be ignored, as well as more rigorously with inclusion of the charge of the methylene carbon atom. The potential gradient model accounts for a greater positive (negative) charge on the antiperiplanar atom than on the gauche ones (staggered conformer) and on the gauche compared to synperiplanar (eclipsed conformer) for positively (negatively) charged X. The model also gives a rationalization for the change in charge observed on the Z atoms when X = EHnq is replaced by X = EFnq. Comparisons of antiperiplanar and eclipsed conformers of CH3CH2X and CF3CH2X show that these conformational changes have little effect on the charges of the atoms in the methylene (CH2X) fragment.