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

Abstract

The results of ab initio calculations with the 6-31+G* basis set are reported on over sixty compounds of the type CZ 3 CH 2 X (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 CH 2 X group. The length of the vectors from the atoms in the CH 2 X 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 ) EH n q is replaced by X ) EF n q . Comparisons of antiperiplanar and eclipsed conformers of CH 3 CH 2 X and CF 3 CH 2 X show that these conformational changes have little effect on the charges of the atoms in the methylene (CH 2 X) fragment.

Aromatic Trifluoromethylation with Metal Complexes

“…For instance, the Mulliken and NAO negative charges on the carbon atom of the CH 3 group are considerably larger in CH 3 CF 3 than in ethane . The computed charges on the central carbon atom of propane, CF 3 CH 2 CH 3 , and CF 3 CH 2 CF 3 are similar, which is in accord with the fact that CF 3 CH 2 CF 3 is not nearly as CH-acidic as CF 3 CH 2 CN . Apparently, the β-effect might well take place in trifluoromethyl complexes of metals, as can be seen from the remarkably similar charge distribution trends shown in Schemes and .…”

Quantified electrostatic preorganization in enzymes using the geometry of the electron charge density

“…Studies have shown similar magnitudes of changes in position of bond CPs on small organic molecules due to the addition of electron withdrawing and donating groups. 34 , 35 Perturbations to wavefunctions that move CPs are due primarily to the orientations and positions of dipole moments from each atom in the surrounding protein, and will be most prominent for charged and polar residues, including the K + present in the 7 Å model for configurations 1 and 2.…”