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Is a totally eclipsed Newman projection the most stable conformation for a 2 carbon compound, with one carbone bonded to more electronegative atoms (making them partial negative), and the other carbon with something less electronegative (making it partial positive)?

For example, let's imagine CF3CH3. If they are eclipsed, the fluorine side is partial negative and the hydrogen side is partial positive, so wouldn't they attract?

Or do the electron clouds always repel each other, no matter what atom is used (making a staggered conformation the most stable, not eclipsed)?

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  • $\begingroup$ Electron clouds are always negatively charged and repel if they come too close, no matter if the atoms they belong to have an overall attraction to each other. $\endgroup$ – Karl Sep 29 '18 at 6:39
  • $\begingroup$ Hmmm, how do hydrogen bonds (or intermolecular forces) exist in that case? Wouldn't the atoms repel there too? I'm getting confused haha. $\endgroup$ – F16Falcon Sep 30 '18 at 17:08
  • $\begingroup$ All bonds have the lenght where long range attraction and short range repulsion cancel each other. $\endgroup$ – Karl Sep 30 '18 at 17:21
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Generally staggered form is more stable. Same is the case here. Staggered form is more stable. This is because the partial positive charge on hydrogen is too low that electronic repulsion dominates. It is considerable only when hydrogen is bonded to highly electronegative elements like F,O,N (chlorine and sp as well as sp² carbon in some cases), that is in case of hydrogen bonding.

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For 1,1,1-trifluoroethane the two extremes are the staggered and the eclipsed conformations. I have done some molecular mechanics calculations which indicate that the staggered conformation has an energy of 5.7 kCal per mole while the eclipsed it is 1.31 kCal per mole.

This is the result for the molecular mechanics, if we consider the charges calculated in extended Huckel theroy then the flourines and hydrogens do not have large partial charges.

The carbon bearing the three flourines has a charge of +0.865378, the flourines have a charge of -0.261741, the other carbon has a charge of -0.209224 and the hydrogens have a charge of + 0.0430

I have not done the calculations for the electrostatic attraction, but I think in this molecule that if we were to treat the atoms as point charges that the main effect would the flourine atoms and the carbon which they are attrached to. I think that a change of the conformation will make very little difference to the electrostatic attraction energy.

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