Eclipsed vs. Staggered. Tetrahedral Carbons

It has been observed that ethane prefers a staggered conformation. Unfortunately, experiments do not tell us why this preference exists. 

Step through the sequence of structures depicting bond rotation in ethane. Plot energy (vertical axis) vs. HCCP torsion angle (horizontal axis). Do the minima correspond to staggered structures Do the maxima correspond tc eclipsed structures If not, to what do they correspond  

Estimate the cost of nonbonded HH repulsion as function of distance by plotting energy (vertical axis) vs HH separation (horizontal axis) for methane+metham (two methanes approaching each other with CH bond head on ). Next, measure the distance between the nearest hychogens in eclipsed ethane. What is the HI repulsion energy in the methane chmer at this distance Multiplied by three, does this approximate the rotatioi barrier in ethane  

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