Among the butane conformers, which occur at energy minima on a graph of potential energy versus dihedral angle?

A) gauche only
B) eclipsed and totally eclipsed
C) gauche and anti
D) eclipsed only
E) anti only

The anti conformer is the most stable of all the butane conformers and occurs at the energy minimum with a dihedral angle of 180 degrees. The gauche conformer occurs at a secondary energy minimum with a dihedral angle of 60 degrees or 300 degrees, while the eclipsed and totally eclipsed conformers occur at higher energy levels and are unstable. Therefore, the correct answer is C, as both the anti and gauche conformers occur at energy minima on the graph of potential energy versus dihedral angle.