Ethane C₂H₆ can rotate rather freely being a smaller compound.  The sigma bond can overlap while the ends of the molecule rotate; the two methyl groups are not held in fixed positions relative to one another.  The different spatial arrangements formed by these rotations about a single bond are called conformations or conformers.  We rotate the molecule bond by degrees from 0-360.  There are an infinite number of conformations about any sigma bond so we focus on the most significant; eclipsed and staggered conformations.  Staggered conformations are almost always favored.  In an eclipsed conformation the carbons are aligned so that the hydrogens are lined up with each other.  This creates steric hindrance between them.  In a staggered conformation the atoms are all equally spaced from each other.  Ethane being rather small and only having simple hydrogens to rotate has only two major conformations.  The staggered and eclipsed conformations repeat every 120 degrees.

The eclipsed conformation of ethane is less stable than the staggered conformation.    Energies for these rotations are measured using Chem 3D,  drawing an ethane molecule and minimizing its energy will start it in the staggered conformation.  The staggered conformation is the most stable of all possible conformations of ethane, since the angles between C-H bonds on the front and rear carbons are maximized which minimizes the energy.   The Total Energy is visualized on the graph by the green curve.  The minimums can be seen on the graph at 60, 180 and 300 degrees.  In the eclipsed form, the electron densities on the C-H bonds are closer together than they are in the staggered form. When two C-H bonds are brought into a dihedral angle of zero degrees, their electron clouds experience repulsion, which raises the energy of the molecule. The eclipsed conformation of ethane has three such C-H eclipsing interactions, they can be seen on the graph at 0/360, 120,  300 degrees.

Eclipsing interactions are an example of a general phenomenon called steric hindrance, which occurs whenever bulky portions of a molecule repel other molecules or other parts of the same molecule. The steric hindrance causes resistance to rotation, also called torsional strain.  This strain was also measured by the Chem 3D program and is visualized in blue on the graph.  As shown on the graph the maximums and minimums also correspond to the eclipsed and staggered conformations as before.  The red curve is the measurement of the 1,4 Van der Waals forces; which again correspond to the eclipsed and staggered conformations.

These two forms of the ethane molecule do not exist independently of each other;  the molecule ist in constant motion.  The eclipsed and staggered conformation are not considered isomers because of their rapid interconversion.

Butane, C4 H10, has more conformations than ethane since it has four carbon atoms and  its dihedral angles could vary across three C-C bonds. We focus on the central C2 -- C3  bond and treat the end carbons generally as methyl groups.  Conformationally,  butane is more complex than ethane because instead of rotating alike hydrogens as in ethane, butane rotates two hydrogrens and one methyl group.  The methyl group creates more steric hindrance especially as they approach each other.  We analyze the conformational energies as we did ethanes’.  Butane will have four conformations of importance; two different eclipsed conformations and two different staggered conformations.  These conformations differ by the relative positions of the two methyl substituents. The butane molecule is drawn in Chem 3D and energy minimized.  This places the molecule at its optimal energy which corresponds to the staggered conformation where the two methyl groups are furthest from each other.   This is visualized at 180 dergrees on the graph and is the most stable conformation.  This particular staggered conformation is called anti. The other staggered conformation has the methyl groups at a  dihedral angle of 60 and 300 degrees on the graph. This is called the gauche conformation. The gauche form is less stable than the anti form  due to steric hindrance between the two methyl groups but still is more stable than the eclipsed formations.  Such an interaction is often referred to as a gauche-butane interaction because butane is the first alkane discovered to exhibit such an effect.   The eclipsed formations have two energy levels as well.  At the 0 or 360 angle of rotation the steric energy is at its maximum and is the least favorable conformation.  In this conformation the methyl groups are overlapped with one another.  The other eclipsed conformation occurs at the angles of 60 and 300 degrees.  In these conformations the methyl groups are eclipsed by single hydrogens and thus have less steric interaction. However it is still less favorable than either of the staggered conformations.  The Newman Projections which illustrate these interaction by sighting directly down the C2 -- C3  bond showing the conformations discussed.  The graph illustrates the total energy in the purple curve and the torsion (blue curve), non 1,4 Van der Waals (red curve), 1,4 Van der Waals (green curve) separately as well.