Eclipsed conformation

An eclipsed conformation places a pair of groups on adjacent bonded atoms at zero torsion angle: their bonds overlap in a projection looking along the bond joining those atoms. In the idealized Newman projection of an ethane-like group, three front/rear bond pairs can be simultaneously eclipsed. Projected overlap does not mean that two atoms occupy the same point in three-dimensional space.^[ref]

For a concrete construction, draw three front bonds at $0^{\circ }$, $120^{\circ }$ and $240^{\circ }$. Draw the rear bonds at the same three angles. Their projected directions coincide, giving an eclipsed conformation. Turning the rear set through $60^{\circ }$ gives a staggered conformation. Turning both sets together merely rotates the whole drawing and leaves their relative torsion angles unchanged.

The distinction is useful in discussing a rotational energy barrier. A deliberately simplified threefold model,has a maximum $B$ at the eclipsed angle $\theta =0$ and a minimum at $\theta =\pi /3$. Thus continuous rotation can preserve every bond while changing potential energy. This model defines its own barrier; it does not measure a barrier in a particular substance. The eclipsed conformation is the geometry at the chosen position, while the barrier height and the rate of interconversion require further physical information. J. D. Kemp and K. S. Pitzer's ethane study provides an early thermodynamic setting for the problem of hindered internal rotation.^[ref]