= Eclipsed conformation
An \b[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.<Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 by IUPAC>[P-94.2 and P-94.3.1.1: zero torsion angle, eclipsed and staggered descriptions.]{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,
$$
V(\theta)=\frac{B}{2}(1+\cos3\theta),
$$
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.<The Entropy of Ethane and the Third Law of Thermodynamics. Hindered Rotation of Methyl Groups by J. D. Kemp and Kenneth S. Pitzer>[Originating 1937 paper, title and bibliographic record.]{ref}
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