= Staggered conformation
A \b[staggered conformation] is a <conformation> in which groups on adjacent bonded atoms are positioned between the groups on the other atom when viewed along the bond. In the idealized <Newman projection> of an ethane-like arrangement with three equally spaced groups on each carbon, the rear bonds bisect the angles between the front bonds. This is a geometric description relative to the viewing bond, not a change in molecular connectivity.<Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 by IUPAC>[P-94.2 and P-94.3.1.1: torsion angles and staggered/eclipsed conformations.]{ref}
Put the three front bonds at angular positions $0^\circ$, $120^\circ$ and $240^\circ$ in a <Newman projection>. Rear bonds at $60^\circ$, $180^\circ$ and $300^\circ$ give a <staggered conformation>. Rotate the rear group by $60^\circ$ and the rear bonds line up with front bonds, giving an <eclipsed conformation>. These positions describe projections: the atoms are still separated along the central bond.
A simple illustrative <potential energy> for a threefold rotor is
$$
V(\theta)=\frac{B}{2}\left(1+\cos3\theta\right),\qquad B>0,
$$
with <torsion angle> $\theta$ measured from an eclipsed position. It gives $V(\pi/3)=0$ for a staggered position and $V(0)=B$ for an eclipsed position. Here $B$ is an assumed barrier parameter, not a measured universal constant. Different substituents can give distinct energies to different staggered arrangements, so a geometric label alone is not a complete energy ranking. The historical connection between thermodynamics and hindered methyl-group rotation was examined by <J. D. Kemp> and <K. S. Pitzer>.<The Entropy of Ethane and the Third Law of Thermodynamics. Hindered Rotation of Methyl Groups by J. D. Kemp and Kenneth S. Pitzer>[JACS 59,276–279(1937), originating title and scope; no numerical barrier attributed.]{ref}
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