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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]

J. D. Kemp is a chemist credited with K. S. Pitzer on the 1937 paper The Entropy of Ethane and the Third Law of Thermodynamics. Hindered Rotation of Methyl Groups by J. D. Kemp and Kenneth S. Pitzer. The paper connects molecular internal rotation with entropy and the third law of thermodynamics; the originating record identifies Kemp by initials.^[ref]

The physical question is whether rotation around a carbon-carbon bond can be treated as free or whether angle-dependent potential energy must be included. For example, a model $V(\theta )=0$ assigns the same energy to every orientation; a threefold model $V(\theta )=B(1+\cos 3\theta )/2$ distinguishes staggered conformations from eclipsed conformations. Such models predict different populations of orientations and therefore different contributions to entropy. The example explains the thermodynamic relevance of the paper's question; it is not a reconstructed numerical result from the 1937 experiment.

Kemp's verified contribution here is coauthorship of that focused physical-chemistry study. The bibliographic evidence does not assign a separate apparatus design or calculation to each coauthor, and it does not establish a full name or a portrait.

A 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.^[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 iswith 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.^[ref]

Torsional strain is an increase in the energy of a molecule associated with an unfavourable torsion angle, relative to a stated reference conformation. Rotation about a bond can change the relative orientations of substituents without changing which atoms are bonded. The IUPAC definition of strain explicitly includes unfavourable dihedral angles alongside bond-length and bond-angle distortions.^[ref]

Consider ethane viewed along its central carbon–carbon bond. An eclipsed conformation aligns projected carbon–hydrogen bonds, whereas a staggered conformation places them between one another. An idealized threefold potential energy model iswith $\varphi =0$ chosen eclipsed. It gives $V(0)=V_3$ and $V(\pi /3)=0$: a finite rotational barrier separates equivalent minima. The model is a teaching parametrization; $V_3$ must be obtained independently rather than read from the drawing.

The 1937 paper by J. D. Kemp and K. S. Pitzer explicitly connected the entropy of ethane and the third law of thermodynamics with hindered methyl-group rotation.^[ref] This historical connection shows why internal rotation is a measurable molecular degree of freedom, not simply unrestricted motion around a single bond. In a ring, geometric closure can restrict accessible torsion angles; ring strain can therefore include torsional strain as well as other contributions. Assigning a separate torsional energy is model-dependent, rather than a complete account of every electronic interaction.