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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]

A Newman projection views a molecule along a chosen bond, showing substituents on the front atom and rear atom. It makes relative torsions visible without assuming that the two centres lie in the plane of a skeletal drawing.

A is the predicted major diastereomer. At the carbon adjacent to the ketone, take phenyl as the large group, vinyl as medium, and hydrogen as small. There is no adjacent heteroatom donor that would force a five-membered chelate. The appropriate first model is therefore the non-chelating Felkin-Anh model.

View along the carbonyl-carbon–stereocentre bond. Rotate to place the large alpha phenyl group approximately perpendicular to the carbonyl plane. Of the two resulting approaches opposite that large group, the favourable Bürgi-Dunitz trajectory passes nearer hydrogen rather than vinyl. Propyl transfer to the carbonyl and protonation of its oxygen give the configuration labelled A: in the printed projection the new OH projects forward and the propyl bond backward, while the original vinyl bond remains backward. Rotation used to construct a Newman projection must not be mistaken for inversion of the original stereocentre.

This is a relative-selectivity prediction, not a numerical product ratio. Simple attack on the face that happens to look less crowded in the initial flat drawing is unreliable because the relevant C–C single bond can rotate. B is the competing product from the less favourable approach beside vinyl. The model and its preference for the trajectory beside the small group are described in the institutional treatment.^[ref]

View the backbone along N–C${}_{\alpha }$ to display $\varphi =\angle (C_{i-1}^{\prime },N_i,C_{\alpha i},C_i^{\prime })$ in a Newman projection. The front amide nitrogen carries the preceding carbonyl and H, while the back C${}_{\alpha }$ carries the next carbonyl, H and side chain. Rotation changes the contacts among these groups. A corresponding view along C${}_{\alpha }$–C${}^{\prime }$ displays $\psi$. The peptide C${}^{\prime }$–N bond itself is approximately planar and is not the freely rotating bond in either definition.

glycine replaces the side chain by a second hydrogen. Removing C${}_{\beta }$ removes many contacts that exclude backbone arrangements for other residues, so its allowed $\varphi ,\psi$ region is much broader and includes substantial positive-$\varphi$ space. Its alpha carbon is also achiral, so the steric map is approximately mirror-symmetric between positive and negative torsions in the simple isolated-residue model.

proline closes a five-membered ring from C${}_{\alpha }$ through three methylenes back to nitrogen. In the N–C${}_{\alpha }$ projection, the nitrogen's side-chain arm and C${}_{\alpha }$'s C${}_{\beta }$ arm are tied together, preventing unrestricted relative rotation. Its $\varphi$ is consequently confined near the usual negative-$60^{\circ }$ region rather than exploring the broad range of an acyclic amino acid. proline also lacks a backbone N–H donor when incorporated into an ordinary internal peptide. Ring closure restricts $\varphi$ strongly, but does not freeze every backbone or ring degree of freedom. These differences are naturally represented by distinct Ramachandran plots; the original torsion/steric analysis established the basis for such comparisons.^[ref]

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]