Rotation and energy profile diagrams

An amide bond has double-bond character from conjugation. This tends to prevent rotation, but rotation does happen slowly, and can be measure by NMR.

Depending on the relative sizes of attached R groups, we can expect one form to be more stable than another. This can be shown on an energy profile diagram:

The maximum at 90 degrees is from the lone pair on nitrogen not having the right symmetry to overlap with the pi* of the C=O.

When both substituents on N are the same, we can expect equal energies:

Energy profile diagrams can also be used with alkenes:

The energy at 90 degrees going off the scale makes sense, since cis and trans alkenes are not observed to intercovert.

One way to measure these energies is to use the heat of hydrogenation:

We expect movement to a lower energy state to be spontaneous, ignoring entropy effects. Hence we can often consider the higher energy state to be a reactant, and the lower energy state to be a product.