Rotation around C2-C3 in n-butane


So far, we have seen the rotation around the carbon-carbon axis of ethane and propane. In this exercise, we will be looking at the rotation around the C2-C3 axis of n-butane. Going from ethane to propane, nothing really unexpected happened; the energy profile looked quite the same, except for the slightly larger rotational barrier in propane.
For butane however, we get a really different energy profile:



Whereas the energy profiles for rotation in ethane and propane were purely sinusoidal (i.e. only one maximum and one minimum energy, both repeated every 120°), the profile for butane contains local minima and maxima. Let's step through them, starting with the easy ones:



Energies are from AM1 calculations and are for qualitative discussion only. Better calculations would give different quantitative results. This page was adapted by Bob Hanson for Internet Explorer 4 and Netscape 7 from the original, written by Hens Borkent