I’d like to clarify the problem that I added to the homework assignment due Wednesday.

I’m sure you all understand what to do: calculate HF/3-21G equilibrium geometries for three conformers of 1, 3-butadiene: planar s-trans, planar s-cis, and skew s-cis.

I suspect, though, that you are not so sure what to do after that. I said you should “rationalize” your results, but what does that mean?

First, I would like you to notice which conformer is most stable, which is least stable, and so on. Second, I would like you to rationalize the energy ordering. Why is the most stable conformer, the most stable? Why is the least stable, the least stable? As you know, I expect you to link energies with structural features and electronic interactions.

Third, all three models are produced by “equilibrium geometry” calculations, but are all three models energy minima? As we saw today, symmetry constraints can make a geometry optimization produce a false minimum. Are any of these models false minima? One way to answer this question is to construct an energy profile for internal rotation and that’s just what I would like you to do. Once you have a profile, and can trace the ups and downs of the energy curve as a function of dihedral angle, you will be able to state unequivocally which models, if any, are minima and which are something else. Which brings me to your last task: explain why certain structures turn out to be false minima, i.e., why the energy profile depicted them as maxima even though the geometry optimization decided they were minima.