Mol4D Ethane & Propane
From WikidChem
[edit]Slide 4
This slide introduces us to a jmol application that can show how atoms rotate around a bond in three-dimensions. It is a good alternative to ball and stick models if we do not feel like buying them, but there is actually nothing quite as good as being able to rotate it yourself. That being said, it does have a very useful feature in the energy graphs which show the change in energy as the atom spins. By clicking on different points on the graph one can see how the atom would look at that point. Or, one can repeatedly press the “Play” button to watch it spin. If one does not like the way the 3D figure looks, he can spin it around by clicking on it and dragging to get different points of view.
Slide 5
This should show (it did not on a computer I used in Conneticut hall, because QuickTime and a TIFF decompressor were needed; if others have this problem we may want to email Professor McBride about it or just click on the link to see it) a picture of ethane. The slide points out that the lowest points, when the graph says the energy is 0, are when the two atoms are staggered. The highest points are when it is eclipsed. The change in energy from a minimum to a maximum is the eclipsed barrier because the atom needs that much energy to go from staggered to eclipsed. As useful as this is qualitatively, the program does not seem to work quantitatively. The program says that the eclipsed barrier is about 5.2 kJ/mol, which is 1.24 kcal/mol. We learned during an earlier lecture that the actual eclipsed barrier is 2.9 kcal/mol. Therefore, “let the buyer beware!”
Slide 6
Notice that the graph of the spin of one end carbon atom in propane has an energy graph quite similar to ethane. Staggered is still the minimum energy and eclipsed the max. The barrier this time is 3.3 kcal/mol, which is slightly more than that of ethane, but still not enough to make the spinning unlikely.
(Sorry for explaining Slide 4, but I realized that I didn't have to do it afterwards and it seemed like a bother and a waste to erase it all.--KJA)
From what I remember in class (I can't see the images either), everything sounds good. The slides are off by one, though. The propane slide is actually number 7. -RLM
