Energy Barrier to Rotation in 1-Iodopropane

[jnimagine]

What's an energy barrir to rotation?
I have data on the enrgy at anti and gauche conformations of 1-iodopropane.
I have also graphed the data (steric energy vs dihedral angle)
from the graph, do I just subtract the trough at anti from the trough at gauche?
so from anti to gauche, it'd be a positive number whereas from gauche to anti would be a negative number??
What about for gauche to gauche? since the energies are the same at the two gauche conformations, would it be 0??

 

[Gannon]

I've never had to do graphs, but that sounds right. Anti usually has less steric and torsional strain than anti; therefore rotating from gauche leads to a decrease in energy (negative) while pushing an anti conformation into the gauche position is energetically unfavorable (which requires energy, and change will be positive). From gauche to gauche, let's say the energy barrier is 10kcal/mol. Gauche1 to anti1 is -10kcal/mol, and then anti1 to gauche2 is +10kcal/mol. Therefore the net energy change is zero. Keep in mind, though, that anti to anti, although the net change is zero, still requires the energy needed to reach the first gauche position.