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Ethane rotation at room temperature

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abinit
#1
Mar23-13, 11:42 AM
P: 2
The energy available at room temperature is 0.593 kcal/mol (wikipedia) so why is it that Ethane is said to freely rotate from staggered to eclipse if it has a rotational energy barrier of 2.9 kcal/mol (wikipedia)? What am I missing here?
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Borek
#2
Mar23-13, 12:03 PM
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Quote Quote by abinit View Post
The energy available at room temperature is 0.593 kcal/mol (wikipedia)
Energy available to what? No idea where does this number come from (it can have some sense, I just don't know).
mfb
#3
Mar23-13, 12:06 PM
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0.593 kcal/mol is the average energy per degree of freedom. You always have some molecules with more and some with less energy.

DrDu
#4
Mar23-13, 12:10 PM
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Ethane rotation at room temperature

He means kT at room temperature.
The rotation is certainly not a free rotation. However thermal energy at room temperature is enough to overcome the rotation barrier in a considerable fraction of the molecules so that interconversion of the conformers is very rapid.
abinit
#5
Mar24-13, 10:14 AM
P: 2
Quote Quote by Borek View Post
Energy available to what? No idea where does this number come from (it can have some sense, I just don't know).
Thanks for the comment Borek, my question was not at all clear. My issue was with the commonly found comment "the barrier to rotation about the C-C bond in ethane is approximately 3 kcal/mol. This energy is easily accessible at room temperature." I want to know why this is easily accessible. I assume (although I have not seen this explicitly written in any of the examples I've read) that we are talking about ethane gas at room temperature.

- The driving force for rotation is found through collisions with other ethane molecules? Ethane has 3N degrees of freedom and so a total internal energy of 12kT ~ 7kcal/mol. So it is assumed that many collisions can transfer the required 3kcal/mol? Quick side question, the 3N degrees of freedom has three rotational (overall molecule) but what about internal rotation? Is this factored in?

- How would this picture change in solution? References for this would be greatly appreciated.
DrDu
#6
Mar25-13, 02:26 AM
Sci Advisor
P: 3,593
You could try to estimate the velocity of isomerisation using some kinetic theory, e.g. RRK:

http://www.google.de/url?sa=t&rct=j&...41abbA&cad=rja


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