Ethane rotation at room temperature

AI Thread Summary
The discussion centers on the apparent contradiction between the energy barrier for rotation in ethane (2.9 kcal/mol) and the energy available at room temperature (0.593 kcal/mol). It clarifies that the 0.593 kcal/mol figure represents average energy per degree of freedom, specifically kT at room temperature, indicating that while individual molecules may not have enough energy to overcome the barrier, a significant fraction can due to thermal fluctuations. The rapid interconversion of ethane conformers is attributed to frequent molecular collisions that can transfer energy sufficient to surpass the rotational barrier. The conversation also touches on the degrees of freedom in ethane, noting that it has multiple modes of motion, including internal rotation. Additionally, the impact of solvent on the rotational dynamics of ethane is questioned, suggesting a need for further exploration into how these dynamics change in different environments.
abinit
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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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abinit said:
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).
 
0.593 kcal/mol is the average energy per degree of freedom. You always have some molecules with more and some with less energy.
 
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.
 
Borek said:
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.
 
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