Ethane rotation at room temperature

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Discussion Overview

The discussion centers on the rotational dynamics of ethane at room temperature, specifically addressing the apparent contradiction between the energy barrier for rotation and the thermal energy available to the molecules. Participants explore the implications of thermal energy on molecular rotation and the factors influencing this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the accessibility of the rotational energy barrier of 2.9 kcal/mol given the thermal energy of 0.593 kcal/mol at room temperature.
  • Another participant clarifies that 0.593 kcal/mol refers to the average energy per degree of freedom, suggesting variability in molecular energy.
  • A different participant notes that while rotation is not entirely free, thermal energy at room temperature allows a significant fraction of molecules to overcome the rotational barrier, leading to rapid interconversion of conformers.
  • One participant expresses confusion about the statement that the rotational barrier is "easily accessible," seeking clarification on the mechanisms that allow for this energy transfer, particularly in the context of molecular collisions.
  • Another participant suggests estimating isomerization velocity using kinetic theory, referencing RRK theory for further exploration.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of the energy dynamics involved in ethane rotation. Multiple viewpoints exist regarding the implications of thermal energy and the mechanisms facilitating rotation.

Contextual Notes

Participants express uncertainty about the definitions and assumptions underlying the energy values discussed, particularly concerning the conditions of ethane gas versus other states. The role of molecular collisions in facilitating energy transfer is also questioned.

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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