Ethane rotation at room temperature

  1. 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?
  2. jcsd
  3. Borek

    Staff: Mentor

    Energy available to what? No idea where does this number come from (it can have some sense, I just don't know).
  4. mfb

    Staff: Mentor

    0.593 kcal/mol is the average energy per degree of freedom. You always have some molecules with more and some with less energy.
  5. DrDu

    DrDu 4,157
    Science Advisor

    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.
  6. 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.
  7. DrDu

    DrDu 4,157
    Science Advisor

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