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Homework Help: Full Excitation of Molecular Vibration Modes

  1. Sep 3, 2012 #1
    1. The problem statement, all variables and given/known data
    Given the relationship Cp/R as a function of Temperature, approximate the number of fully excited vibration modes at a given temperature for a given molecule.

    2. The attempt at a solution
    I have looked through several text books and many web sites, but I can't find this type of problem discussed anywhere. vaguely similar problems I have found only assume given excitation states. It seems like a relatively obscure problem, but I assume there is a generic solution process.
  2. jcsd
  3. Sep 4, 2012 #2


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    Gold Member

    I assume you are talking about degree of freedom, the total ways in which the molecule can exhibit motion, seeing you mentioned Cp/R.

    Anyway, can you give a sample molecule on which you would like to attempt this question on?
  4. Sep 13, 2012 #3
    Let's think specifically about Cp/R

    What is the value of Cp/R for a monatomic gas? is it the same for all monatomic gases?

    Now what is it for a linear rotor? Is it the same for N2, O2, CO2?

    What about I2 -- that value is a little bit different -- why?

    What is the value of Cp/R for water? Methane? Could you tell whether SO2 was linear or bent by looking at Cp/R?

    How and why does Cp/R differ for a really floppy molecule like 1-propanol and a really rigid molecule like benzene?

    And if you can work your way through all of that you should have a really good idea about the relationship between Cp/R and "fully excited" vibrational modes.
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