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Activation energy from dynamic NMR

  1. Apr 25, 2013 #1
    1. The problem statement, all variables and given/known data

    I need to calculate the activation energy (barrier of rotation) of an amide bond. I have NMR spectra at various temperatures. I know that once I have the rates I can do an Arrhenius plot to find the activation energy, but I'm having trouble with the ``finding the rates'' part.

    I found some equations and I want to confirm that I am doing the right thing

    2. Relevant equations

    Slow exchange (defined peaks)

    k = pi*(va - vo)

    where va is the full width at half height of the peak that has experienced some widening due to exchange. vo is the initial full width at half height of a peak due to no exchange (essentially the FWHH of a peak at low temperatures).

    intermediate exchange

    k = (pi * (do^2 - da^2))/2^.5

    where do is the difference between the peaks when no exchange occurs (again at low temps). da is the difference between the peaks in spectrum.

    fast exchange

    k = pi * do^2 /(2(va - vo))


    3. The attempt at a solution

    I went through and did the calculation and came up with an Ea of 18 kcal/mol which is reasonable considering the Ea of the very similar peptide bond is about 20 kcal/mol. The problem occurs in the frequency factor which is off the charts (e^32) so this is giving me reason to believe I might not be using the correct equations.

    Any help is appreciated.
     
  2. jcsd
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