Activation energy from dynamic NMR

[Chickenpoxpie]

Homework Statement

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

Homework 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))

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.

 

[nate808]

Thank you for your post. It seems like you have the right idea in terms of using NMR spectra to calculate the activation energy of an amide bond. However, I would like to offer some suggestions to help you confirm your calculations.

Firstly, make sure that you are using the correct units for your calculations. The activation energy is typically measured in joules/mol or kilocalories/mol. Make sure that all your other values, such as the frequency factor and rate constant, are also in the correct units.

Secondly, it's always a good idea to double check your calculations and equations to ensure that you are using the correct ones. You may also want to consult with a colleague or supervisor to confirm your approach.

Lastly, it's important to keep in mind that the equations you are using may have certain assumptions or limitations, so make sure that they are applicable to your specific case. It may also be helpful to compare your results with previous studies on amide bond activation energy to see if they are in the same range.

I hope this helps and good luck with your calculations! Don't hesitate to reach out if you have any further questions.