I've calculated the energy barrier for keto enol tautomerization of glyceraldehyde by using a Bronsted Acidic site (SiOH) of Al-Zeolite. DFT based B3LYP theory level with 6-31g(d) basis set was used for my calculations, with implicit water as solvent at 310K. The obtained IRC matches with the corresponding intermediates on either side. I have doubts as to whether deprotonation can have such a huge activation barrier (42 kcal/mol - enthalpy) because in earlier reports the value is much less in the range of 10-20 kcal/mol for deprotonation from SiOH group of zeolites.
I have even tried the same with MP2 calculations and the activation energy was found to be higher. Can anyone suggest why this barrier is too high in this case?
Dear Sinisa
Thank you for your comments.
1. The proposed reaction proceeds further, if the proton transfer from the SiOH group of zeolite takes place
2. I tried with different possible way to modelling the reaction to get the activation barrier. I observed 42kcal/mol.
Even i tried with MP2, the energy barrier found to be higher.
I've used implicit water molecule it doesnot affect much in the reaction.
Since I'm elucidating the reaction mechanism, i will consider the relative energy values for my intermediates and transition state structures.
Chethan,
What is the experimental barrier? That will tell you if you are close to a correct model.
Use the attached excel spreadsheet to calculate the barrier. (NOTE: This is a rough calculation assuming many variables, but should be close enough for most reactions.)
Indeed such a high value of Ea is strange. Are you sure of your calculations?
Could it be that this Ea value is in fact in kJ and nor in kCal.?
Usually activation energy for this kind of reaction is in the range 10-20 kCal (less than 100 kJ).
See for instance: reaction of keto-enol tautomerism of p-hydroxyphenylpyruvic acid Ying Huang et al., J. Sep. Sci. 2009, 32, 4155–4160
they found Ea = 71.9 KJ/mol.
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