I am currently making reaction profile by means of DFT calculations. I am able to correctly draw a whole reaction profile. However, I am struggling to find what exactly is the energy barrier of a reaction. It is pretty difficult to find this kind of information on Internet or in literature because most described systems are often one-step simple cases. When multi-step reactions are described, I noticed researchers seem to follow different rules ?

To give an example, here is a reaction pathway with the energies attributed to the intermediates and TSs. The pre/post-reactive complexes were determined by IRC followed by optimization.

enter image description here

We see that the separate reactants are defined as the reference (0 kJ/mol) for the reaction pathway. The pre-reactive complex has an energy of 10 kJ/mol and follows a TS of 30 kJ/mol. We then arrive to an intermediate of 20 kJ/mol. The same intermediate in a different configuration following the pre-reaction has an energy of 25 kJ/mol. Finally, the last TS has an energy of 40 kJ/mol. The post-reactive complex of this second step has an energy of -5 kJ/mol. The products have an energy of -2 kJ/mol. This is clear.

Now here is where I am questioning:

  1. What is the energy barrier of the forward reaction ? Is it 20 kJ/mol corresponding to the energy barrier of the first elemental step ? Is it 30 kJ/mol it is the energy of the first elemental step vs reactants ? Is it 40 kJ/mol as this energy is the highest of the pathway ? This is kind if misleading as the second step has a smaller elemental energy barrier of 15 kJ/mol.

  2. What is the energy barrier of the reverse reaction ? Is it 40.0 + 5.0 = 45.0 kJ/mol or 42.0 kJ/mol (compared to the products ?

Overall, I would say I am pretty lost if I have to select the separate reactants / products as references for calculating barriers and how to select the rate-determining step when the second TS has a larger value of absolute energy but is a small elemental step.

Researchers in the literature have tendency not to report energy barriers when they compare pathways, or just mention these of elemental steps. Others take account of the energy barrier of an elemental step (example here: 20 kJ/mol) to define the global energy barrier but others take the highest value (40 kJ/mol) here.

If researchers do different things, isn't there a rule ? Is it misleading for most people ? So many people doing DFT nowadays to support mechanisms without having ground rules or proper tutorials for organic chemists is clearly missing to my view.

I already thank you for your explanations.

  • $\begingroup$ Not sure exactly how one would find the rate-determining step with that data. For example, the single biggest barrier is from pre-reactive complex 1 to TS1, 20 kj,/mol, but that does not describe how quickly that reaction occurs. It would be a rate limiting step, in that until 20 kj/mol is available the reaction cannot proceed. $\endgroup$ Mar 8 at 18:32


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.