Negative activation energies

vitalys

Hi,
I studied a chemical reaction (A + B ___> [TS] ___> P), computationally using B3LYP, B3LYP(GD3BJ), and M06-2X, but I didn't understand the results, because I obtained positive activation everyone enegies under B3LYP, and negative energies under B3LYP(GD3BJ) and M06-2X, which made me in confused.
Does the results reliable? Did I miss something? Or my results are wrong?

Any advice is greatly appreciated, thank you very much.

wzkchem5

Negative activation energies are actually possible, for example when:
(1) A and B forms a non-covalent complex exothermically, and the barrier from the non-covalent complex to the TS is smaller than the energy released upon forming the non-covalent complex. In this case, if you calculate the activation energy from the energies of the separate reactants A and B, you get a negative activation energy.
Or
(2) you are not using the same level of theory for the geometry optimization and single point energy calculations. Even if the activation energy is positive at the level of theory used in the geometry optimization, there is clearly no guarantee that it is still positive at the level of theory used in the single point energy calculations, because you changed the level of theory.

vitalys

wzkchem5 发表于 2024-1-22 06:11
Negative activation energies are actually possible, for example when:
(1) A and B forms a non-coval ...

Thank you very much for the valuable informations, It's really helpful.
Considering what you explained, I believe I have situation (1) [non-covalent complex], but I don't know to consider the complex to included in my calculation.
Moreover, is it totally normale to keep the results in case to publish a journal article ?

Just an additional note: I used another computational method (wB97XD), and I get nearly same result (negative activation energies).

Please guide me.

wzkchem5

vitalys 发表于 2024-1-22 11:29
Thank you very much for the valuable informations, It's really helpful.
Considering what you expl ...

You can calculate the non-covalent complex in the same way as you calculate any molecule: just draw the structure of the complex, optimize it, calculate frequencies, and calculate the single point energy. But note that you will probably have to do a configurational search, if you don't already know what the binding configuration is.
With so large negative activation energies, I would recommend you to try to locate the non-covalent reactant complex. Then, if you still observe negative activation energies whose magnitude is within e.g. 1 kcal/mol, you can explain it away by my explanation (2), and this is normally accepted.