Hi all!
I'm computing the energy profile for a Rh catalyzed organic reaction and I got the optimizied structure at BP86/6-31G**/SDDALL in gas phase. I have the whole body of the catalytic cycle and now I was connecting it to the starting material A. The first step is coordination of the catalyst B_Solvent to the substrate A with concomitant lost of the molecule of the Solvent. To do this I compared the sum of the energies (A + B_Solvent) with (A_B + Solvent) expecting an increase in energy. Unfortunately I got the reverse result and the coordination give a -26 kcal/mol. Also, in the case of sums of energies of molecules there is a constant difference between the zero point corrected energies and the free enegies. I tried to include solvent and also to repeat the single point calculations with def2TZVP on all atoms but the problem is not solved. Does anyone knows how to proceed in this case? Any suggestion would be welcome!
There are also a few other items that you may need to consider. First, you indicate that you start with the gas phase optimized structure and then you start discussing solvated species. Your missing energy may simply be the difference between the gas phase optimized structure and the solvated structure. Second, you indicate you "lost of the molecule of the solvent." This implies that you only included one discrete solvent molecule. This is very unlikely the case as there are always multiple solvent molecules occupying multiple solvation shells. Unfortunately, accurate solvent treatment is a common problem that doesn't (in my experience) have a fast or easy fix. I would recommend trying to optimize your initial structure with at least some consideration of the dielectric constant of your solvent in question. PCM is one possibility, but there are certainly others. Also, you may need to consider multiple solvent molecules as the desolvation of the various species involved will need to be considered.
Best of luck!
CBM
It may also be the dispersion correction that can influence the coordination energy. Depending on the structures of A, B, and Solvent, it may have significant contribution as well. For instance, for the formation of pi-complexes (coordination of an alkene to a metal center) this can change the negative value of energy to a positive one. I would try to add a dispersion correction (D3 with Becke-Johnson damping) to the BP86 functional and recalculate SPEs of the reagents and the products. This is of no extra computational cost.
Good luck!
Sergey
I can't really tell if this is relevant from what I have read so far since it is unclear why the decrease in energy is not an acceptable result, but I would suggest keeping in mind that there is a free energy increase in swapping out a solvent molecule that is not accounted for computationally: the concentration of the solvent is very large compared to the substrate, so there is a concentration-gradient-based free energy penalty in having a solvent product. That would only be relevant if you are trying to compare the computations to some type of experimental data, though.
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