I am trying to calculate the free energy of formation of Ferrous-phenanthroline 1:1 complex in water using Gaussian 09.
I need to consider solvation of Ferrous ion explicitly and have calculated the free energy of reactant Ferrous ion with 6 water molecules surrounding it in octahedral geometry in water medium (using keyword CPCM).
For the organic ligand, I have done the calculation in solvent medium without explicit consideration of the water molecules.
The problem comes in calculating the free energy of the product. Since two water molecules around ferrous ion is replaced by phenanthroline ligand in the complex, these two should move out of the hydration sphere around ferrous ion. Is it okay to model the system by moving those two water molecules to the second hydration sphere?
Mr/Miss/Mrs,
The explicit treatment is computaionally expensive and frequently fail to correlate good to the experiment. Despite this comment, can you specify in more detail your model, because of there are set of different possibilities in predicting free Gibbs energy by Gaussian.
Dear madam Bojidarka,
Thank you for your kind attention.
I am optimizing the reactant Ferrous ion in low spin configuration with six water molecules around it in octahedral geometry. I am using CPCM keyword to add water solvent implicitly as well.
The reactant Phenanthroline ligand is optimized without explicit consideration of water around it, but using CPCM keyword.
The free energies of both reactants are calculated.
As the product, I am using the Phenanthroline:Ferrous=1:1 complex, with four water molecules occupying the rest of the octahedral sites (two are occupied by the bidentate phenanthroline ligand). This calculation is also done with CPCM.
But calculating these three structures is errorsome as reactant side has six water molecules but product side only has four.
Mr/Miss/Mrs,
Please pay attention to ref. 1. There are shown details, allowing you to solve your problem.
Ref. 1. Explicit Solvation Shell Model and Continuum Solvation Models for Solvation Energy and pKa Determination of Amino Acids, M. Gupta, E. da Silva, H. Svendsen, J. Chem. Theory Comput., 2013, 9 (11), pp 5021–5037.
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