Calculation of pH dependence for small molecules
No quantum chemistry program can directly give the dominant forms of a molecule under different pHs, but only Gibbs free energies of the relevant species, because it is trivial to calculate the pKa and then the pH dependence of the dominant form of your molecule from the Gibbs free energy data by hand.
- calculate Gibbs free energies of the relevant species;
- convert the pKa values to Ka values (equilibrium constants), and the pH value to [H+];
- use the definition of the equilibrium constants to calculate the ratios of the different species;
- converted the ratios to the relative proportions of each species with respect to all species;
- If the one is the largest, it is the dominant form at the given pH;
酸碱pK
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