Hi everybody,

I would like to calculate the theoretical pKa values of a set of simple organic molecules using Gaussian. I have located some information online, including a tutorial by McKee & Pogorelov for the pKa calculation of phenol in water (see file attached).

Unfortunately, this tutorial appears to have some inconsistencies:

1.) It appears that the information for the gasphase values of phenol have been left out on page 4. Furthermore, the text states that one shall use the "Sum of Electronic and Thermal Energies". This is done for all available entries except for the solvated phenoxide anion where the information on page 4 lists "-306.924685" while the Excel spreadsheet on page 5 shows an entry of "-306.929919". Not sure if this a typo or if there was a conversion performed

2.) Initially, things looked good:

• I was able to reproduce the gasphase value of 0.289765 Hartree (equal to 181.8301454 kcal/mol) by subtracting the sum of the gasphase values for phenol and water on the one hand from the sum of the gasphase values for the hydronium ion and the phenolate anion on the other hand.
• I was also able to obtain the Delta Gs values for phenoxide and phenol by subtracting the respective gasphase value from the solvated value.

But now things come off the rails - how to proceed from here onwards? 

The problem appears to be a list of inconsistencies with regards to how values are referred to (e.g. is Delta G(aq) meant to be the same as Delta G(sol) etc.?) and, generally, which values to enter where now.

Could somebody show how this example in the tutorial attached was worked, step by step, with values given in the example inserted at each turn, please?

Thank you in advance