I found in gaussian that gibbs free energy is sum of the total electronic energy and correction to the gibbs free energy but in some articles its sum of just gibbs free energy in the output of freq.
[reactants]->[products]
For the equation above (AH+H2O -> A- + H3O+) you would have:
Greaction=[G(A-)+G(H3O+)]-[G(AH)+G(H2O)]
And the four Gs can be obtained from the four different opt freq calculations in Gaussian, just like I said before. Also, Hammett equation or not, please take into account that vacuum H3O+ or even H3O+ calculated with PCM solvation is not a realistic description for a hydrated proton.
Good luck in your endeavor.
see the webpage
http://www.gaussian.com/g_whitepap/thermo.htm
Keyword: OPT+Freq
Follow the Gaussian whitepaper on thermo given to you by Mohammad. Open the log file with a text editor and look for the string "Sum of electronic and thermal Free Energies="
The value next to it is your total Gibbs free energy (including electronic energy) in Ha. But take into account that the errors might be high. Plus, how do you want to estimate the pKa? Transferring a proton just to water (AH+H2O -> A- + H3O+) is not a realistic description of what is happening during the actual proton transfer, as you would at least need to use a water cluster.
i know all of them and done it before,but i want to calculate pKa for hammet equation,in the Gaussian whitepaper write total free gibbs energy is the difference between the product and reactant in the reaction. my calculation is in gas phase and i don't know whats my reactant and product,plz help me in this case.
[reactants]->[products]
For the equation above (AH+H2O -> A- + H3O+) you would have:
Greaction=[G(A-)+G(H3O+)]-[G(AH)+G(H2O)]
And the four Gs can be obtained from the four different opt freq calculations in Gaussian, just like I said before. Also, Hammett equation or not, please take into account that vacuum H3O+ or even H3O+ calculated with PCM solvation is not a realistic description for a hydrated proton.
Good luck in your endeavor.
i obtained this gibbs energy you re said and found out the pka,but in some articles G difine as :
G(gass phase) = G(HA+) - G(H+) - G( A)
tnx so much dear Mihai.
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