I am trying to calculate solvation energy for acetic acid and acetate ion.I am taking the difference between the gas phase(opt+freq HF 6-31 G(d)) and solvated phase (opt+freq HF 6-31G(d)/CPCM/water).The values I obtained are-
Ggas(Acid)= -227.770611 Hartree
Gsol(Acid)= -227.779303 Hartree
Ggas(Acetate)= -227.200572 Hartree
Gsol(Acetate)= -227.305984 Hartree
Substracting respective values gives
Gsolvation(Acid)= -5.454312574 Kcal/mol
Gsolvation(Acetate)= -66.147031414 Kcal/mol
While literature gives following values for the same calulations
Gsolvation(Acid)= - 6.93 cal/mol
Gsolvation(Acetate)= -76.58 Kcal/mol
Is something wrong with the calculation?
Any help would be appreciated.
these papers might be usefull for you
(1) J. Phys. Chem. B 2008, 112, 9709–9719 Calculation of Solvation Free Energies of .............Mixed Cluster/Continuum Models
Vyacheslav S. Bryantsev, Mamadou S. Diallo, and William A. Goddard ,
(2)Pure Appl. Chem. , Vol. 76, No. 1, pp. 231–240, 2004. © 2004 IUPAC 231
Calculation of the free energy of solvation from molecular dynamics simulations*
Paulo F. B. Gonçalves and Hubert Stassen
Mr. Parashar,
Can you specify how you can obtain these DGs by Gaussian outputs?
Are you sure you are looking at G (Gibbs energy) and not E (internal energy) ? That value showed by GaussView in the summary window is E (in the ouput file is will be referred as E(SCF) and not G. You will have to open the output file using Notepad and go to Thermochemistry section to find the value of G. Refer to the attached file.
If you are correctly using G, then I would suugest redoing the calculations with 6-31+G(d) or 6-31+G(d,p) basis sets. As you have an anion in the system it might be helpful to use diffused basis sets for your calculations.
Mr. Parashar,
Let us start with the fact that E(SCF) and the term 'G' according to your formalism are different from 0.002932 a.u. This error corresponds to calculation errors for many other methods. Depending on methodology there is unable to distinguish between E(SCF) value from the last optimization cycle and so-called 'sum of electronic and thermal free energies'. Furthermore, in solution, dominates electronic term. The molecular motion one can be neglected. This is confirmed by ZVPE value, which is 0.020658 a.u. Or within the framework of a less accurate methods, like yours, for example, the data can not be compared with those in ref. J. Am. Chem. Soc. 2001, 123, 7314-7319. But the latter ones, you would like to reproduce, in fact (link: https://www.researchgate.net/post/Can_someone_help_me_with_Pka_calculation_of_acetic_acid_in_water_using_Gaussian_09).
Probably you have used (according to the values from your posting) a model of dimeric acid: 2CH3COOH = 2H+ + 2CH3COO-
However, in the real system these dimers are cyclic. There are experimental spectroscopic data in solution; Many crystallographic data in solid-state; And many theoretical data. And according to your formalism G should be "0". Therefore, thermodynamic cycles 1 or 2 from ref. J. Am. Chem. Soc. 2001, 123, 7314-7319 become inapplicable (CH3COOH1.CH3COOH2=CH3COOH2.CH3COOH1, cyclic dimer).
In addition, CH3COOH is a weak acid which leads to CH3COO- ions in concentrated solution. But to concentrated solutions, the thermodynamic cycles 1 and 2 are also inapplicable. Because of you should account for surface forces and interactions. There are inapplicable H2O as model solvent, viscosity and many other parameters, too.
Obviously, the authors have used other formalism to obtain G, because of, there is more than one equation to G and thus there are more than one ways of obtaining G from a Gaussian output-file.
Chemistry is an experimental science. Theoretical chemistry, and in parallel, computational chemistry allow obtaining of one and the same term via different manners, depending on various formalisms.
The data in your (two) postings, however, rather assume a lack of knowledge associated with experimental analytical chemistry (equilibria in solution) and structural analytical chemistry along with thermodynamic phenomenology. You have used only formalism from Gaussian's manual, which in general, is applicable to gas-phase systems and/or systems of strong electrolites of simple (as electronic structure) inorganics.
In other words, you still have not reproduced the data reported to J. Am. Chem. Soc. 2001, 123, 7314-7319, adapting a theoretical model applicable to a weak electrolite in concentrated solution, but using thermodynamic cycles 1 and 2 of strong electrolites in diluted solutions.
Furthermore, there are more then one molecular interacting ensembles of CH3COO-/CH3COOH, yielding to an energy difference 10 kical.mol-1, values between your current theoretical model (including accuracy of your theoretical level) and the experimental data. The difficulty associated with modeling in solution is due to solute-solvent, solute-solute and solvent-solvent interactions which depending on the system could have opposite contributions to energetics.
HF (1mol/L, gas ) -> HF (1mol/L, solution)
The solvation free energy can be calculated with following input file:
----------
#p hf/6-31g(d) scrf=(smd,solvent=water,read,externaliteration,dovacuum)
Structure: HF; solvation free energy calculate at HF/6-31g(d) level
0 1
F 0.00000000 0.00000000 0.09148400
H 0.00000000 0.00000000 -0.82335900
cav
dis
rep
--------
Search DeltaG in the output file， you will find the DeltaG (Solv) line. The number is what you wanted.
...
(a.u.) = -99.350482
(a.u.) = -99.491545
(a.u.) = -99.491545
(a.u.) = -99.350482
(a.u.) = -99.491545
Total free energy in solution:
- with all non electrostatic terms (a.u.) = -99.487890
--------------------------------------------------------------------
(Unpolarized solute)-Solvent (kcal/mol) = -88.52
(Polarized solute)-Solvent (kcal/mol) = -88.52
Solute polarization (kcal/mol) = 0.00
Total electrostatic (kcal/mol) = -88.52
--------------------------------------------------------------------
Cavitation energy (kcal/mol) = 4.46
Dispersion energy (kcal/mol) = -2.52
Repulsion energy (kcal/mol) = 0.35
Total non electrostatic (kcal/mol) = 2.29
DeltaG (solv) (kcal/mol) = -86.22 #this line
--------------------------------------------------------------------
Dipole moment (Debye):
in vacuo : X= 0.0000 Y= 0.0000 Z= 0.0000 Tot= 0.0000
in solution : X= 0.0000 Y= 0.0000 Z= 0.0000 Tot= 0.0000
...
If you need to calculate the DeltaG for idea gas, you need to do some convertion:
DeltaG(1 M, gas) = DeltaG(1 atm, gas) + 1.98
Please see the Going Deeper box in the page 108-109, 《Exploring Chemistry with Electronic Structure Methods》3Ed.
Dear Gao Keng Xiao
I used your input file without CAV option because it ran into error with that.
My result were:
SMD-CDS (non-electrostatic) energy (kcal/mol) = 2.03
Total non electrostatic (kcal/mol) = 2.03
DeltaG (solv) (kcal/mol) = -4.68
which is quite different from yours.
Any suggestions?
Dear Gao Keng Xiao,
I wonder how can you calculate Cavitation energy:? I tried but it gave me errors. Can you show us in details please? Thank you.
The popular solvent model should be used to be IEF-PCM. Please refer to a outstanding book as follow: https://chemistry.fudan.edu.cn/_upload/article/files/81/27/040931c841b49c61ab6c2f7e144a/924bdd80-e6a1-40b5-8546-ab535bac2e92.pdf
- Badges
- Science topic