I'd like to calculate relative energies for some molecules with taking a solvent into account. Reaction takes place in sulfuric acid, which is not included in the list of defined solvents in Gaussian. Could somebody recommend what to do in this case? Maybe someone did calculations in H2SO4 environment?
Dr. Badohglu is correct. When defining a new solvent using Gaussian software, you need to know the values for the following parameters: (i) EPS - the static dielectric constant - you can generally find this number somewhere within the Gaussian manual... or from some other reputable source!, (ii) EPSINF - the optical (or dynamic) dielectric constant. This number is calculated by squaring the refractive index (n) of the solvent, (iii) RSOLV - The molecular radius of the solvent. I calculate this number using the Stearn-Eyring equation, and (iv) DENSITY - The density of the solvent. This number needs to be in units of particles/(Angstroms^3). Just find the density of the solvent in g/(cm^3) and convert it to particles/(Angstroms^3). As a test, For DMF, 0.944 g/(cm^3) = 0.0078 particles/(Angstroms^3). I hope this information helps. Good luck!
Hi Tatyana,
Even in concentrated sulfuric acid the solvent molecules will predominantly be water.
The pH can be taken into account though by considering protonation and/or deprotonation of acidic/basic sites in your molecules. This is something like taking a microscopic view of the reaction where solutes are surrounded only by water solvent molecules, but a macroscopic view of the acid-base chemistry, assuming it is at equilibrium.
Hope this is useful,
Gabriel
Hi Gabriel,
thank you very much for answer! I'll try to make calculations in water.
Hi, Tatyana, I can not know exactly but, I think, You can calculate the molecule and acid together, and can find total energy. Then, The energy of molecule and acid can be calculated as single. Then after, the total energy (acid and molecule together) can subtraction from the energy of acid.
When defining a new solvent, Eps itself is not enough. To define a solvent, Gaussian software uses three or four parameters at the same time. If any of them are missing in your definition, then the software borrows the missing parameters from the default solvent which is water.
Dr. Badohglu is correct. When defining a new solvent using Gaussian software, you need to know the values for the following parameters: (i) EPS - the static dielectric constant - you can generally find this number somewhere within the Gaussian manual... or from some other reputable source!, (ii) EPSINF - the optical (or dynamic) dielectric constant. This number is calculated by squaring the refractive index (n) of the solvent, (iii) RSOLV - The molecular radius of the solvent. I calculate this number using the Stearn-Eyring equation, and (iv) DENSITY - The density of the solvent. This number needs to be in units of particles/(Angstroms^3). Just find the density of the solvent in g/(cm^3) and convert it to particles/(Angstroms^3). As a test, For DMF, 0.944 g/(cm^3) = 0.0078 particles/(Angstroms^3). I hope this information helps. Good luck!
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