I have made some experimental calculation in acetonitrile water mixture solvent. Now, I want to calculate optimization, frequency, NMR, UV , NBO charge and excited state properties theoretically using Gaussian 09W in the same solvent using solvent effect. The di-electric constant value of the solvent mixture is 43.2. How can I perform ? Is it really possible?
Dear Suvendu,
First you have to run the calculations in the gas phase. Then you run the calculations withsolvent. The calculations with the incorporation of a solvent are performed using the integral equation formalism model of the Polarizable Continuum Model (PCM). In this model the cavity is created via a series of overlapping spheres. The radii type employed is the United Atom Topological Model on radii optimized for the PBE0/6-31G (d) level of theory.
You should use the dielectric constant of your mixture of solvents (43.2).
1. Cancès M.T., Mennucci B., Tomasi J. (1997) A new integral equation formalism for the polarizable continuum model: theoretical background and applications to isotropic and anisotropic dielectrics. J Chem Phys;107:3032-3041.
2. Mennucci B., Tomasi J. (1997) Coninuum solvation models: A new approach to the problem of solute’s charge distribution and cavity boundaries. J Chem Phys; 106:5151.
3. Mennucci B., Cancès M.T., Tomasi J. (1997) Evaluation of solvent effects in isotropic and anisotropic dielectrics and in ionic solutions with a unified integral equation method: Theoretical bases, computational implementation, and numerical applications. J Phys Chem B;101:10506-10517.
4. Tomasi J., Mennucci B., Cancès M.T. (1997) The IEF version of the PCM solvation method: an overview of a new method addressed to study molecular solutes at the QM ab initio level. J Mol Struct. (Theochem);464:211-226.
Hoping this will be helpful,
Rafik
If I've understood correctly, you have a solute in a mixture of H2O and CH3CN and you want to calculate the spectroscopic and electronic properties of this system with PCM.
However, between H2O and CH3CN molecules H-bonds actually forms, and these bonds cannot be modeled with PCM.
H-bonds are strong have a slight but not-negligible covalent character, so PCM isn't suited to reproduce them.
If you want to model the intercation between solute and solvent engaged in H-bonds, then you have to explicitly put in your system at least some solvent molecules.
By averaging of dielectric constant and using the Onsager method is possible to calculate the effect of solvent at this level of calculation
Dear Francesco Muniz-Miranda,
Generally I do DFT calculations in the Gas phase, with 2-3 water molecules and using PCM model. Among the 100 studies that I conducted I did NOT find a significant difference in energies between that with 2-3 water molecules compared to that in the PCM model.
For examples, you can see papers (2009-2014) contained in my ResearchGate profile.
In addition, please note that using 2-3 molecules of H2O and 2-3 of acetonitrile along with the molecule to be studied will demand a long computation time if a good method with adequate basis set is to be used.
Hoping this will clarify my first answer,
Rafik
Thanks to all of you for your kind response. Mr. Tayarri, I am planning this kind of calculation using Onsager model. But, How do I use the dielectric constant? What is the connection between dipole moment and dielectric constant?
Dear Sayyed Faramarz Tayyari
Can you please send me an input file of mixed solvents on an example molecule.
In Onsager Method first obtain the radius of molecue by using the volume option. The by using SCRF=dipole and adding the Radius and dielectric constant (epsilon) of solvent at after the geometry section the calculation will be done!!
- Badges
- Science topic