Introduction to Computational Quantum Chemistry
Lesson 08: Implicit Solvations: PCM and COSMO, SMD
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 1
Modeling Systems in Solution
so far, all the calculations we’ve one have been doing is in
vacuum
while gas phase predictions are appropriate for many
purposes, there are also a variety of systems and
environments where they fail to reproduce the chemistry
adequately.
a substantial part of interesting chemical processes occur
in solution.
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 2
Solvation Models
Implicit solvation (sometimes termed continuum solvation) represent
solvent as a continuous medium instead of individual Explicit solvent
molecules
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 3
Self-Consistent Reaction Field (SCRF) methods
the solvent is treated as a continuous, uniform dielectric medium
characterized by its dielectric constant ε.
the solute is typically modeled as a single molecule/complex,
corresponding to a very dilute solution.
the solute is placed inside an empty cavity within the solvent dielectric
medium.
the interaction between the solute and the solvent consists primarily of
electrostatic interactions: the mutual polarization of the solute and the
solvent.
the charge distribution of the solute inside the cavity polarizes the
dielectric continuum, which in turn polarizes the solute charge
distribution.
because of the mutual polarization of the solute and solvent, they are
members of the family of self consistent reaction field methods.
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 4
Cavity Shapes
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 5
single sphere. (top)
a surface of constant electron
density, also known as an
isodensity surface. (middle)
the superposition of interlocked
spheres centered on the atoms in
the molecule. The radii of the
individual spheres are close to the
van der Waals values. (bottom)
Polarizable Continuum Model (PCM)
Cavity
overlapping van der Waals spheres (for PCM and CPCM)
solvent accessible surface
isodensity surface (IPCM, SCIPCM)
electrostatic potential from solute and polarization of
solvent must obey Poisson equation
electrostatic interactions calculated numerically
in Gaussian: SCRF=IEFPCM (default)
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 6
Solvation Model Based on Density (SMD)
a universal continuum solvation model where “universal”
denotes its applicability to any charged or uncharged
solute in any solvent or liquid medium
includes corrections for CDS (cavitation, dispersion and
solvent structure effects) while PCM does not
gives more accurate solvation energies compared to PCM
(IEFPCM)
in Gaussian: SCRF=SMD
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 7
Conductor-like Screening Model (COSMO)
COnductor-like Screening MOdel
Cavity
based on solvent accessible surface (SAS)
electrostatic interactions are treated in more approximated
manner
solvent treated as conductor not dielectric (ε = ∞)
good approximation in very polar solvents
NOTE: conductor-like PCM (C-PCM) in which the
continuum is conductor-like similar to COSMO Solvation
Model.
in Gaussian: SCRF=CPCM
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 8
ACTIVITY 1: Properties in Solution
Formaldehyde IR Spectrum in Acetonitrile (using Gaussian)
perform an Optimizization and Freq calculation of formaldehyde
in acetonitrile and vibrational frequencies.
do each for Gas Phase and Solution (acetonitrile) in
SCRF=IEFPCM (default in Gaussian)
# APFD/6-311+G(2d,p) Opt Freq SCRF
# APFD/6-311+G(2d,p) Opt Freq SCRF(Solvent=Acetonitrile)
complete the table:
collect the Gas Phase and Solution Vibration Values, and the
changes ∆ (Shift)
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 9
ACTIVITY 2: Predicting Free Energies in Solution
IEFPCM vs SMD
solubility of Acetic Acid in Chloroform and in Water
perform an Optimization and Freq Calculation on Acetic Acid
under chloroform and water (4 SETS OF CALCULATIONS)
# APFD/6-311+G(2d,p) Opt Freq SCRF(IEFPCM &
SMD,Solvent=Chloroform & Water )
NOTE: the starting structure for acetic acid should have the
dihedral angles H-O-C=O and H-C-C=O equal to 0.0 (in the same
plane)
in order to predict the free energy of solvation in each
environment, optimize the geometry of acetic acid in the gas
phase and in each solvent (with the second optimization starting
from the gas phase-optimized structure).
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 10
ACTIVITY 2: Predicting Free Energies in Solution
(Cont.)
we will compute ∆G of solvation as the difference of the predicted
Gibbs free energy values in the gas phase and in solution, taken
from the two frequency calculations.
complete the entire table:
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 11
ACTIVITY 3: Reaction Free Energy
IEFPCM vs SMD vs COSMO
Hydrolysis of Methyl Acetate
(base catalyzed) reaction where R=R’=CH3: methyl acetate and
hydroxide ion going to acetate ion and methanol
perform an Optimization and Freq Calculation all species of the
reaction in Gase Phase and Water (below)
# APFD/6-311+G(2d,p) Opt Freq SCRF(IEFPCM & SMD &
CPCM,Solvent=Water )
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 12
ACTIVITY 3: Reaction Free Energy, (Cont.)
to predict the Gibbs free energy of the reaction, you need to have free
energies of the four compounds in question.
compare it with the experimental value, complete the table:
NOTE: It is expected that there’s a big descrepancy of the computational
results vs. the experiment, most likely due to effects not included in the
SCRF model. The solute-solvent interactions treated too simply by the
solvation model and the non-static nature of the solvation shell. Thus the
superiority of EXPLICIT SOLVATION over IMPICIT ONE is without
question.
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 13
Extra Activity: (Explicit Solvation)
NOTE: OPTIONAL, THIS IS NOT A PART OF THE LESSON which
deals with IMPLICIT SOLVATION
take the reaction of Activity No.3
explicitly add two water molecules per oxygen in the reactants and
products as shown below:
use SCRF=SMD, Opt=CalcAll (for Gas), Opt=CalcFC (for SMD),
Opt=Tight
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 14
Extra Activity: (Explicit Solvation), Cont.
these calculations are longer
water molecules change drastically during the course of
the optimization thus be careful with your guess structures
(better start with HF optimized structures)
compare the improvement of the results in Activity 3
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 15
END
(Prepared by Radek Marek Research Group)
Lesson 08 - Implicit Solvations: PCM and COSMO, SMD 16