Introduction to Computational Quantum Chemistry Lesson 7: Solvation models Martin Novák (NCBR) Solvation models November 1, 2016 1/19 Imlicit vs. Explicit solvation • Implicit solvation • Dielectric continuum • No water molecules perse © Wavefunction of solute affected by dielectric constant of solvent 9 At 20 °C: Water - e = 78.4; benzene: e = 2.3 ... • Explicit solvation © Solvent molecules included (i.e. with electronic & nuclear structure) • Used mainly in MM approaches • Microsolvation: only few solvent molecules placed around solute o Charge transfer with solvent can occur Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 2/19 mplicit Models Martin Novák (NCBR) Solvation models November 1, 2016 3/19 Basic assumptions • Solute characterized by QM wavefunction o Born-Oppenheimer approximation • Only interactions of electrostatic origin o Isotropic solvent at equilibrium • Static model Martin Novák (NCBR) Solvation models November 1, 2016 4/19 • Solute is placed in a void of surrounding solvent called "cavity" • Size of the cavity: • Computed using vdW radii of atoms (from UFF, for example) • Taken from the electronic isodensity level (typically 0.001 a.u.) • The walls of cavity determine the interaction interface (Solvent Excluded Surface, SES) • Size of the solvent molecule determines the Solvent Accessible Surface (SAS) Solvent □ = ► Solvation models November 1, 2016 5/19 Visualizing cavity o Geomview software (in the modules). • Works only with Gaussian 03 o SCRF=(read) in the route section of the job • "geomview" in the SCRF specification • Visualize the "tesserae.off" file Martin Novák (NCBR) Solvation models November 1, 2016 6/19 Electrostatic Interactions • Self-consistent solution of solute-solvent mutual polarizations • Solute induces polarization at the interface of cavity • This polarization acts back on the solute changing its wavefunction a Various solvation models use different schemes for evaluation of solvation effects • Problems arise when electrostatics do not dominate solvent-solute interactions Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 7/19 Polarizable Continuum Model (PCM) Treats the solvent as polarizable dielectric continuum Implemented in Gaussian, GAMESS Martin Novák (NCBR) Solvation models November 1, 2016 8/19 Solvation Model "Density" (SMD) • Full solute density is used instead of partial charges • Implicitly treats dispersion • Lower unsigned errors against experimental data than other models Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 9/19 COnductor-like Screening MOdel (COSMO) • Solute in virtual conductor environment • Charge q on molecular surface is lower by a factor /(e): Q = f(e)q* 0) • where /(e) = (e - l)/(e + x)\ x being usually set to 0.5 or 0 a Implemented in Turbomole, ADF Martin Novák (NCBR) Solvation models November 1, 2016 10/19 Beyond basic models Anisotropic liquids Nonequilibrium solution (Vertical excitations, TDDFT) Concentrated solutions Martin Novák (NCBR) Solvation models November 1, 2016 11/19 Explicit Models Martin Novák (NCBR) Solvation models November 1, 2016 12/19 Two models • Microsolvation • Few solvent molecules (1 to 3) put at chemically reasonable place • Water close to exchangeable protons (OH, NH2...) • Macrosolvation • First (sometimes second) solvent layer around the whole molecule • Usually snapshots from MD Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 13/19 Pros & Cons • +++ Modelling of real interactions with solvent (this can be crucial for exchangeable protons in protic solvents) • - Microsolvation lacks sampling • - Computationally more demanding • - For macrosolvation only single point calculations - the geometry is as good as forcefield Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 14/19 Practical task Martin Novák (NCBR) Solvation models November 1, 2016 15/19 Reaction • cd /scratch/USERNAME* • Model the CI" + CH3Br -> CH3CI + Br" • Find the energy barrier for the reaction • Select any solvent from Gaussian library (be not concerned about solubility of species or chemical relevance) • Assume Sni and Sn2 reaction pathways • Use "SCRF=(solvent=XY)" in the route section of the calculation * If you are using INFINITY, you can ignore this step Martin Novák (NCBR) Solvation models □ SP - = 5 Q, o November 1, 2016 16/19 Procedure • Use B3LYP 6-31+g(d,p) method • Usage of difuse functions when dealing with anions is crucial! • Use ultrafine integration grid • Use Frequency calculations to be sure where on PES you are • For the scan use the distance between C and CI as RC o Negative value of step defines two atoms approaching Martin Novák (NCBR) Solvation models □ SP - = 5 ^0 Q, o November 1, 2016 17/19 Module "qmutil • Extraction of values from gaussian runs: • extract-gopt-ene logfile • extract-gopt-xyz logfile • extract-gdrv-ene logfile • extract-gdrv-xyz logfile o extract-xyz-str xyzfile framenumber o extract-xyz-numstr xyzfile • Values ready for plotting in your favorite software □ Solvation models November 1, 2016 18/19 furbomole o Prepare job using define module (see presentation 6 for help) • Setup COSMO using cosmoprep module • Set epsilon to 78.4 and rsolv to 1.93 • Leave all other values at their default • Define radii of atoms using "r all o" for optimized values • Optimize all geometries Martin Novák (NCBR) Solvation models □ SP - = 5 ^0 Q, o November 1, 2016 19/19