Applications III: Excited States, Solutions, Surfaces

1. Applications III: Excited States, Solutions, Surfaces Lecture CompChem 7Chemistry 347Hope College

2. Excited State Methods • Difficult to compute,as methods tend to find the ground state • ZINDO: semi-empirical • CIS (CI-Singles): fast, robust, qualitative • TD (Time Dependent): fast • CASSCF (Complete Active Space SCF): slow, tricky, more accurate (full CI of a subset of orbitals)

3. CIS • Promote a single electron electron from occupied to unoccupied orbitals • Wavefunctions are built up from these interacting electron configurations • Lowest energy wavefunction is the ground state; higher energy wavefunctions are excited states • Energy differences between states correspond to electronic transitions in the UV-Vis spectrum

4. Solution Methods • Default calculations are gas phase (isolated molecule) • Onsager (Dipole and spherical cavity) • PCM (Polarized Continuum Model) • atomic spheres • isodensity surface • SCF isodensity surface • COSMO (Conductor-like Screening Model) • can compute energy derivatives

5. Solution Calculations • Solvent is characterized by their dielectric constant • Water=78.4, acetonitrile=35.4, cycloheaxane=2.0 • Solute is characterized by a molecular volume • Esolvation = Esolution - Egas phase • Ions are effectively solvated by polar solvents due to favorable ion-dipole electrostatic interactions

6. Surfaces (MOViewer) • Electron Density: 0.003 = “size” of molecule • Electrostatic Potential: map EP onto ED using color code: red=negative « blue=positive • Electrophilic Frontier Density: map HOMO density onto ED • Nucleophilic Frontier Density: map LUMO density onto ED red =small « blue = large