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

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)

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

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

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 E solvation = E solution - E gas phase Ions are effectively solvated by polar solvents due to favorable ion-dipole electrostatic interactions

Surfaces (MOViewer) Electron Density: = “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