Applications III: Excited States, Solutions, Surfaces

Lecture CompChem 7Chemistry 347Hope 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

• Esolvation = Esolution - Egas phase

• Ions are effectively solvated by polar solvents due to favorable ion-dipole electrostatic interactions

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