Applications III: Excited States, Solutions, Surfaces

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Applications III Excited States, Solutions,
Surfaces

• Lecture CompChem 7Chemistry 347Hope College

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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)

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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

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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

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Solution Calculations

• Solvent is characterized by their dielectric
  constant
• Water78.4, acetonitrile35.4, cycloheaxane2.0
• 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

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Surfaces (MOViewer)

• Electron Density 0.003 size of molecule
• Electrostatic Potential map EP onto ED using
  color code
• rednegative bluepositive
• Electrophilic Frontier Density map HOMO density
  onto ED
• Nucleophilic Frontier Density map LUMO density
  onto ED
• red small blue large