Potential energy surfaces, Model chemistries, Asking the right questions

Potential energy surfaces

Model chemistries

  • Schrödinger equation and stat. mech. provide a mathematical model of chemistry
  • cannot solve equations exactly, except for trivial problems
  • need to make approximations
  • model chemistry - approximate mathematical model
  • models need to be validated
  • types of models: molecular mechanics, semi-empirical, ab initio
  • Asking the right questions

  • molecular modeling can answer some questions easier than others
  • phrase questions in terms of energy differences, energy derivatives, geometries, electron distributions
  • trends easier than absolute numbers
  • gas phase much easier than solution
  • structure and electron distribution easier than energetics
  • vibrational spectra and NMR easier than electronic spectra
  • bond energies, IP, EA, activation energies are hard (PA not quite as hard)
  • excited states much harder than ground states
  • solvation by polarizable continuum models (very hard by dynamics)
  • stability and reactivity are not precise concepts - need to give a specific reaction
  • similar difficulties with other general concepts - resonance, nucleophilicity, leaving group ability, VSEPR, etc.