Last hour: Shortcomings of MO theory: Overestimates ionic character of bonding Underestimates electron correlation Valence Bond Theory: construct molecular function by exchange of electron pairs  more correlation “built in”, but overestimates correlation Better still mixed approaches Polyatomic molecules: Hybridization sp hybrid orbitals (from 2s and 2p): Hybrid orbitals are directional Use Walsh diagrams to determine bond angles, e.g., for triatomic molecules

sp2 hybrid orbital in BH3 from McQuarrie & Simon “Physical Chemistry”

General shape of sp2 hybrid orbitals from McQuarrie & Simon “Physical Chemistry”

 bond in ethene from McQuarrie & Simon “Physical Chemistry”

 bond in ethene

 system in butadiene

from Demtröder “Molecular Physics”

sp3 hybrid orbitals in CH4

from McQuarrie & Simon “Physical Chemistry”

Learning Goals for Chapter 24 – Electronic States of Polyatomic Molecules After this chapter, the related homework problems, and reading the relevant parts of the textbook, you should be able to: explain hybrid orbitals; use Walsh diagrams to determine molecular geometry and electron configurations for small molecules;