Last hour: Shortcomings of MO theory: Overestimates ionic character of bonding

1. 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

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

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

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

5.  bond in ethene

6.  system in butadiene

7. from Demtröder “Molecular Physics”

8. sp3 hybrid orbitals in CH4

9. from McQuarrie & Simon “Physical Chemistry”

10. 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;