Molecular orbital theory

• how many hybrid sp3 orbitals are there?

  4, formed from 2s and three 2p atomic orbitals
• how many sp2 hybridisation are there?

  2s and two 2p
• what does in phase combination lead to?

  bonding orbitals
• what does out of phase combination lead to?

  antibonding orbitals
• which has more energy antibonding or bonding orbitals?

  Antibonding orbitals
• Atomic orbitals

  • Must have the correct symmetry to combine to molecular orbitals
• Number of molecular orbitals


  Must equal the number of atomic orbitals combined
• Bonding molecular orbitals

  Are lower in energy than the atomic orbitals that are combined
• Antibonding molecular orbitals

  Are higher in energy than the atomic orbitals that are combined
• Filling molecular orbitals

  1. Follows the same rules as filling atomic orbitals

  2. Lowest energy orbitals first

  3. Hund's rule

  4. Pauli principle
• Bond order


  1/2 (number of electrons in bonding molecular orbitals - number of electrons in antibonding molecular orbitals)
• Atomic orbitals

  • For best interaction, the atomic orbitals that are combined should be the same size
• Only atomic orbitals of sufficiently similar energy interact to form molecular orbitals
• what is Heteronuclear diatomic?

  small energy difference, smaller interaction between AOs, covalent bond with some ionic character

  
• what is homonuclear diatomic?

  Perfect energy match, Large interaction between AOs, purely covalent bond

  
• Molecule orbital theory of methane:

  • Hybridisation of C in CH4 is tetrahedral sp3 as 4 other atoms bound to it
  • C-atom contributes 4 valence e- 4 Sp3 each H-atom 1 e- in one 1s orbital -> 8 AOs contribute -> there must be 8 MO 4 sigma and 4 Sigma*
  • MO diagram: AO levels of C-atom on one side and AO levels of 4 H-atoms on other
  • As usual, MOs formed from overlap of AOs go in the middle
• when is a system conjugated?

  Two double bonds within an organic molecule are separated by just one single bond the double bonds are said to be conjugated
• Properties of conjugated double bonds:

  • stability of benzene
  • reactivity of compounds such as 1,3-butadiene and acrolein
  • colour in compounds such as beta-carotene
• where are the pi-electrons in benzene?


  Not localised in alternating double bonds but are spread out (delocalised) and shared over the whole ring system