iii molecular orbital theory wade chapters 2 and 15 n.
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III. Molecular Orbital Theory (Wade chapters 2 and 15). Dr. Clower CHEM 4201. Sigma Bonding. Electron density lies between the nuclei A bond may be formed by s — s , p — p , s — p , or hybridized orbital overlaps. Molecular orbitals (MOs)

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sigma bonding
Sigma Bonding
  • Electron density lies between the nuclei
  • A bond may be formed by s—s, p—p, s—p, or hybridized orbital overlaps.
  • Molecular orbitals (MOs)
    • Produced when atomic orbitals on different atoms interact
    • The bonding molecular orbital is lower in energy than the original atomic orbitals.
    • The antibonding MO is higher in energy than the atomic orbitals
s bonding mo
s Bonding MO
  • Formation of a s-bonding MO
  • When the 1s orbitals of two hydrogen atoms overlap in phase with each other, they interact constructively to form a bonding MO.
  • The result is a cylindrically symmetrical bond (s bond).
s antibonding mo
s* Antibonding MO
  • Formation of a s* antibondingMO
  • When two 1s orbitals overlap out of phase, they interact destructively to form an antibonding (*) MO.
  • Result in node separating the two atoms
h 2 s s overlap
H2: s—s Overlap
  • Bonding MOs are lower in energy than the atomic orbitals.
  • Antibonding MOs are higher in energy than the atomic orbitals.
  • In stable molecules, bonding orbitals are usually filled and antibonding orbitals are usually empty.
pi bonding
Pi Bonding
  • p molecular orbitals are the sideways overlap of p orbitals.
  • p orbitals have two lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charges.
  • When lobes overlap constructively (+ and +, or - and -), a bonding MO is formed.
  • When + and - lobes overlap (destructive), waves cancel out and a node forms; this results in an antibonding MO.
p bonding and antibonding
p Bonding and Antibonding

The sideways overlap of two p orbitals leads to a p bonding MO and a p*antibondingMO. Electron density is centered above and below the s bond.

ethylene pi mos
Ethylene Pi MOs
  • The combination of two p orbitals gives two molecular orbitals.
  • Constructive overlap is a bonding MO.
  • Destructive overlap is an antibonding MO.
mos of 1 3 butadiene
MOs of 1,3-butadiene
  • p orbitals on C1 through C4
  • Four MOs (2 bonding, 2 antibonding)
  • Represent by 4 p orbitals in a line
  • Larger and smaller orbitals are used to show which atoms bear more of the electron density in a particular MO
1 mo for 1 3 butadiene
1 MO for 1,3-Butadiene
  • Lowest energy
  • All bonding interactions
  • Electrons are delocalized over four nuclei
  • Contains first pair of p electrons
2 mo for 1 3 butadiene
2 MO for 1,3-Butadiene
  • Two bonding interactions
  • One antibondinginteraction
  • One node
  • A bonding MO
  • Higher energy than1 MO and not as strongly bonding
  • Contains second pair of p electrons
3 mo for 1 3 butadiene
3* MO for 1,3-Butadiene
  • Antibonding MO
  • Two nodes
  • Unoccupied in the ground state
4 mo for 1 3 butadiene
4* MO for 1,3-Butadiene
  • Strongly antibonding
  • Very high energy
  • Unoccupied in ground state
mo for 1 3 butadiene and ethylene
MO for 1,3-Butadiene and Ethylene
  • The bonding MOs of both 1,3-butadiene and ethylene are filled and the antibonding MOs are empty.
  • Butadiene has lower energy than ethylene. (stabilization of the conjugated diene).
pericyclic reactions and mos
Pericyclic Reactions and MOs
  • The Diels–Alder reaction is an example of a pericyclic reaction
    • Concerted forming and breaking of bonds within a closed ring of interacting orbitals
    • Single transition state
    • Activation energy supplied by heat (thermal induction) or UV light (photochemical induction)
  • Woodward and Hoffmann (1965) predicted reaction products using their theory of conservation of orbital symmetry
    • MOs must overlap constructively to stabilize the transition state
    • Drastic changes in symmetry may not occur
symmetry allowed reaction 4 2
Symmetry-Allowed Reaction [4+2]
  • Diene donates electrons from its highest energy occupied molecular orbital (HOMO)
  • Dienophile accepts electrons into its lowest energy unoccupied molecular orbital (LUMO)
  • Butadiene HOMO and ethylene LUMO overlap with symmetry (constructively)
forbidden 2 2 cycloaddition
“Forbidden” [2+2] Cycloaddition
  • [2 + 2] cycloaddition of two ethylenes to form cyclobutene has antibonding overlap of HOMO and LUMO.
  • For reaction to occur, one of the MOs would have to change its symmetry (orbital symmetry is not conserved)
photochemical induction
Photochemical Induction
  • Absorption of correct energy photon will promote an electron to a higher energy level
  • This state is called the excited state
  • The LUMO is now the HOMO* (HOMO of excited molecule)
2 2 cycloaddition
[2 + 2] Cycloaddition
  • HOMO* of excited ethylene and LUMO of ground state ethylene have same symmetry
  • The [2+2] cycloaddition can now occur
  • Photochemically allowed, but thermally forbidden
2 2 cycloadditions and skin cancer
[2+2] Cycloadditions and Skin Cancer
  • Dimerization of thymine in DNA
  • Exposure of DNA to UV light induces the photochemical reaction between adjacent thymine bases
  • Resulting dimer is linked to development of cancerous cells