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III. Molecular Orbital Theory (Wade chapters 2 and 15)

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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III. Molecular Orbital Theory (Wade chapters 2 and 15)

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  1. III. Molecular Orbital Theory(Wade chapters 2 and 15) Dr. Clower CHEM 4201

  2. 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

  3. 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).

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. 1 MO for 1,3-Butadiene • Lowest energy • All bonding interactions • Electrons are delocalized over four nuclei • Contains first pair of p electrons

  11. 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

  12. 3* MO for 1,3-Butadiene • Antibonding MO • Two nodes • Unoccupied in the ground state

  13. 4* MO for 1,3-Butadiene • Strongly antibonding • Very high energy • Unoccupied in ground state

  14. 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).

  15. 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

  16. 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)

  17. “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)

  18. 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)

  19. [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

  20. [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 http://chm234.asu.edu/reallife/332thymine/thymine.html

  21. IV. Ultraviolet Spectroscopy

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