Lecture 3 - MO Benzene Butadiene, C 4 H 6 Alternating...

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Theories of Chemical Bonding –  Lecture 3
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Review Molecular Orbitals MOs transcend the entire molecule sigma and pi MOs Bond order
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MOs can explain observed physical  properties e.g. bond energies, bond lengths, magnetic properties
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π 2p σ 2p π 2p σ * 2p σ * 2s σ 2s antibonding bonding
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π 2p σ 2p π 2p σ * 2p σ * 2s σ 2s N 2 , BO = 3
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π 2p σ 2p π 2p σ * 2p σ * 2s σ 2s N 2 + , BO = 2.5
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N 2 N 2 + bond order 3 2.5 bond energy (kJ/mol) 945 841 bond length (nm) 0.110 0.112
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π 2p π 2p σ * 2p σ * 2s σ 2s σ 2p O 2 , BO = 2
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π 2p σ * 2p σ * 2s σ 2s σ 2p π 2p O 2 + , BO = 2.5
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O 2 O 2 + bond order 2 2.5 bond energy (kJ/mol) 498 623 bond length (nm) .121 .112
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Molecule BO Bond Bond Energy Length (kJ/mol) (nm) F 2 1 154 .144 B 2 1 290 .16 O 2 2 495 .121 C 2 2 620 .131 N 2 3 942 .110
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Remember NO? N = O or N = O : : ?? : : .. .. . .
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2p 2p 2s 2s σ * 2s σ 2s π 2p π 2p σ 2p σ * 2p N O NO
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Magnetism paramagnetic - unpaired electrons are attracted to the magnetic field diamagnetic - paired electrons are repelled by the magnetic field
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π 2p σ 2p π 2p σ * 2p σ * 2s σ 2s O 2 , BO = 2 Unpaired! (Hund’s Rule)
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Benzene, C 6 H 6 C C C C C C H H H H H H
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Benzene - resonant forms = = Kekulé structures
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Unformatted text preview: MO Benzene Butadiene, C 4 H 6 Alternating single and double bonds “conjugation” σ-bonds π-bonds Carbon Dioxide Filled Empty Filled Highest Occupied MO (HOMO) Lowest Unoccupied MO (LUMO) Conjugation may lead to colour π π Unconjugated System Large energy gap HOMO LUMO π π Conjugated System Smaller energy gap HOMO LUMO h υ h υ UV absorption (VIS absorption, coloured!) β-carotene chlorophyll xanthin Band Theory of Solids Band Theory of Solids HOMO LUMO Metals conduct electricity… LUMO HOMO Negative potential raises the energy of the LUMO LUMO HOMO HOMO Lead Diamond Summary ● MO’s are delocalized ● no need for resonance forms ● can predict bond order ● correlations with bond length, bond energy, magnetic and optical properties...
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This note was uploaded on 11/08/2011 for the course CHEM 1001 taught by Professor Burk during the Fall '09 term at Carleton CA.

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Lecture 3 - MO Benzene Butadiene, C 4 H 6 Alternating...

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