W2013CHM2311 Part 5 Notes

W2013CHM2311 Part 5 Notes - CHM2311 Introduc0on to...

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CHM2311’ Introduc0on’to’Structure’&’Bonding’ Part’5’ Molecular’Orbital’Theory’for’ Molecules’with’ π-Systems’
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Organic’Molecules’with’ π-Bonding’ Many’organic’and’inorganic’species’have’ π-electrons.’Many’of’these’are’delocalized’ structures’for’which’resonance’descrip0ons’are’used.’ Our’treatment’of’this’type’of’species’will’begin’by’focusing’on’some’organic’chemistry’ examples.’ Each’carbon’is’sp 2 ’hybridized.’ The’singly’occupied’sp 2 ’orbitals’are’all’used’to’create’sigma’( σ)’bonds.’ The’ σ’framework’is’therefore’made’up’of’the’sp 2 ’orbitals.’ Ethylene (C 2 H 4 ): Valence Bond Descrip;on C C H H H H C C H H H H z H H H H sp 2 hybrid orbital C C C σ framework
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Organic’Molecules’with’ π-Bonding’ Many’organic’and’inorganic’species’have’ π-electrons.’Many’of’these’are’delocalized’ structures’for’which’resonance’descrip0ons’are’used.’ Our’treatment’of’this’type’of’species’will’begin’by’focusing’on’some’organic’chemistry’ examples.’ The’ unhybridized’p-orbitals’on’the’two’carbon’atoms’will’overlap’side-to-side’to’form’ one’ π’MO’and’one’ π*’MO.’ Since’they’are’singly’occupied’we’end’up’with’2’e - ’in’the’ π’MO .’ Ethylene (C 2 H 4 ): C C H H H H C C H H H H
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Organic’Molecules’with’ π-Bonding’ Computa0onally’determined’molecular’orbitals’of’ethylene.’Rela0ve’energies’are’ given’in’ eV.’ ’ -33.15 (a g ) -21.89 (b 2u ) -15.79(b 3u ) -14.30 (a g ) -11.83 (b 1g ) -10.55 (b 1u ) 1.45 (b 3g ) 4.01 (b 3u ) 4.40 (b 2u ) 5.08 (a g ) 5.56 (b 1g ) 5.79 (b 2u )
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Organic’Molecules’with’ π-Bonding’ In’order’to’make’an’MO’diagram,’the’orbitals’need’to’be’ordered’in’terms’of’energy’ and’then’the’electrons’included.’ Ethylene’has:’ 12’valence’AOs’and’therefore’12’MOs’ 12’valence’electrons’ -33.15 (a g ) -21.89 (b 2u ) -15.79 (b 3u ) -14.30 (a g ) -11.83 (b 1g ) -10.55 (b 1u ) 1.45 (b 3g ) 4.01 (b ) 4.40 (b ) 5.08 (a g ) 5.56 (b ) 5.79 (b ) HOMO LUMO π π * Energy Corresponds’to’ previous’slide’ The’ Hückel Molecular Orbital (HMO) method ’is’a’simple’LCAO’MO’method’ for’determining’energies’of’molecular’ orbitals’ of’ π-electrons’ in’ conjugated’ hydrocarbon’systems.’
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HMO’Treatment’of’Ethylene’ C C H H H H E n e r g y C C H H H H C C H H H H C C H H H H p x p x π π∗ Ethylene: a linear 2-carbon π-system When’only’considering’ π-bonding’in’ethylene,’the’situa0on’is’analogous’to’the’MO’ treatment’of’ diatomics’and’construc0on’of’the’MO’diagram’is’quite’simple.’ The’ unhybridized’p-orbitals’on’the’two’carbon’atoms’will’overlap’side’to’side’to’form’ a’ π’MO’and’a’ π*’MO.’ ’ LUMO HOMO
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HMO’Treatment’of’Ethylene’
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