W2013CHM2311 Part 4b Notes

W2013CHM2311 Part 4b Notes - CHM2311 Introduc0on to...

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CHM2311’ Introduc0on’to’Structure’&’Bonding’ Part’4’ Molecular’Orbital’Theory’
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Successes’of’Molecular’Orbital’Theory’ 1. MO’theory’can’predict’which’ homonuclear’ diatomics’will’form’and’which’will’not’ ’e.g.,’H 2 ’exists,’He 2 ’does’not’ ’ ’ ’ Note:’Just’because’MO’theory’predicts’that’a’diatomic’species’can’exist’ DOES NOT mean’that’species’is’the’most’stable’form’of’the’molecule.’ ’e.g.,’Graphite,’diamond’and’fullerenes’are’more’stable’forms’of’carbon’ than’C 2 2.’MO’theory’can’accurately’predict’the’ paramagne0sm’of’oxygen’and’other’ compounds’where’Lewis’and’Valence’Bond’theories’do’not.’
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Successes’of’Molecular’Orbital’Theory’ As’the’number’of’electrons’increases,’the’bond’length’shortens’and’the’bonds’become’ stronger.’ This’con0nues’up’to’1±’valence’electrons’where’the’trend’reverses.’Why?’ 3.’Bond’Lengths’in’ Homonuclear’Diatomic’Molecules:’ ’ ’
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Successes’of’Molecular’Orbital’Theory’ 4. Colours of Halogen Molecules: ’ MO’theory,’and’a’knowledge’of’the’energe0c’posi0ons’of’the’MO’levels,’can’help’ to’explain’why’halogens’are’coloured.’ 2s 2s σ g σ u * σ u * σ g π g * π g * π u π u 2p 2p energy MO’Diagrams’for’Halogens’ X 2 ’[ σ g 2 π u 2 π u 2 π* g 2 π* g 2 ]’ ΔE’
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Successes’of’Molecular’Orbital’Theory’ 4. Colours of Halogen Molecules: ’ MO’theory,’and’a’knowledge’of’the’energe0c’posi0ons’of’the’MO’levels,’can’help’ to’explain’why’halogens’are’coloured.’ Since’the’energy’diFerence’between’the’ π g *’(p)’and’ σ u *’(p)’levels’(i.e.,’ the’energy’ diFerence’between’the’HOMO’and’LUMO )’ is’such’that’visible’light’can’be’absorbed,’ these’molecules’coloured.’The’energy’gap’is’lowest’for’I 2 ’then’increases’from’Br 2 ’to’ Cl 2 ’to’± 2 .’As’a’result’the’colours’go’from’dark’purple’for’I 2 ’to’red’brown’for’Br 2 yellow’for’Cl 2 .’±luorine’absorbs’light’in’the’UV’region’and’so’is’colourless. I 2 Br 2 Cl 2 σ u * σ g π g * π g * π u π u ΔE’
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Ioniza0on’occurs’when’maCer’interacts’with’light’of’sufficient’energy’ (Heinrich’Hertz,’1886)’(Einstein,’A.’Ann.’Phys.’Leipzig’19±5,’17,’132-148.)’ E h ν ’=’electron’kine0c’energy’+’electron’binding’energy’ h ν e-’ e-’ e-’ Photoelectron spectroscopy uses this phenomenon to probe the electronic structure of ma4er. It is par7cularly powerful for probing the (molecular orbital) energy levels in molecules Photoelectric’EFect’&’Photoelectron’ Spectroscopy’
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+ - h ν A e - (E kin ) FÖRE EFTER A + h + E A E A
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W2013CHM2311 Part 4b Notes - CHM2311 Introduc0on to...

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