W2013CHM2311 Part 4c Notes

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

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CHM2311’ Introduc0on’to’Structure’&’Bonding’ Part’4’ Molecular’Orbital’Theory’
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MO’Theory’for’ Triatomic Molecules’ Deriving’ molecular’ orbitals’ for’ diatomic’ molecules’ (AB)’ was’ a’ simple’ maJer’ of’ combining’symmetry’and’energy’allowed’atomic’orbitals.’ This’ task’ is’ simpliFed’ by’ the’ fact’ that’ the’ geometry’ of’ diatomic’ molecules’ is’ predetermined.’ i.e.,’linear’ ±or’polyatomic’molecules,’deriving’molecular’orbitals’becomes’more’complicated.’In’ these’cases,’we’are’faced’with’problems’such’as:’ a) What’are’meaningful’and’allowed’LCAOs’that’lead’to’the’M Os?’ b) How’do’the’shapes’and’energies’of’these’M Os’relate’to’the’structure’and’ proper0es’of’the’molecule?’ MO’diagrams’for’triatomic’molecules’(AB 2 )’may’be’derived’by’employing’the’same’ method’previously’established’for’ diatomics.’
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The’Concept’of’Group’Orbitals’ SALC: Symmetry Adapted Linear Combina3ons of Atomic Orbitals Determine’how’the’sets’of’group’orbitals’(GOs)’interact’with’the’AO’of’appropriate’ symmetry’ on’ the’ central’ atom’ –’ this’ is’ where’ CHARACTER’ TABLES’ are’ useful’ for’ developing’ MOs.’ Instead’of’mixing’all’atomic’orbitals’at’once’to’make’molecular’orbitals’of’a’large’ molecule,’smaller’fragments’are’built’and’those’are’combined’together’in-phase’and’ out-of-phase’to’form’the’molecular’orbitals.’ In’molecules’of’the’type’ EX n ’(n’=’2,’…,’9)’consider’the’orbitals’on’the’ central’atom’E’ and’the’set’of’ terminal’ X n ’atoms’ separately.’ s,’ p x ,’ p y ,’ p z ,’ d xy ,’ d xz ,’ d yz ,’d x 2 -y 2 ,’d z 2 groups’of’ symmetry equivalent orbitals’ e.g.,’n’×’ p x ’and’linear’combina0ons’thereof’such’as’ p x (a)’+’ p x (b)’ and’ p x (a)’-’ p x (b)’ E’ X’
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MO’Theory’for’ Triatomic Molecules’ Using’BeH 2 ,’a’very’simple’triatomic,’as’an’example,’Lewis/VSEPR’theory’describes’this’ species’as’a’linear’molecule.’ In’regards’to’MO’theory,’since’hydrogen’only’has’1s’orbitals,’they’can’only’interact’with’ the’Be’(2s)’and’Be’(2p z )’since’they’have’the’correct’symmetry’to’give’in-phase’and’out-of- phase’overlap.’ But…’this’is’star0ng’to’get’more’complex.’ Be H H Be H a H b Can’we’use’the’ideas’of’Group’Theory’to’help’us’out?
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MO’Theory’for’ Linear Triatomics &’ Group’Theory The’problem’can’be’simpliFed’by’breaking’it’into’two’parts.’
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W2013CHM2311 Part 4c Notes - CHM2311 Introduc0on to...

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