Lecture 16 C1403 October 31, 200518.1 Molecular orbital theory: molecularorbitals and diatomic molecules18.2 Valence bond theory: hybridized orbitals andpolyatomic moleculesBond order, bond lengths, connections of MO theoryand VB theory with Lewis structures+ + + ++ ++++______Making of a !z and !z*orbital from overlap oftwo 2pz orbitalsMaking of a "x and "x*orbital from overlap of two2px orbitalsMaking of a "y and "y*orbital from overlap of two2py orbitalsPotential energy curves for the ! and !* orbitals of adiatomic moleculeDistance dependence of the energy of a ! and !* orbitalThe reason for the“switch” in the s and pMOsLarger gap between!2s and !2p withincreasing ZSwitchO2Bond length = 1.21ÅO2+ Bond length = 1.12 ÅO2-Bond length = 1.26 ÅO22-Bond length = 1.49 ÅBond order?Compare the Lewis and MO structures of diatomicmoleculesC2N2O2F2What is the bond order of NO in Lewis terms and MOtheory?Double bonds and triple bonds18.2 Polyatomic moleculesValence bond versus molecular orbital theoryHybridization of atomic orbitals to form molecularorbitalssp, sp2 and sp3 hybridized orbitalsHybridized orbitals and Lewis structures andmolecular geometries18.218.2Bonding in Methane andBonding in Methane andOrbital HybridizationOrbital Hybridizationtetrahedraltetrahedralbond angles = 109.5°bond angles = 109.5°bond distances = 110 pmbond distances = 110 pmbut structure seems inconsistent withbut structure seems inconsistent withelectron configuration of carbonelectron configuration of carbonStructure of MethaneStructure of MethaneElectron configuration of carbonElectron configuration of carbon22ss22pponly two unpaired only two unpaired electronselectronsshould form should form !! bonds to only two bonds to only two hydrogen atomshydrogen atomsbonds should be bonds should be at right angles to at right angles to one anotherone another22ss22ppPromote an electron from the 2Promote an electron from the 2s s to the 2to the 2pp orbital orbital spsp33 Orbital Hybridization Orbital Hybridization22ss22pp22pp22ssspsp33 Orbital Hybridization Orbital Hybridization22pp22ssspsp33 Orbital Hybridization Orbital HybridizationMix together (hybridize) the 2Mix together (hybridize) the 2ssorbital and the three 2orbital and the three 2pp orbitals orbitals22pp22ssspsp33 Orbital Hybridization Orbital Hybridization2 2 spsp334 equivalent half-filled4 equivalent half-filledorbitals are consistentorbitals are consistentwith four bonds andwith four bonds andtetrahedral geometrytetrahedral geometryShapes of orbitalsShapes of orbitalsssppNodal properties of orbitalsNodal properties of orbitalssspp++––++Shape of spShape of sp33 hybrid orbitals hybrid orbitalssspp++––++take the take the ss orbital and place it orbital and place iton top of the on top of the pp orbital orbitalShape of spShape of sp33 hybrid orbitals hybrid orbitalss + ps + p++––++reinforcement of electron wave inreinforcement of electron wave inregions where sign is the sameregions where sign is the samedestructive interference in regionsdestructive interference in regionsof opposite signof opposite signShape of spShape of sp33 hybrid orbitals hybrid orbitalssp sp hybridhybridorbital shown is orbital shown is spsp hybrid hybridanalogous procedure using three analogous procedure using three ss orbitals and orbitals andone one p p orbital gives orbital gives spsp33 hybrid hybridshape of shape of spsp33 hybrid is similar hybrid is similar++––Shape of spShape of sp33 hybrid orbitals hybrid orbitalssp sp hybridhybridhybrid orbital is not symmetricalhybrid orbital is not symmetricalhigher probability of finding an electron on onehigher probability of finding an electron on oneside of the nucleus than the otherside of the nucleus than the otherleads to stronger bondsleads to stronger bonds++––––++––The CThe C——H H !! Bond in Methane Bond in Methanespsp33ssCHHH——C C !!CHgives a gives a !! bond. bond.In-phase overlap of a half-filled 1In-phase overlap of a half-filled 1ss orbital of orbital ofhydrogen with a half-filled hydrogen with a half-filled spsp33 hybrid orbital of hybrid orbital ofcarbon:carbon:++++Justification for Orbital Hybridization Justification for Orbital Hybridization consistent with structure of methaneconsistent with structure of methaneallows for formation of 4 bonds rather than 2allows for formation of 4 bonds rather than 2bonds involving bonds involving spsp3 3 hybrid orbitals are stronger hybrid orbitals are stronger than those involving than those involving ss--s s overlap or overlap or pp--pp overlap overlap18.218.2spsp33 Hybridization Hybridization and Bonding in Ethaneand Bonding in EthaneStructure of EthaneStructure of EthaneCHCH33CHCH33CC22HH66tetrahedral geometry at each carbontetrahedral geometry at each carbonCC——H bond distance = 110 pmH bond distance = 110 pmCC——C bond distance = 153 pmC bond distance = 153 pmIn-phase overlap of half-filled In-phase overlap of half-filled spsp33 hybrid hybridorbital of one carbon with half-filled orbital of one carbon with half-filled spsp33hybrid orbital of another.hybrid orbital of another.Overlap is along Overlap is along internuclear internuclear axis to give a axis to give a !! bond. bond.The CThe C——C C !! Bond in Ethane Bond in EthaneThe CThe C——C C !! Bond in Ethane Bond in EthaneIn-phase overlap of half-filled In-phase overlap of half-filled spsp33 hybrid hybridorbital of one carbon with half-filled orbital of one carbon with half-filled spsp33hybrid orbital of another.hybrid orbital of another.Overlap is along internuclear axis to give a Overlap is along internuclear axis to give a !! bond. bond.18.218.2spsp22 Hybridization Hybridization and Bonding in Ethyleneand Bonding in EthyleneCC22HH44HH22C=CHC=CH22planarplanarbond angles: bond angles: close to 120°close to 120°bond distances: bond distances: CC——H = 110 pmH = 110 pmC=C = 134 pmC=C = 134 pmStructure of EthyleneStructure of Ethylene22ss22ppPromote an electron from the 2Promote an electron from the 2s s to the 2to the 2pp orbital orbital spsp2 2 Orbital HybridizationOrbital Hybridization22ss22pp22pp22ssspsp22 Orbital Hybridization Orbital Hybridization22pp22ssspsp2 2 Orbital HybridizationOrbital HybridizationMix together (hybridize) the 2Mix together (hybridize) the 2ssorbital and two of the three 2orbital and two of the three 2pp orbitals orbitals22pp22ssspsp22 Orbital Hybridization Orbital Hybridization2 2 spsp223 equivalent half-filled3 equivalent