《结构化学 Structural Chemistry》课程教学资源（课件讲稿）Chapter 5 Structures of Polyatomic Molecules（Part I）

Chapter 5 Structures of Polyatomic Molecules ()IntroductionMolecular Orbital Theory vs.Valence Bond TheoryVB theory: focusing on the (localized) bonds formedbetweenvalence electrons/atomicorbitalsof neighboringatom(s),easiertovisualize/imagineVB modelofamolecule, i.e.,“of chemical intuition!".e.g., CH4Mo theory: more powerful and more sophisticated thanVB theory in many aspects, e.g., description of electrondelocalizationin Benzene,but sometimes not so easytovisualize/imagineaMOmodele.g.,for CH4!

Chapter 5 Structures of Polyatomic Molecules (I) Molecular Orbital Theory vs. Valence Bond Theory • VB theory: focusing on the (localized) bonds formed between valence electrons/atomic orbitals of neighboring atom(s), easier to visualize/imagine VB model of a molecule, i.e., “of chemical intuition!”. e.g., CH4 • MO theory: more powerful and more sophisticated than VB theory in many aspects, e.g., description of electron delocalization in Benzene, but sometimes not so easy to visualize/imagine a MO model e.g., for CH4 ! Introduction

Comparison of MO and VB theoriesVBTheoryMolecular orbital theory. The electrons pair to localize in a. MOs are formed by the overlap ofbond. ybond ~AO,AO2AOs. yMO ~ Zc,AO;Electrons are delocalized"within·Demands hybridization of AOMOs consisting of AOs·Basis of Lewis structures,Electrons fill up the MOsresonance,and hybridizationaccording to the aufbau principle· Good theory for predictingGive accuratebond dissociationmolecular structureenergies,IP, EA,and spectral data: Sometimes not so easy to·Easierto visualize/imagine VBvisualize/imagine a MO modelmodel for a molecule, i.e., “ofchemicalintuition!"foramolecule!

Comparison of MO and VB theories VB Theory Molecular orbital theory • Demands hybridization of AO • MOs are formed by the overlap of AOs. MO  ciAOi • The electrons pair to localize in a bond. bond  AO1 AO2 • Good theory for predicting molecular structure. • Basis of Lewis structures, resonance, and hybridization. • Easier to visualize/imagine VB model for a molecule, i.e., “of chemical intuition!”. • Electrons fill up the MOs according to the aufbau principle. • Electrons are “delocalized” within MOs consisting of AOs. • Sometimes not so easy to visualize/imagine a MO model for a molecule! • Give accurate bond dissociation energies, IP, EA, and spectral data

Electron Delocalizationin Benzene三个节面节有C节面VB description:have to introduce resonance of无节面localizedVB structuresMO description:inherently describingelectron delocalization!

Electron Delocalization in Benzene MO description： inherently describing electron delocalization! VB description: have to introduce resonance of localized VB structures

Molecular Orbital (MO)Theory Treatmentof Polyatomic Molecules LCAO-MO & group Theory (in part B of Chapter 3)Mean-field ApprInitial guessBasis set:& LCAO-MO(SALC)yCMO = Zc;ΦAOs ()SCF-HF[8, ,=Zc,Canonical molecularorbitals of CH(valence electrons only!)CAOsH ls AOsCMOW(A))±= ca(Φ1 + Φ2 + Φ3 + Φ4 )/2 ± C,C2syV(T2)x±= c(Φ1 - Φ2 - Φ + Φ4 )/2 ± caC2pxV(T2)± = c(Φ1 - Φ2 + Φ3 - Φ4 )/2 ± cCaC2pydV(T2)± = cc(Φ1 + Φ2 -Φ3 -Φ4 )/2 ± CaC2pAlldelocalized!

Molecular Orbital (MO) Theory Treatment of Polyatomic Molecules • LCAO-MO & group Theory (in part B of Chapter 3) Basis set: AOs {i } Mean-field Appr. & LCAO-MO(SALC) Initial guess CMO = cii SCF-HF {j , j = ci (j)i } Canonical molecular orbitals of CH4 (valence electrons only!) z x y a b c d All delocalized! (A1 ) = ca (1 + 2 + 3 + 4 )/2  cbC2s (T2 )x = cc (1 – 2 – 3 + 4 )/2  cdC2px (T2 )y = cc (1 – 2 + 3 – 4 )/2  cdC2py (T2 )z = cc (1 + 2 – 3 – 4 )/2  cdC2pz CMO H 1s AOs C AOs

How to get the CMOs of a molecule?CH, TaGroup theory treatment- SALCs:2s(A),(T2)2px,2py,2pz4H:1sΦn2Z2ETa8C,3C,65.60dpn111A.11-AETT-111V22010301-1(R..R..R)X30111(x,y. z)3Φh3０2I(4H) 41 0h4(4H)= A +TPsALc=Plg=Zx(R)ROZg(R)x(R)x(R)=R0=(Φ+Φ2+Φ3+Φ4)/2 ~As± = ca(Φ,+Φ2+Φ3+Φ4 )/2 ± c,C2sb= (Φ1-Φ2-Φ3+Φ4)/2 ~ x-like Tx± = cc(Φ1-Φ2-Φ3+Φ4 )/2 ± caC2px0。= (Φ1-Φ2+Φ3-Φ4)/2 ~ y-like T2V± = cc(Φ1-Φ2+Φ3-Φ4 )/2 ± caC2p)a= (Φi+Φ2-Φ3-Φ4)/2 ~ z-like T2± = c(Φ+Φ2-Φ3-Φ4 )/2 ± caC2p

How to get the CMOs of a molecule? • Group theory treatment– SALCs: h1 h2 h4 h3 z x y 1 2 3 4 C: 2s (A1 ), (T2 ) 2px , 2py , 2pz CH4 , Td 4 H: 1s (4H1s) 4 1 0 0 2 1 a g( R) ( R) ( R ) h R i    i  4H  A1  T2 ( ) i R j j i j j SALC R R h l P    ˆ ( )   ˆ   s = ca (1+2+3+4 )/2  cbC2s x = cc (1–2–3+4 )/2  cdC2px y = cc (1–2+3–4 )/2  cdC2py z = cc (1+2–3–4 )/2  cdC2pz a= (1+2+3+4 )/2 ~A1 b= (1–2–3+4 )/2 ~ x-like T2 c= (1–2+3–4 )/2 ~ y-like T2 d= (1+2–3–4 )/2 ~ z-like T2

PESSpectrumofMethaneC4H2t213a1LUMO2p1sHOMO2s1t2N/2a12a122.42.21.81.61.41.2lonizationEnergy(eV)MOTheory explainsthe PESquitewell!

C 4H 2p 2s 1s 2a1 3a1 1t2 2t2 1t2 2a1 MO Theory explains the PES quite well!

2t2C4H3a12p1sH2py2px2P1t22s1t2MO model2a12s / 2a1VB modelTheMOmodelofCH.doesnotsigma bondsLexplicitlyreflect the4equivalent Cbetweensand sp3orbitalsHo-bonds of"chemical intuitionastheVBmodeldoes!UVB=HO,(1)·Φ(2)+HO(2)·Φ(1)CHHowcantheMOmodel becomechemicallyintuitiveastheVBmodel does ?http://www.science.oregonstate.edu/~gablek/CH334/Chapter1/methaneMOs.htm

VB model http://www.science.oregonstate.edu/~gablek/CH334/Chapter1/methane_MOs.htm C 4H 2p 2s 1s 2a1 3a1 1t2 2t2 MO model 2a1 1t2 The MO model of CH4 does not explicitly reflect the 4 equivalent C￾H -bonds of “chemical intuition” as the VB model does ! How can the MO model become chemically intuitive as the VB model does ? (1) ( 2 ) ( 2 ) (1) CHi i i i i VB   HO   HO 

Now considerthe linear combinations of thefour CMOsY=(,+,+,+)/2Y,=(s + x-V,- W)/2Let Cb= CdY3=(s-Wx- V, + W)/2Y4=(-++ V,- )/2& ca=CcY1 = [(c,C2s+ca(C2p,+C2p,+C2p.)+(ca+3c)Φi+(ca-C)(Φ2+Φ3+Φ4) ]/2Y,=cl(C2s+C2p,+C2p,+C2p.)/2+ca*ΦLocalized molecularY2= Cb(C2s-C2px-C2p,+C2p-)/2 + caΦ2orbitals(LMOs)describing C-H bondsY3= cb(C2s-C2p,+C2p,-C2p.)/2 + caΦ3Y4 = cb(C2s+C2px-C2p,-C2p.)/2 + ca'Φ4中h2dh14 sp3-hybridized orbitals on Cdh3The4LMOsareineffect similartothose4covalent C-H bonds described in VB theory

Now consider the linear combinations of the four CMOs, h1 h2 h4 h3 z x y a b c d 1= (s + x + y + z )/2 2= (s + x – y – z )/2 3= (s – x – y + z )/2 4= (s – x+ y – z )/2 1 = [(cbC2s+cd (C2px+C2py+C2pz )+(ca+3cc )1+(ca -cc )(2+3+4 )]/2 Let cb= cd & ca=cc 1 = cb (C2s+C2px+C2py+C2pz )/2+ca 1 2 = cb (C2s–C2px–C2py+C2pz )/2 + ca 2 3 = cb (C2s–C2px+C2py–C2pz )/2 + ca 3 4 = cb (C2s+C2px–C2py–C2pz )/2 + ca 4 Localized molecular orbitals (LMOs) describing C-H bonds 4 sp3-hybridized orbitals on C The 4LMOs are in effect similar to those 4 covalent C-H bonds described in VB theory

主观题10分设置Are thefour LMOs of CH4 equal in energy?If so,please prove that after class!Q1:Notethat hybrid orbitals areused inboththeVBand LMO descriptions ofa covalent bond (e.g.,in CH4)Whatisthedifference inthetwo models?Q2:Howtoconstructhybridized atomicorbitalsforfurtherconstructionofLMOsofamolecule?正常使用主观题需2.0以上版本雨课堂作答

Are the four LMOs of CH4 equal in energy? If so, please prove that after class! 作答 正常使用主观题需2.0以上版本雨课堂 Q1: Note that hybrid orbitals are used in both the VB and LMO descriptions of a covalent bond (e.g., in CH4 ). What is the difference in the two models? Q2: How to construct hybridized atomic orbitals for further construction of LMOs of a molecule ? 主观题 10分

Content of this chapterHybrid Orbital Theoryg-bonds/o-frameworkofa molecule (Qualitative2.VSEPRModel(after-class)MO&VBtheory)元-bonds of a molecule3.Delocalized元-coniugationVB&MOtheories.and Delocalized MO TheoryQualitative)4.Hickel MO Theory and(Semi-QuantitativelSemiempiricalMO)Coniugated Systems5.Symmetry Rules forMolecularreactions(Qualitative)Molecular Reactions

4. Hückel MO Theory and Conjugated Systems Content of this chapter 1. Hybrid Orbital Theory -bonds/-framework of a molecule (Qualitative, MO&VB theory ) 5. Symmetry Rules for Molecular Reactions -bonds of a molecule (VB & MO theories, Qualitative ) (Semi-Quantitative/ Semiempirical MO) Molecular reactions (Qualitative) 3. Delocalized -conjugation and Delocalized MO Theory 2. VSEPR Model (after-class)