Symmetry and Molecular Orbitals (I) - National Chiao...

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Symmetry and Molecular Orbitals (I)

Transcript of Symmetry and Molecular Orbitals (I) - National Chiao...

Page 1: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Symmetry and MolecularOrbitals (I)

Page 2: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Simple Bonding Modelhttp://chiuserv.ac.nctu.edu.tw/~htchiu/Chemistry/Fall-2005/Chemical-Bonds.htm

Lewis StructuresOctet RuleResonanceFormal ChargeOxidation NumberHypervalence

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Resonance and Formal Charge

A0

A+1

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VSEPR Some Molecules

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VSEPR Some Molecules

Page 6: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

VSEPR Model

Bond LengthBond Strength

Less Repulsion More Repulsion

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Why Sharing Electron FormsCovalent Bond?

Without Electrons, Internuclei Repulsion Forces PushesNuclei Apart.

Sharing Electrons (in some area around the nuclei)Generates Attractive Force to Bind Nucleus Together.

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Why Sharing Electron Forms Anti-Bond?

In Some Area, Sharing Electrons Generates RepulsiveForce.

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Bonding and AntibondingRegion Around The Nuclei has Binding (bonding) andAntibinding (antibonding) Zones.

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Spherical Boundary Surface of s and pOrbitals

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Spherical BoundarySurface of dOrbitals

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Spherical Boundary Surface of fOrbitals

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Valence Bond Theory•VB Theory is a quantum mechanical model

describes the distribution of electrons in bonds.Developed from Lewis theory and the VSEPR model.

•Usually provides bonding picture with localized view.

•Simple VB theory cannot explain bonding inpolyatomic molecules. The concept of Hybridizationis developed to explain different geometry variations.Describes molecular geometry easier.

•Using Delocalization and Resonance to explainextended distribution of electrons over many atoms ina substance.

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Valence Bond Theory

s p

p p

N2

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Square of a Wavefunction isProbability Density

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Constructive and DestructiveInterference

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Hybridization•Linear Combination of Atomic Orbitals of an Atom

forms a new set of orbitals.•Hybridization divides the electron density distribution

of an atom into new areas.

• (sp)A = (1/2)1/2((s) + (p))

• (sp)B = (1/2)1/2((s) - (p))

• (sp2)A = (1/3)1/2(s) + (2/3)1/2(px)

• (sp2)B = (1/3)1/2(s) - (1/6)1/2(px) + (1/2)1/2(py)

• (sp2)C = (1/3)1/2(s) - (1/6)1/2(px) - (1/2)1/2(py)

• (sp3)A = (1/2)((s) + (px) + (py) + (pz))

• (sp3)B = (1/2)((s) + (px) - (py) - (pz))

• (sp3)C = (1/2)((s) - (px) + (py) - (pz))

• (sp3)D = (1/2)((s) - (px) - (py) + (pz))

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Hybridization sp

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Hybridization sp

+ -

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Hybridization sp2

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Hybridization sp2

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Hybridization sp2 + p

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Hybridization sp3

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Hybridization sp3

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Hybridization H2O Bonds

Page 26: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Hybridization C2H2 Bonds

Page 27: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Molecular Orbital Theory

•Electrons occupy orbitals that spread through the entiremolecule.

•Providing bonding pictures with non-localized view.•Deals with entire area surrounding a molecule.

•Hyperchem Litehttp://www.hyper.com/sales/electronic/electronic-lite.htm

•CACAO98 (Beta Version)http://www.chembio.uoguelph.ca/oakley/310/cacao/cacao.htm

Page 28: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Rules of Molecular Orbitals

Rules for forming bonding and antibonding MOsnumber of total molecular orbitals = number of total atomic orbitalsatomic orbitals have the right symmetryatomic orbitals overlap wellatomic orbitals have similar energy

Rules for filling electrons are the same for MOs and AOs.start filling from the lowest energy orbitalfollows Pauli exclusion principle and Hund's rule

Provides molecular energy informationExplains magnetic behaviorCan provide molecular structural explanation, but moredifficult to comprehend than VB theory

Page 29: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Bonding Molecular Orbitals

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Bonding Molecular Orbitals

Constructive InterferenceSymmetric: i

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Antibonding MolecularOrbitals

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Antibonding Molecular Orbitals

Destructive Interference

Anti-symmetric: i

Page 33: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

Molecular Orbital Energy Level Diagram•Better Overlap => Higher E•Bond Order = ½(# of B.O. e - - # of A.O. e -)•Diamagnetic: all e- paired•Paramagnetic: with e- unpaired

EE

0

½

1

½

B.O.

-He2

PH2-

DH2

PH2+

MagneticProperty

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Overlap of MOs

Wrong symmetry

Right symmetryBad OverlapGood Overlap

Wrong symmetry

+

+

+

+

+

+

+

+

++

-

-

-

Page 35: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

MO Symmetrybond: no nodal plane passing through internuclear axisbond: 1 nodal plane passing through internuclear axisbond: 2 nodal planes passing through internuclear axis

+ +

+ +

+ +

+ +

-

++

+-

-

--

-- -

Page 36: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

P Orbitals

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P Orbitals

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MO Symmetry

B.MO

A.MO

g: geradeu: ungerade

+ +

+ +

+

+ +-

- -

-

-

Page 39: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

MO Energies of Period 2 M2 Molecules

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Energy Levels ofMany ElectronAtoms in PeriodicTable

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M2 MO Energy Level Diagrams

Higher Zeff2s 2p more separated2s e- closer to nuclei2s MOs more like 2s AOss-s overlap not effective

2s and 2pz in same spacebetween nucleiGreater 1 u –2 g repulsion2s 2pz MOs same symmetryMore mixing

1g

1u

1g

1u

1g

1g

O2 –F2 Li2 –N2

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MO of Period 2 M2 Molecules

LUMOLowestUnoccupiedMolecularOrbital

HOMOHighestOccupiedMolecularOrbital

DPDDPDDMagnet.

1232101B.O.

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PhotoelectronSpectroscopy

•Ek = h–I

measured known calculated E(MO)

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UV photoelectron Spectrum of N2

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Heteronuclear Diatomic Molecules

•A more electronegative•B less electronegative

+

+

+ +

+ -

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CO MO Energy Level Diagram

2

1

3

2

1u

1g

1g

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CO MO Energy Level Diagram

2

1

3

2

Page 48: Symmetry and Molecular Orbitals (I) - National Chiao …ocw.nctu.edu.tw/upload/classbfs1209120524118880.pdfHF MO Energy Level Diagram AMO NBMO BMO H+ F-H :F::: Bond Order, Strength

CO MO Energy Level Diagram

2

1

3

2

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ICl MO Energy Level Diagram

1g

1u

1g

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HF MO Energy Level Diagram

AMO

NBMO

BMO

H+ F-

H :F:::

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Bond Order, Strength and Length

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Bond Strength and Length

O-O

N-NC-N,C-O

C-C