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Symmetry and MolecularOrbitals (I)
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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
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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 TheoryVB 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 in
polyatomic 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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HybridizationLinear 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
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Hybridization C2H2 Bonds
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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
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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
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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
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Molecular Orbital Energy Level DiagramBetter Overlap =>
HigherEBond Order = (# of B.O. e - - # of A.O. e -)Diamagnetic: all
e- pairedParamagnetic: with e- unpaired
EE
0
1
B.O.
-He2
PH2-
DH2
PH2+
MagneticProperty
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Overlap of MOs
Wrong symmetry
Right symmetryBad OverlapGood Overlap
Wrong symmetry
+
+
+
+
+
+
+
+
++
-
-
-
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MO Symmetrybond: no nodal plane passing through internuclear
axisbond: 1 nodal plane passing through internuclear axisbond: 2
nodal planes passing through internuclear axis
+ +
+ +
+ +
+ +
-
++
+-
-
--
-- -
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POrbitals
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POrbitals
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MO Symmetry
B.MO
A.MO
g: geradeu: ungerade
+ +
+ +
+
+ +-
- -
-
-
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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 1u2g repulsion2s
2pz MOs same symmetryMore mixing
1g
1u
1g
1u
1g
1g
O2F2 Li2N2
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MO of Period 2 M2 Molecules
LUMOLowestUnoccupiedMolecularOrbital
HOMOHighestOccupiedMolecularOrbital
DPDDPDDMagnet.
1232101B.O.
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PhotoelectronSpectroscopy
Ek = hImeasured known calculated E(MO)
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UV photoelectron Spectrum of N2
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Heteronuclear Diatomic Molecules
A more electronegativeB 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
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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
