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Symmetry and MolecularOrbitals (I)
Simple Bonding Modelhttp://chiuserv.ac.nctu.edu.tw/~htchiu/Chemistry/Fall-2005/Chemical-Bonds.htm
Lewis StructuresOctet RuleResonanceFormal ChargeOxidation NumberHypervalence
Resonance and Formal Charge
A0
A+1
VSEPR Some Molecules
VSEPR Some Molecules
VSEPR Model
Bond LengthBond Strength
Less Repulsion More Repulsion
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.
Why Sharing Electron Forms Anti-Bond?
In Some Area, Sharing Electrons Generates RepulsiveForce.
Bonding and AntibondingRegion Around The Nuclei has Binding (bonding) andAntibinding (antibonding) Zones.
Spherical Boundary Surface of s and pOrbitals
Spherical BoundarySurface of dOrbitals
Spherical Boundary Surface of fOrbitals
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.
Valence Bond Theory
s p
p p
N2
Square of a Wavefunction isProbability Density
Constructive and DestructiveInterference
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))
Hybridization sp
Hybridization sp
+ -
Hybridization sp2
Hybridization sp2
Hybridization sp2 + p
Hybridization sp3
Hybridization sp3
Hybridization H2O Bonds
Hybridization C2H2 Bonds
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
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
Bonding Molecular Orbitals
Bonding Molecular Orbitals
Constructive InterferenceSymmetric: i
Antibonding MolecularOrbitals
Antibonding Molecular Orbitals
Destructive Interference
Anti-symmetric: i
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
Overlap of MOs
Wrong symmetry
Right symmetryBad OverlapGood Overlap
Wrong symmetry
+
+
+
+
+
+
+
+
++
-
-
-
MO Symmetrybond: no nodal plane passing through internuclear axisbond: 1 nodal plane passing through internuclear axisbond: 2 nodal planes passing through internuclear axis
+ +
+ +
+ +
+ +
-
++
+-
-
--
-- -
POrbitals
POrbitals
MO Symmetry
B.MO
A.MO
g: geradeu: ungerade
+ +
+ +
+
+ +-
- -
-
-
MO Energies of Period 2 M2 Molecules
Energy Levels ofMany ElectronAtoms in PeriodicTable
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
MO of Period 2 M2 Molecules
LUMOLowestUnoccupiedMolecularOrbital
HOMOHighestOccupiedMolecularOrbital
DPDDPDDMagnet.
1232101B.O.
PhotoelectronSpectroscopy
Ek = hImeasured known calculated E(MO)
UV photoelectron Spectrum of N2
Heteronuclear Diatomic Molecules
A more electronegativeB less electronegative
+
+
+ +
+ -
CO MO Energy Level Diagram
2
1
3
2
1u
1g
1g
CO MO Energy Level Diagram
2
1
3
2
CO MO Energy Level Diagram
2
1
3
2
ICl MO Energy Level Diagram
1g
1u
1g
HF MO Energy Level Diagram
AMO
NBMO
BMO
H+ F-
H :F:::
Bond Order, Strength and Length
Bond Strength and Length
O-O
N-NC-N,C-O
C-C