第三章
Molecular Shape and
Structure
3-3 Molecular Orbital Theory -1
Reviews Bond Theories
Lewis Structure :
Bonding Structures around the Center Atoms
Formal Charge
VSEPR :
Electron Arrangement
Molecular Geometry
Valence-Bond theory :
Insights of Bonding Structures in terms of
Hybridization Orbitals, σ- and π- Bonds
Limitation 1 of Lewis Model
Please write the Lewis
structure of O2 and
predict the bond type of
O2 in terms of σ- and πbonds.
Diradical (Biradical)
Molecules?
Paramagnetic O2
Gouy Balance
How do we know that the electrons are not paired?
Limitation 2 of Lewis Model
H H
What is the Lewis
structure of B2H6?
H B B H
Electron Deficient
Molecules?
H H
B
B
H
H
H
H
H
H
Limitation 3 of Lewis Model
為什麼物質有顏色？
胡蘿蔔素本身是紅色
為什麼物質會導電？
導體：溫度越高導電性越差
半導體：溫度越高導電性越佳
Molecular Orbital Theory的原理
的原理
AO (1S) ＋ AO (1S) MO (σ1s)
Energy Correlation of MO (σ1s)
Linear Combination of 1s AOs
Linear Combination of
Atomic Orbitals (LCAO)
Boding orbital
σ1s
ψ = ψA1s + ψB1s
Antibonding orbital
ψ = ψA1s - ψB1s
σ∗1s
MO Energy-Level Diagram
H2
He2
Bond Order
Bond Order
= 0.5 × (number of electrons in bonding MO –
number of electrons in antibonding MO)
= 0.5 × (N-N*)
Bond Order of H2
Bond Order of He2
Examples
What are MO energy-level diagram and bond
order of H2+ ?
What are MO energy-level diagram and bond
order of He2+ ?
MO of Period 2
MO energy-level diagram and bond order of Li2
MO energy-level diagram and bond order of Be2
Review 2P Atomic Orbital
ψ2p
MO from Overlap of 2p AOs
MO Energy-Level from 2s & 2p
MO Energy-Level of O2 and F2
Examples
What is the MO Energy-Level diagrams and
bond orders of B2 and C2?
What is the MO Energy-Level diagram and
bond order of N2?
What is the MO Energy-Level diagram and
bond order of NO?
Influence of 2s and 2p Mixing
Heteronuclear Diatomic Molecules
ψ ≠ψA + ψ B
ψ = CAψA + CBψB
Nonpolar Covalent Bond
Ionic Bond
CA = CB
(CA ,CB) = (1,0) or (0,1)
Polar Covalent Bond
If χA > χB
Bonding： CA >CB
Antibonding： C’A < C’B
Molecular Orbitals of HF
Antibonding
Nonbonding
Bonding
Bonding
Molecular Orbitals Diagram of NO
?
Molecular Orbitals Diagram of CO
?
Example
What is the MO Energy-Level diagrams and
bond orders of CN-?
Energy
Polyatomic Molecule H2C CH2
π∗
π
Combination of VB and MO
2Pz
VB Theory
2Pz
VB Model of C6H6
MO Model of C6H6
π∗
π∗
π
π
HOMO and LUMO
HOMO
Highest Occupied
Molecular Orbital
LUMO
Lowest Unoccupied
Molecular Orbital
π∗
π
Energy Required for π π*
Structure
Energy
(kcal/mol)
173
132
107
92
Retinol and β-Carotene
HOMO LUMO LUMO+1
LUMO + 1
LUMO
HOMO
Electron Conductivity
Obtained From Engineering Sciences 154 maintained by
Professor R. Victor Jones
N-type & P-type semiconductor
Si + P or As
Si + In or B
P-N Junction