an explanation for observed chemical and spectroscopic ... · chemical bonding theory an...
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Chemical Bonding TheoryChemical Bonding Theory
An explanation for observed chemical and spectroscopic behavior
Descriptions of BondingDescriptions of Bonding• Valence-bond theory
– Localized electrons– Convenient for presentation– “Hybrid Atomic Orbitals” are used to form
“independent” Valence Bonds• Molecular orbital theory
– Delocalized electrons– More difficult to conceive and present– “Hydrogen-like Atomic Orbitals” are used to form
Molecular Orbitals
Atomic Atomic OrbitalsOrbitals
Atomic Atomic OrbitalsOrbitals
ValenceValence--bond Theory: Hbond Theory: H22
H H H H+ =
H H+ = H H
Pictorial VB Theory: sp HybridsPictorial VB Theory: sp Hybrids
=+
=
Pictorial VB Theory: sp HybridsPictorial VB Theory: sp Hybrids sp
hybrid
Pictorial VB Theory: spPictorial VB Theory: sp22 HybridsHybrids
Pictorial VB Theory: spPictorial VB Theory: sp33 HybridsHybrids
Pictorial VB Theory: spPictorial VB Theory: sp33 HybridsHybrids
Pictorial VB Theory: Pictorial VB Theory: EtheneEthene
C CH
HH
H
Pictorial VB Theory: Pictorial VB Theory: EtheneEthene
C
H
C
H
C
H
C
H
C CH
H H
H
C C
C CH
HH
H
C CH
HH
HPictorial VB Theory: Pictorial VB Theory: EtheneEthene
Pictorial VB Theory: Pictorial VB Theory: EtheneEthene
CH2H2CCH2H2C
Pictorial VB Theory: Pictorial VB Theory: EtheneEthene
Sigma and pi bonding in ethane, Sigma and pi bonding in ethane, ethylene and acetyleneethylene and acetylene
ConjugationConjugation• Conjugated bonds: p atomic orbitals form a
continuous system• Isolated bonds: p atomic orbitals do not form a
continuous system
Need Molecular Orbital Theory
Molecular Orbital Theory: HMolecular Orbital Theory: H22
HH - = H H
H H+ = H H
Molecular Orbital Theory: HMolecular Orbital Theory: H22
Molecular Orbital Theory: HMolecular Orbital Theory: H22
Conjugation in Conjugation in ππ--Systems:Systems: EtheneEthene
one node
no nodes
π
π∗
Conjugation in Conjugation in ππ--Systems:Systems: EtheneEthene
one node
no nodes
π
π∗
Conjugation in Conjugation in ππ--Systems:Systems: AllylAllyl
π∗
π
n
Conjugation in Conjugation in ππ--Systems:Systems:ButadieneButadiene
π
π∗
π∗
π
Conjugation in Conjugation in ππ--Systems:Systems:PentadienylPentadienyl
n
π
π∗
π∗
π
Conjugation in Conjugation in ππ--Systems:Systems:HexatrieneHexatriene
π
π∗
π∗
π
π∗
π