Lecture 16 C1403October 31, 2005

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Lecture 16 C1403 October 31, 2005
18.1 Molecular orbital theory molecular orbitals
and diatomic molecules
18.2 Valence bond theory hybridized orbitals and
polyatomic molecules
Bond order, bond lengths, connections of MO
theory and VB theory with Lewis structures
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Making of a ?z and ?z orbital from overlap of
two 2pz orbitals
Making of a ?x and ?x orbital from overlap of
two 2px orbitals
Making of a ?y and ?y orbital from overlap of
two 2py orbitals
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Potential energy curves for the ??and ? orbitals
of a diatomic molecule
Distance dependence of the energy of a ? and ?
orbital
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The reason for the switch in the s and p MOs
Larger gap between ?2s and ?2p with increasing Z
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Bond order?
O2 Bond length 1.21Å
O2 Bond length 1.12 Å
O2- Bond length 1.26 Å
O22- Bond length 1.49 Å
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Compare the Lewis and MO structures of diatomic
molecules
C2
N2
O2
F2
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What is the bond order of NO in Lewis terms and
MO theory?
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18.2 Polyatomic molecules
Valence bond versus molecular orbital theory
Hybridization of atomic orbitals to form
molecular orbitals
sp, sp2 and sp3 hybridized orbitals
Hybridized orbitals and Lewis structures and
molecular geometries
Double bonds and triple bonds
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18.2Bonding in Methane andOrbital Hybridization
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Structure of Methane

• tetrahedral
• bond angles 109.5
• bond distances 110 pm
• but structure seems inconsistent withelectron
  configuration of carbon

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Electron configuration of carbon

• only two unpaired electrons
• should form s bonds to only two hydrogen atoms
• bonds should be at right angles to one another

2p
2s
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sp3 Orbital Hybridization
2p

• Promote an electron from the 2s to the 2p
  orbital

2s
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sp3 Orbital Hybridization
2p
2p
2s
2s
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sp3 Orbital Hybridization
2p

• Mix together (hybridize) the 2s orbital and the
  three 2p orbitals

2s
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sp3 Orbital Hybridization
2p
2 sp3

• 4 equivalent half-filled orbitals are consistent
  with four bonds and tetrahedral geometry

2s
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Shapes of orbitals
p
s
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Nodal properties of orbitals
p



s
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Shape of sp3 hybrid orbitals
p

• take the s orbital and place it on top of the p
  orbital

s
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Shape of sp3 hybrid orbitals
s p

• reinforcement of electron wave in regions where
  sign is the same
• destructive interference in regions of opposite
  sign

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Shape of sp3 hybrid orbitals
sp hybrid

• orbital shown is sp hybrid
• analogous procedure using three s orbitals and
  one p orbital gives sp3 hybrid
• shape of sp3 hybrid is similar

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Shape of sp3 hybrid orbitals
sp hybrid

• hybrid orbital is not symmetrical
• higher probability of finding an electron on one
  side of the nucleus than the other
• leads to stronger bonds

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The CH s Bond in Methane
In-phase overlap of a half-filled 1s orbital of
hydrogen with a half-filled sp3 hybrid orbital of
carbon

sp3
s
H
C
gives a s bond.

HC s
C
H
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Justification for Orbital Hybridization

• consistent with structure of methane
• allows for formation of 4 bonds rather than 2
• bonds involving sp3 hybrid orbitals are stronger
  than those involving s-s overlap or p-p overlap

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18.2sp3 Hybridization and Bonding in Ethane
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Structure of Ethane
C2H6
CH3CH3

• tetrahedral geometry at each carbon
• CH bond distance 110 pm
• CC bond distance 153 pm

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The CC s Bond in Ethane

• In-phase overlap of half-filled sp3
  hybridorbital of one carbon with half-filled
  sp3hybrid orbital of another.
• Overlap is along internuclear axis to give a s
  bond.

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The CC s Bond in Ethane

• In-phase overlap of half-filled sp3
  hybridorbital of one carbon with half-filled
  sp3hybrid orbital of another.
• Overlap is along internuclear axis to give a s
  bond.

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18.2sp2 Hybridization and Bonding in Ethylene
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Structure of Ethylene
C2H4 H2CCH2

• planar
• bond angles close to 120
• bond distances CH 110 pm CC 134 pm

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sp2 Orbital Hybridization
2p

• Promote an electron from the 2s to the 2p
  orbital

2s
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sp2 Orbital Hybridization
2p
2p
2s
2s
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sp2 Orbital Hybridization
2p

• Mix together (hybridize) the 2s orbital and two
  of the three 2p orbitals

2s
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sp2 Orbital Hybridization
2p
2 sp2

• 3 equivalent half-filled sp2 hybrid orbitals plus
  1 p orbital left unhybridized

2s
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sp2 Orbital Hybridization
p
2 of the 3 sp2 orbitalsare involved in s
bondsto hydrogens the otheris involved in a s
bondto carbon
2 sp2
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sp2 Orbital Hybridization
s
p
s
2 sp2
s
s
s
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p Bonding in Ethylene
the unhybridized p orbital of carbon is involved
in p bondingto the other carbon
p
2 sp2
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p Bonding in Ethylene
p
2 sp2

• each carbon has an unhybridized 2p orbital axis
  of orbital is perpendicular to the plane of the s
  bonds

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p Bonding in Ethylene
p
2 sp2

• side-by-side overlap of half-filledp orbitals
  gives a p bond
• double bond in ethylene has a s component and a
  p component

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Hybridization and methane CH4
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sp3 hybridization and ethylene H2CCH2
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Ground state gt excite one electron gt mix orbitals
one s orbital and one p orbital two sp orbitals
Acetylene
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Other examples of sp2 and sp hybridized carbon
Formaldehyde H2CO
Carbon dioxide OCO
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d2sp3 hybridization
dsp3 hybridization
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Hybrid orbitals are constructed on an atom to
reproduce the electronic arrangement
characteristics that will yield the experimental
shape of a molecule
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BeF2 SN 2 sp
BF3 SN 3 sp2
CH4 SN 4 sp3
PF5 SN 5 sp3d
SF6 SN 6 sp3d2
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Describe the bonding for ethane, ethene and
acetylene in terms of overlap of hybridized
orbitals
Ethane
Ethylene
Acetylene