Heteronuclear Molecules - Georgia Institute of...
Transcript of Heteronuclear Molecules - Georgia Institute of...
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Heteronuclear Molecules
• The relative energy of the bonding orbitals determines themagnitude of the covalent bond energy (∆Ecov):
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Energy Level D
iagram of C
O
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Hydrogen Fluoride
• In H-F the 1s orbital of H is energetically well above the 1sand 2s orbitals of F–> it interacts only with the 2pz orbital (all remainingelectrons are in non-bonding orbitals!)
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Lithium Fluoride
• As the polarity difference between two atoms increases, theorbital energy difference also increases–> electrons shift towards the more electronegative atom
• Limiting case: Ionic compounds
Ionic crystal:Ions are held together in a3-dimensional lattice bycombination of electrostaticand covalent interactions
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Homonuclear Bond Energies
Note: Values can vary considerably depending on compounds involved
• Bond becomes weaker with increasing atom size• But: First row elements of group 15, 16 and 17 form weaker bonds than
2nd row elements (repulsion between lone pairs more pronounced)• Group 1&2: Destabilizing δ+ charges when mutually bonded
• Energy of C–C to N–N sharply decreased due to presence of lone pairs
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Heteronuclear Bond Energies
• Heteronuclear bond energies much higher (stronger bonds) than homonuclearenergies (due to electronegativity differences)
• Group 2 bonds stronger than group 1 (ionic bond: lattice energies proportional to ioncharges)
• B–F stronger than Al–F due to π interaction
• Si–F stronger than C–F: large difference in electronegativity, π interaction with vacantd orbitals of Si
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Multiple Bond Energies
• N2 triple is very strong (only the CO triple bond is stronger)• N=N is more than twice the bond energy of a N–N single bond
But: P=P is less than twice the bond energy of the P–P bond–> π bond interaction for 2nd and 3rd row elements weaker (due to larger and more diffuse porbitals)–> P–P single bond considerably stronger than N–N (or O–O) (less repulsion of lone pairelectrons)
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Molecular Orbitals of Polyatomic Molecules
• Concept of linear combination can be also applied to polyatomicmolecules–> the resulting MOs are delocalized over the entire molecule
• Symmetry analysis by group theory predicts those linearcombinations, which lead to bonding, anti-bonding or non-bonding MOs
• The energy of the resulting MOs is measured via photoelectronspectroscopy or estimated with quantum chemical calculations
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Beryllium Dihydride (BeH2)
• VSEPR analysis: linear geometry
• Set of AOs:Be: H:Note: the 2px and 2py orbitals yield non-bonding interactions
• Form group orbitals with 1s orbitals of H and H’:
Ψ Ψ ΨA s sH H= +1 1( ) ( ' )
Ψ Ψ ΨB s sH H= −1 1( ) ( ' )
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Group Orbitals Interact with Be AOs:
ΨA interacts with the 2s orbital of Be to form a bonding (σs) andanti-bonding (σs*) orbital:
ΨB interacts with the 2pz orbital of Be to form a bonding (σp)and anti-bonding (σp*) orbital:
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Energy Level Diagram
Group orbitals
ΨA
ΨB
energy
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• VSEPR analysis: bent geometry
• Set of AOs:O: H:
• Note: the 2py orbital results a non-bonding interactions
• Form goup orbitals with 1s orbitals of H and H’:
MOs of Water (H2O)
Ψ Ψ ΨA s sH H= +1 1( ) ( ' )
Ψ Ψ ΨB s sH H= −1 1( ) ( ' )
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The Two ΨA Group Orbital Interactions:
ΨA interacts with two AOs of Oxygen: The 2s and 2pz orbital:
–> this results in one bonding, one anti-bonding and onenon-bonding (approximately) orbital:
ΨA2pz
2s
Oxygen AOs
(Group Orbital)
σs,z*
σs,z
σs,z nb
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The ΨB Group Orbital Interaction:
ΨB interacts with the 2px orbital of oxygen to form a bonding (σs)and anti-bonding (σs*) orbital:
ΨB2px
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To Give the Final Energy Level Diagram...
ΨA
ΨB
energy
Group orbitals
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Diagram
with M
O shapes
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Hybridization
• Molecular Orbital Theory:–> Electrons are delocalized over entire molecule (including core shellelectrons!)
• Hybrid Orbital Description:–> Valence bond approach–> Bonds described as localized interactions of TWO electrons
Bonding between two atoms can be also described as overlap of twohybrid orbitals, which represent the correct valence geometries
– A hybrid orbital is a linear combination of AOs of a SINGLE atom
– Different linear combinations will result different geometries
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Hybrid O
rbitals
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Linear Geometry
• Beryllium Hydride
promotion
Linear combination of one 2s and one 2p orbitals results two sphybrid orbitals:
sp s pz( ) ( )112
2 2= + sp s pz( ) ( )212
2 2= −
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Linear Geometry
• Ethyne (HCCH)
• The two remaining p orbitals contain one electron each–> Delocalization of these two electrons results two orbitalswith π symmetry (with 90° angle between each other):
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Trigonal Geometry
• Boron Trihydride:
• The remaining 2pz orbital is perpendicular to the the threehybrid orbitals and is not occupied
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Trigonal Geometry
• Ethene (H2CCH2):
• The remaining two 2px orbitals is perpendicular to the the threehybrid orbitals and contain one electron each–> Delocalization results one additional orbital with π symmetry
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Tetrahedral Geometry
• Methane (CH4)
• The 2s and three 2p orbitals of carbon result four sp3 hybridorbitals
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How useful are hybrid orbitals?
• Provides a qualitative picture of the bonding around an atom
• Purely mathematical concept
• Hybridization arguments are not predictive, just descriptive!
• MO theory is predictive, but complicated to use
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Frontier Orbitals
• Many properties of molecules can be interpreted through theuse of the molecular orbital model, and in particular bylooking at the frontier orbitals:– the HOMO (highest occupied molecular orbital)
–> Donor orbital– the LUMO (lowest unoccupied molecular orbital)
–> Acceptor orbital
• The energy difference between the HOMO and LUMOcorresponds to the lowest excitation energy
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Conjugated Systems
• When two or more double (or triple) bonds are close to one another, thevalence π electrons tend to “mingle together”
–> the MO model with its emphasis on delocalization explains this effectbetter than the localized valence bond approach:
• Ethene:
π
π*
H
H H
H
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Conjugated Sytems
• Butadiene:
π
π
π*
π*
1 node
2 nodes
3 nodes
no node
H
H
H
H
H
H
Carbon: sp2 hybridized
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Energy Levels
• With increasing number of orbitals, the energy levels getcloser and closer–> the energy difference between HOMO and LUMO isdecreasing–> excitation by a small electric potential or by light will moveelectrons into the LUMO orbitals, ready to convey a current(if a potential difference is imposed)–> semiconducting properties
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Cyclic Structures
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August Kekulé had a dream...
• Benzene was isolated 1823 from distillation of whale oil byMichael Faraday (named ‘bicarburet of hydrogen’)
• The structure was an unsolved puzzleuntil 1865, when Kekulé dreamed of“carbon-chain snakes” and finallyproposed the correct structure
Some older “versions” of benzene:
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Resonance Structures of Benzene
• There are two possible resonance structure for benzene:
–> each carbon has sp2 hybridization, the remaining 6p-orbitals combine to give 6 delocalized MO π orbitals
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MO Energy Diagram
π
π
π*
π*
1 node
2 nodes
3 nodes
no node
π
π*
Energy
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Large Conjugated Sytems
• Graphite consists of layers of fused 6-membered carbon rings (sp2 hybridized) with aninterlayer spacing of 335 pm (sum of carbon radii)
• The remaining unhybridized p-orbitals participate in extensive π bonding, with electron densitydelocalized over the layers
• Bond distance: 142 pm = bond order of 1.333
• At very high pressure graphite canbe converted to diamonds, anotherallotrope of carbon
• All carbon atoms are sp3 hybridized,there are no delocalized electrons–> diamond is an insulator
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Buckminster Fullerene
• In 1985 another allotrope of carbon (C60) was discovered via pulsed laservaporization of graphite
• It was named after the architect Buckminster Fuller
• All 60 carbons are sp2 hybridized, leaving 60 p-orbitals to give 60 MOs(spread over both sides of the surface)
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Natural Products
• Many colored compounds in nature consist of large, delocalized π-systems
• The HOMO-LUMO energy difference in these molecules is small enough toabsorb light in the visible region
N
NN
N
O
O
H3CO
O
O
H
H3C
Mg
Chlorophyll (green)
beta-carotene (orange)
Note: The color of a material is complementaryto the absorbed light–> beta carotene is orange, and therefore itabsorbs strongly in the blue and violet region ofthe spectrum
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Photosynthesis
• The green color of plants is due to absorption of light bychlorophyll pigments–> the absorbed energy is used to convert CO2 to carbohydrates(sugars), oxygen is produced as a “side product”