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Chapter 18 Molecular orbitals and spectroscopy
18.1 Diatomic molecules
18.2 Polyatomic molecules
18.3 Conjugation of bonds and resonance structures
18.4 The interaction of light and matter (spectroscopy)18.5 Buckyballs
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18.1 Diatomic molecules
Constructing molecular orbitals from atomic orbitals
Orbital correlation diagrams
Homonuclear and heteronuclear diatomic molecules
Constructive and destructive interference of wavesBonding and antibonding molecular orbitals
MO energies, AO parentage, Bond order
Diamagnetism of N2 and paramagnetism of O2
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18.1 Diatomic Molecules
Molecular orbitals: orbitals that span two or more atoms.
Atomic orbitals: orbitals that are localized on single atoms.
Constructing molecular orbitals (MOs) by overlapping atomic orbitals (AOs)
bonds: electron density of MO directed along bond axis
bonds: electron density of MO has a nodal plane that contains the bond axis
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Constructive and destructive interference of waves
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The orbital correlation diagram for overlap of two 1s orbitals
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Correlation diagram for the hydrogen molecule, H2
The electronic configuration of a H2 molecule is 1s2
The subscript (1s) tells which AOs are combined, the superscript (2) tells how many electrons are in the MO
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Answer: any diatomic with three electronsIsoelectronic valence = (1s)2(1s*)1
Plausible diatomics possessing only combinations of H or He atoms: H2
-, He2+, HHe and the configuration
(1s)2(1s*)1
What hypothetical diatomic molecules might fit this electron configuration?
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Bond order (BO): the net number of bonds, allowing for the cancellation of bonds by anti-bonds
BO = 1/2(N - N*) where N = number electrons in bonding orbitals and N* = number of electrons in
anti-bonding orbitals
Example: (1s)2(1s*)1
N = 2, N* = 1BO = 1/2(N - N*) =1/2Any diatomic molecule with BO > 0 is considered stable relative to the two dissociated atoms.
We can deduce molecular stability from BO
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A shared pair of electrons make a single covalent bondElectrons in bonding orbitals enhance bonding, electrons in anti-bonding orbitals reduce bonding
Bond order is a measure of the bonding between two atoms: 1/2[(e in bonding MOs) - [(e in anti-
bonding MOs)]
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What is the bond order of the first electronically excited state of H2?
The electronic configuration of the first excited state of H2 is (1s)1(*1s)1.
Bond order = 1/2(1 - 1) = 0
Photochemical excitation of H2 makes it fly apart into 2 H atoms.
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Building up the MOs of simple diatomic molecules
Mix atomic orbitals (AOs) of the same or similar energies to form molecular orbitals
(MOs)
2s + 2s = 2s + 2s2pz + 2pz = 2p + 2p
2px + 2px = 2p + 2p 2py + 2py = 2p + 2p
Total of 8 MOs which can hold up to 16 electrons
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The constructive overlap of two 2pz orbitals on neighboring atoms to produce a 2pz bonding orbital
2pz + 2pz = 2p + 2p
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The destructive overlap of two 2pz orbitals on neighboring atoms to produce a 2pz bonding orbital
2pz + 2pz = 2p + 2p
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Constructive and destructive overlap of 2 p orbitals to form and orbitals
(a) Bonding orbital; (b) Antibonding orbital
Overlap along the internuclear axis is termed overlap. The resulting orbitals are called and orbitals
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The destructive overlap of two 2px orbitals on neighboring atoms to produce a 2px bonding orbital
2px + 2px = 2p + 2p
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The destructive overlap of two 2px orbitals on neighboring atoms to produce a 2px anti-bonding
orbital
2px + 2px = 2p + 2p
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(a) Bonding orbital; (b) Antibonding orbital
Constructive and destructive overlap of 2 p orbitals to form and orbitals
Overlap perpendicular to the internuclear axis is termed overlap. A nodal plane that contains the bond axis. The resulting orbitals are called and orbitals
Remember: + and - refer to mathematical symbols, not charge
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Now we have the orbitals, what are their relative energies?
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Li, Be, B, C, N O, F, Ne
Orbital energies for the 3Li-10Ne
Note: (1) The energy of the 2p and 2p orbitals switch energy places between N and O; (2) The electron configuration for any isoelectronic
valences is the same
Switch
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Correlation diagrams for B2 (left) and O2 (right)
VEs)2(s*)2(p)2(p)2 (p*)0(p*)0 VEs)2(s*)2(p)2(p)4 (p*)2 (p*)0
What are the bond orders? What are the magnetic properties?
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Electron configurations for the diatomic molecules:
B2, C2, N2, O2, F2, Ne2
VE: 6 8 10 12 14 16
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The molecular orbital electronic configuration of 7N2Group V: Valence electrons = 10. Ignore core electrons
What is the bond order of N2?
BO = 1/2(N - N*)
N = 8, N* = 2, N - N* = 6
BO = 1/2(6) = 3
How does the Lewis structure compare to the MO structure?
N N
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The molecular orbital electronic configuration of 8O2
Group VI: Valence electrons = 12. Ignore core electrons
What is the electronic configuration of O2?
What is the bond order of O2?Bond order = 1/2(N - N*)1/2(8 - 4) = 2
(2s)2(*2s)2(2p)2(2p)4(*2p )2
O2 possesses a net double bond and is paramagnetic
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Electron configurations of the valence electrons of N2 and O2
N N
Lewis structure
O O O O
Lewis structures
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Paramagnetic: O2: (2s)2(*2s)2(2p)2(2p)4(*2p )2
Diamagnetic: N2: (2s)2(*2s)2(2p)2(2p)4
O2 is paramagnetic
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The molecular orbital electronic configuration of 9F2
Group VII: Valence electrons = 14
What is the bond order of F2?
What is the valence electronic configuration of F2?
(2s)2(*2s)2(2p)2(2p)4(*2p)4
BO = 1/2(8 - 6) = 1
F2 possesses a net single bond
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Molecular orbital configurations of homonuclear diatomics
Bond orders, bond lengths and bond energies
The bond orders from MO theory agree with Lewis
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18.2 Polyatomic molecules
Valence bond theory and molecular orbital theory
Hybridization of orbitals to make stronger bonds
Hybridization and molecular geometry
Hybridization and bond order. Single, double and triple bonds
Valence bond theory: localized bonds, like Lewis structures
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Valence bond: Overlap of two s orbitals to produce a bond
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Valence bond: Overlap of an s orbital and a pz orbital to produce a bond
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Valence bond: Overlap of two pz orbitals to produce a bond
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Valence bond: Overlap of two px orbitals to produce a bond
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Hybridization
If more than two atoms are involved in a molecule, the shapes of the orbitals must match the shape of the bonds that are needed (trigonal, tetrahedral, etc.). The atomic orbitals do not have these shapes, and must be mixed to achieve the needed shapes
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Shapes of hybridized atomic orbitals
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The hybridization of a s orbital and two p orbitals to produce three sp2 orbitals
ThreeAtomicorbitalsAos2s + two 2p
ThreehybridOrbitalsHAOssp2
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The orientations of four sp3 orbitals
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The hybridization of a s orbital and a p orbital to produce two sp hybrid orbitals
2s orbital could be + or -
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