Molecular Orbitals of Heteronuclear Diatomics
The molecular orbitals of heteronuclear diatomics (HF, CO, CN-, etc.)
can be predicted using the same principles that we used to construct
the molecular orbitals of homonuclear diatomics:
i) Ignore the core electrons
ii) Remember that the total number of MOs = total number of AOs
iii) Only AOs of similar energy combine.
iv) Only AOs of compatible symmetry combine.
ie. -type AOs (s and pz orbitals) make  MOs
-type AOs (px and py orbitals) make  MOs
Molecular Orbitals for HF
Valence Atomic Orbitals of Isolated H and F
Molecular Orbitals for HF
Valence Atomic Orbitals of H next to F along the z-axis
3 *
1
2
1
2pz
2px, 2py
Bonding in HF
- 2pz(F) + 1s(H)
3 *
1
2px(F)
2py(F)
2pz(F) + 1s(H)
2s(F)
2
1
Anti-bonding MO
Localized on F
Non-bonding
Bonding MO
Localized on F
Non-bonding
Bonding in HF
3 *
LUMO
LP
:
LP LP
1  HOMO
H-F : LP
:
BP
2
LP
BP
122214
LP
1
NB
B
1LP
1BP
NB
2LP’s
Molecular Orbitals for CO
Valence AO’s for C and O aligned along the z-axis
2pz
2pxy 2pz
2pxy
2s
2s
2s(O)
Core
1
1s(C) & 1s(O)
Not MO’s but AO’s
Molecular Orbitals for CO
Valence AO’s for C and O aligned along the z-axis
2pz
2pxy 2pz
2py(C) - 2 py(O)
2px(C) - 2 px(O)
2
2
2py(C) + 2 py(O)
2px(C) + 2 px(O)
1
1
2pxy
2s
2s
2s(O)
Core
1
1s(C) & 1s(O)
Not MO’s but AO’s
Molecular Orbitals for CO
Valence AO’s for C and O aligned along the z-axis
2pz
2pxy 2pz
2pxy
2s
2s
2pz(C) + 2 pz(O)
2py(C) - 2 py(O)
2px(C) - 2 px(O)
4
2
2
2pz(C) - 2 pz(O)
2py(C) + 2 py(O)
2px(C) + 2 px(O)
2s(C) + 2pz
3
1
1
2
2s(O)
1
Core
1s(C) & 1s(O)
Not MO’s but AO’s
Molecular Orbitals for CO
4
2pz
2
3
2pz
1
2
1
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Molecular Orbitals for CO
4* ?
2*
2 pz
2 pxy
3
2 pz
2 pxy
1
2s
2* ?
1
12221432
LP
LP
2BP 1BP
2s
:C
O:
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Actual Molecular Orbitals for CO from Hyperchem
Node =
Node =
*
2pz(C)+2pz(O)
*
Node =
*
Bond =
2py(C)-2py(C)
2px(C)-2px(O)

2pz(C)-2pz(O)
2px(C)+2px(O)
Bond =
Bond =
2py(C)+2py(O)


Node =
*
2s(C)+2pz(O)
2s(O)
Bond = BMO
Electron Configuration
for CO using MO
AB
4 *
AB
AB
2 *
B
3 Sets of Bonding Pairs
3 B
B
1 B
2 * AB
1
B
122*21432 : C
LP
LP
2BP’s
BP
O:
Electron Configuration of N2
4*
:N
1*
3
N:
1
2*
1
122*21432
LP
LP
2BP’s
BP
Computating MOs
Ab initio calculations :
“from the beginning” and refers to calculations made from first principles.
1) consider all electrons in a molecule. (core & valence)
2) considers all interactions. (n-e, e-e & n-n)
3) Uses Born-Oppenheimer Approximation.
4) Simplifies e-e interactions to make the equations solvable.
Semi-empirical calculations
1) Consider only the valence electrons, replacing the nucleus and core electrons with a
“core potential” which represents their effect on the valence electrons.
2) Valence MO’s are calculated just as in Ab-initio methods where the core potential is
added along with the Coulombic interactions.
Faster than ab initio calculations and give relatively reliable molecular geometries.
MO diagrams are less accurate than ab initio, but the MOs are typically in the
correct order with the right separations.
Predicted geometries can be verified by X-ray crystallography (and other
techniques) and the energies can be verified by spectroscopy.
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