molecular orbitals of heteronuclear diatomics
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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 HFValence Atomic Orbitals of Isolated H and F
Molecular Orbitals for HF
2pz
2px, 2py
Valence Atomic Orbitals of H next to F along the z-axis
1 s
2 s
1 p
3 s*
Bonding in HF
Localized on F
Localized on F
Bonding MO
Anti-bonding MO
Non-bonding
Non-bonding2s(F)
2pz(F) + 1s(H)
2px(F)
2py(F)
- 2pz(F) + 1s(H)
1 s
2 s
1 p
3 s*
Bonding in HF
LP
BP
LP
H-F:
::
LUMO
HOMO
NB B NB
LP LP
LP
LP
BP
1s22s21p4
1LP 1BP 2LP’s1 s
2 s
1 p
3 s*
Molecular Orbitals for CO
2s
2pz
2s
2pxy
2pz
2pxy
2s(O) 1s
Core 1s(C) & 1s(O)
Not MO’s but AO’s
Valence AO’s for C and O aligned along the z-axis
Molecular Orbitals for CO
2s
2pz
2s
2pxy
2pz
2pxy
2s(O) 1s
Core
2px(C) + 2 px(O)2py(C) + 2 py(O)
1p1p
1s(C) & 1s(O)
Not MO’s but AO’s
Valence AO’s for C and O aligned along the z-axis
2px(C) - 2 px(O)2py(C) - 2 py(O)
2p2p
Molecular Orbitals for CO
2s
2pz
2s
2pxy
2pz
2pxy
2s(O) 1s
1s(C) & 1s(O)Core
2px(C) + 2 px(O)2py(C) + 2 py(O)
1p1p
2s(C) + 2pz 2s
2pz(C) - 2 pz(O) 3s
Not MO’s but AO’s
Valence AO’s for C and O aligned along the z-axis
2pz(C) + 2 pz(O) 4s
2px(C) - 2 px(O)2py(C) - 2 py(O)
2p2p
9
1s
2p
1p
2s
3s
Molecular Orbitals for CO
2pz
2pz
4s
10
1s
2 *p
1p
2 * s?
3s
4 * ?s
Molecular Orbitals for CO
2 s
2 s
2 pxy
2 pz
2 pxy
2 pz
1s22s21p43s2
LP LP 2BP 1BP
C O: :
Actual Molecular Orbitals for CO from Hyperchem
2s(O)
2s(C)+2pz(O)
2px(C)+2px(O)
2py(C)+2py(O)
2pz(C)-2pz(O)
2px(C)-2px(O)
2py(C)-2py(C)
2pz(C)+2pz(O)
Node = s*
Bond = BMO
Bond = p
Bond = s
Node =
s*
Node =
p*Node =
p*
Bond = p
B
AB
BB
B
1s22s*21p43s2 C O: :
3 Sets of Bonding Pairs
LP LP 2BP’s BP
1 s
2 s*
1 p
3 s
2 p*
4 s*
B
AB AB
ABElectron Configurationfor CO using MO
3s
1 *p
4 *s
1p
2 *s
1s
Electron Configuration of N2
1s22s*21p43s2
N N: :
LP LP 2BP’s BP
14
Computating MOsAb 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.