Molecular Orbitals of Heteronuclear Diatomics

1. 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

2. Molecular Orbitals for HF Valence Atomic Orbitals of Isolated H and F

3. Molecular Orbitals for HF Valence Atomic Orbitals of H next to F along the z-axis 3 s* 2pz 1 p 2px, 2py 2 s 1 s

4. Bonding in HF - 2pz(F) + 1s(H) 3 s* Anti-bonding MO 1 p 2px(F) Non-bonding Localized on F 2py(F) 2pz(F) + 1s(H) 2 s Bonding MO Non-bonding 2s(F) 1 s Localized on F

5. Bonding in HF LUMO 3 s* LP LP LP : HOMO : H-F 1 p LP : BP 2 s LP BP 1s22s21p4 LP NB B NB 1LP 1 s 1BP 2LP’s

6. Molecular Orbitals for CO Valence AO’s for C and O aligned along the z-axis 2pz 2pxy 2pz 2pxy 2s 2s 1s 2s(O) Core 1s(C) & 1s(O) Not MO’s but AO’s

7. Molecular Orbitals for CO Valence AO’s for C and O aligned along the z-axis 2pz 2p 2py(C) - 2 py(O) 2p 2px(C) - 2 px(O) 2pxy 2pz 2pxy 2s 2py(C) + 2 py(O) 1p 2px(C) + 2 px(O) 1p 2s 2s(O) 1s 1s(C) & 1s(O) Core Not MO’s but AO’s

8. Molecular Orbitals for CO Valence AO’s for C and O aligned along the z-axis 2pz(C) + 2 pz(O) 4s 2pz 2p 2py(C) - 2 py(O) 2p 2px(C) - 2 px(O) 2pxy 2pz 2pxy 2pz(C) - 2 pz(O) 3s 2s 2py(C) + 2 py(O) 1p 2px(C) + 2 px(O) 1p 2s 2s(C) + 2pz 2s 2s(O) 1s Core 1s(C) & 1s(O) Not MO’s but AO’s

9. Molecular Orbitals for CO 4s 2p 2pz 2pz 3s 1p 2s 1s

10. Molecular Orbitals for CO 4s* ? 2p* 2pz 2pxy 3s 2pz 2pxy 1p 2 s 2s* ? 1s : : 2 s C O 1s22s21p43s2 LP LP 2BP 1BP

11. Actual Molecular Orbitals for CO from Hyperchem Node = s* 2pz(C)+2pz(O) Node = p* Node = p* 2py(C)-2py(C) 2px(C)-2px(O) Bond = s 2pz(C)-2pz(O) 2px(C)+2px(O) 2py(C)+2py(O) Bond = p Bond = p 2s(C)+2pz(O) Node = s* 2s(O) Bond = BMO

12. AB Electron Configurationfor CO using MO 4 s* AB AB 2 p* 3 Sets of Bonding Pairs B 3 s B B B 1 p 2 s* AB : : C O B 1 s 1s22s*21p43s2 2BP’s LP LP BP

13. Electron Configuration of N2 4s* 1p* 3s 1p 2s* : : N N 1s22s*21p43s2 1s LP LP 2BP’s BP

14. 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 correctorder with the right separations. Predicted geometries can be verified by X-ray crystallography (and other techniques) and the energies can be verified by spectroscopy.