Chemistry 401

MO Diagrams for Heteronuclear Diatomic Molecules

 

HF

Basis functions: H 1s; F 2s, 2p

Energy of basis functions : 1s Z = 1, F, Z* >1 therefore 2s, 2p lower in energy

BO = 1 σ bond more centered on F so electron density is favors this atom

This is consistent with our ideas about electronegativity

CO

basis, 2s, 2p on each atom; O orbitals a little bit lower

BO = 3 lone pair on C accounts for Lewis basicity

2nb:

1π*

1π

MO diagrams for Polyatomic molecules

H₂O

 

BO = 2

Problem : experimentally, the σ orbitals are of different energy, as are the nb orbitals.

This problem can not be fixed predictably with the methods we have available

Using more sophisticated MO calculations gives:

 

B₂H₆ VBT Lewis Dot structure doesn't have enough electrons:

The B-H-B bonds are called electron deficient bonds (3-center, 2-electron bonds)

MO theory ; break it up into BH₃ fragments