S-p Mixing Flashcards
What is photoelectron spectroscopy?
This is spectroscopy where an electron is emitted from an atom by interaction with photons
It can determine the energy and identity of orbitals
- determine whether MO theory is correct
What causes S-p mixing / sigma pi crossover ?
This is where MOs of the same symmetry and energy can also interact
- sigma 2pz can interact with sigma 2s MOs
What is the effect of sigma pi crossover?
When two orbitals mix, one of the new orbitals moves up in energy whilst the other moves down
The sigma 2pz is destabilised so becomes higher in energy (makes it higher than pi MOs from 2pxy)- crossover
The sigma 2s is stabilised so it is lower in energy
Why do pi MOs not interact with sigma MOs
They are the wrong symmetry
Describe the relationship between energy gap and level of attraction
Orbitals that are close in energy give a large interaction whereas those with larger energy differences give a small interaction
Which diatomics show sigma pi overlap and which do not?
B2, N2 and C2 show pi sigma overlap
O2 and F2 do not
Why do O2 and F2 not show sigma pi overlap?
This is because of the size of the 2s and 2p energy gap
This means that the sigma 2s and sigma 2pz MOs do not have close enough energy to interact
Does the sigma pi cross over only happening with bonding molecular orbitals?
No it can also occur with sigma antibonding orbitals 2s and 2pz
What can a MO diagram tell us?
It can tell you which molecules exist, bond strength, whether the molecule is para or diamagnetic and electronic transitions
Describe an MO diagram construction for a homonuclear diatomic
AOs have the same energy
There S +s, pz+ pz form sigma bonding and antibonding
The px+px and py+py form pi bonding and antibonding
Pi overlap is smaller
There is sp mixing for N2, c2 and B2 not O2 or F2
Describe an MO construction for a heteronuclear diatomic
AOs do not have the same energy, use valence IE or electronegativity of the atom to determine orbital energy
Pz+ S (is similar energy) can interact to form sigma bonding and antibonding
Why is sp mixing present in CO
Oxygen AOs are lower in energy than carbon ones as o is more electronegative and has a higher zeff
Also the 2s-2p energy gap is larger in O than it is in C which means the 2p orbital interacts with the 2s orbital on the carbon
What are the chemical properties of CO?
Toxic
Lewis base so can bond to iron centre in haemoglobin more effectively than O2
HOMO is a c centred lone pair
Donor ligand in complexes with low oxidation state
LUMO is a c centred p* orbital and accepts electrons from transition metal ligands
What is the relationship between bond and bond order
Bonds become short and stronger as bond order increases
Bonds become weaker as they become longer
Bond stretching frequency increases as bond order increases
How can differences in BDE and length be explained by MO theory?
If an electrons is removed from an antibonding orbital, bond order increases therefore bonds are stronger
If an electron is removed from a bonding orbital, bond order decreases and so bonds are weaker
