Transition metal photochemistry Flashcards
What are the selection rules for transitions between electronic states? (Page 9)
∆𝜦 = 𝟎, ±𝟏;
∆𝑺 = 𝟎;
∆𝜮 = 𝟎;
∆𝜴 = 𝟎, ±1
Why is electronic transition between ground and excited states vertical? (Page 9)
Frank-Condon principle, electrons move much faster than nuclei hence nuclei are static on electronic transition and the arrow is vertical.
Why does the excited state have a potential energy curve that lies to the right of the ground state (greater internuclear separation)? (Page 9)
An electron is promoted from a bonding orbital to an antibonding orbital, this weakens the bond and hence lengthens it, therefore there is a greater internuclear separation in the excited state than the ground state.
What does it suggest if a quantum yield is over 1 or 2? (Page 11)
Stark-Einstein law states that not more than one molecule can be decomposed in the primary step (QY is defined as the number of reactant molecules consumed for each photon of light)
If QY > 1 it suggests secondary reactions are occurring as one photon can promote one electron
QY > 2 means chain reaction is occurring
Why does a solvent cage lead to low QY values? (Page 11)
When a molecule dissociates, the two halves can bounce off the solvent cage and recombine (femtoseconds)
The recombined molecule is useless for any chemistry, hence a low quantum yield is obtained
What happens to Δo when ligands are π-donors? (Page 16)
Δo is reduced because new MOs created are closer in energy to the eg orbitals
What happens to Δo when ligands are π-acceptors? (Page 16)
Δo is increased because new MOs created are further away in energy from the eg orbitals
What order of Δo (from small to large) would you expect from a π-donor, weak π-donor, no π-effect, π-acceptor? (Page 16)
(Small Δo) π-donor < weak π-donor < no π-effect < π-acceptor (Large Δo)
How do you determine the ground state out of a number of microstates? (Page 19)
The state with the highest multiplicity is the ground state, if there are multiple states with the same multiplicity then its the state with the highest L quantum number
Why does t2g ^3 –> t2g ^2 eg ^ 1 transition give two peaks? (Page 21)
Because there are 6 possible microstates that arise from this transition. Only one of the microstates leads to an increase in electron repulsion so 5 of the microstates have the same energy and one has different energy. This gives rise to 2 bands with different energies
Does a strong field ligand give rise to a high or low Δo? (Page 23)
High, high Δo gives rise to low spin complexes as the splitting energy is higher than the pairing energy.
What is a LMCT band? (Page 25)
A band deriving from the movement of electrons from predominantly ligand orbitals (MOs close in energy to orbitals on the ligand) to predominantly metal orbitals (MOs close in energy to orbitals on the metal).
What is a MLCT band? (Page 26)
A transition from an orbital with mainly metal character to an orbital with mainly ligand character
What is the molar extinction coefficient for ligand-field transitions? (Page 26)
ε ~ 100 L mol^-1 cm^-1
This can rise to 250 for tetrahedral complexes
What is the molar extinction coefficient for MLCT transitions? (Page 26)
ε ~ 1000-50000 L mol^-1 cm^-1
