UV-vis spectra Flashcards
How are absorption band positions described
- In terms of the wavelength corresponding to absorption maximum Amax
What measures absorbance
- Spectrophotometers measure A as wavelength vaires
What is absorbance given by
- A= log10(lo/l)
- lo= intensity of the incident light on the sample
- l= intensity of the emergent light
What is the molar extinction coefficient
- Is a measure of the intensity (strength) of a transition at Amax
- Given by Beer-Lambert Law
What is Beer-Lambert Law
- emax= Amax/cl = molar extinction coefficient
- c= concentration of the solution in mol L^-1
- l= is the path length of the sample in cm
What are the units of emax
- Lmol^-1 cm^-1
As the molar extinction coefficient increases what happens to the colour intensity
- Increases
What do values of emax range from
- Form close to zero (weak absorption)
2. To >10000 L mol^-1 cm^-1 (intense strong absorption
What rationalises the difference between tetrahedral and octahedral emax values
- How allowed the electronic transitions are
What are the 3 selection rules
- The Spin Selection Rule
- The Orbital (Laporte) Selection Rule
- The Parity Rule
What does it mean when a transition is allowed or not
- If a transition is allowed, it has a high probability of occurring and e will be large
- If a transition is forbidden by a selection rule, it has a low probability of occuring and e will be small
State the Spin Selection Rule
- DS = 0 for spin allowed transitions
Describe the Spin Selection Rule
- The spin of an electron cannot change during an electronic transition
- Sum of all the metal d electron spin Ems when added together must be the same before and after the electronic transition
State the Laporte (orbital) Selection Rule
- Dl= +/-1 for orbital allowed transitions
Describe the Laporte (orbital) selection Rule
- l= secondary/orbital quantum number
- l= 0 –> S, l=1 –> p, l=2 –> d
- s p transitions are allowed
- p d transitions are allowed
- dd transitions are not allowed
State the Parity Rule
- In a molecule/ion with a centre of symmetry, a transition must involve parity change
Describe the Parity Rule
- g–>u = parity allowed
2. g–>g = parity forbidden
Describe parity rule for octahedral complexes
- g in symmetry labels for orbital sets (t2g, eg)
2. so transitions are parity forbidden
Describe parity rule for tetrahedral complexes
- no ‘g’ in symmetry label of orbital sets
2. Crystal field transitions- parity allowed
Describe colour in [Mn(H2O)6]2+
- Spin forbidden
- orbital forbidden- d–>d
- parity forbidden- octahedral so g–>g
- Breaks all 3 rules so colour is very weak
Describe colour in [Ti(H2O)6]3+
- Spin allowed
- orbital forbidden- d–>d
- parity forbidden- octahedral so g–>g
- Breaks 2 rules so colour is weak
Describe colour in [CoCl4]2-
- Spin allowed
- orbital forbidden- d–>d
- parity allowed- tetrahedral
- Breaks all 1 rules so colour is stronger
Describe colour in [TiCl6]2-
- Spin allowed
- orbital allowed- p–>d (from ligand)
- parity allowed- electrons from ligand- not CFT
- Breaks NO rules so colour is very strong
What type of transfer occurs in [TiCl6]2-
- Ligand to metal charge transfer (LMCT)
Summarise results of rules
- All d-d transitions are forbidden (orbital selection rule)
- Tetrahedral complexes have more intense colours than octahedral ones (parity selection rules)
- Colours arising from electrons that move from ligand orbitals onto the metal are most intense (fully allowed)- LMCT bands
How can you diagnose geometry using Molar extinction coefficients
- Octahedral, nearly octahedral and square-planar complexes: emax values in 1 or 2 figures
- Tetrahedral complexes: emax values in the hundreds
- Charge-transfer bands (MLCT or LMCT): emax values range in the thousands
Why do d-d transitions occur
- Octahedral complexes can undergo asymmetrical vibrations which destroy the centre of inversion
- Complexes may not be perfectly octahedral- no real inversion centre
- Orbitals can mix with each other- means transitions may not be 100% pure d-d in nature
- A spin forbidden transition has more impact on reducing the colour intensity (emax) than an orbital forbidden transition
Describe how [Co(en)2F2]+ emax varies with different structures
- Cis complex has a larger emax than trans complex
- Due to partial breaking of parity rule in cis complex
- CFT does not recognise the cis-complex as breaking the parity rule- not sophisticated enough
