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A CH20147 > UV-vis spectra > Flashcards

UV-vis spectra Flashcards

1
Q

How are absorption band positions described

A
  1. In terms of the wavelength corresponding to absorption maximum Amax
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2
Q

What measures absorbance

A
  1. Spectrophotometers measure A as wavelength vaires
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3
Q

What is absorbance given by

A
  1. A= log10(lo/l)
  2. lo= intensity of the incident light on the sample
  3. l= intensity of the emergent light
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4
Q

What is the molar extinction coefficient

A
  1. Is a measure of the intensity (strength) of a transition at Amax
  2. Given by Beer-Lambert Law
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5
Q

What is Beer-Lambert Law

A
  1. emax= Amax/cl = molar extinction coefficient
  2. c= concentration of the solution in mol L^-1
  3. l= is the path length of the sample in cm
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6
Q

What are the units of emax

A
  1. Lmol^-1 cm^-1
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7
Q

As the molar extinction coefficient increases what happens to the colour intensity

A
  1. Increases
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8
Q

What do values of emax range from

A
  1. Form close to zero (weak absorption)

2. To >10000 L mol^-1 cm^-1 (intense strong absorption

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9
Q

What rationalises the difference between tetrahedral and octahedral emax values

A
  1. How allowed the electronic transitions are
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10
Q

What are the 3 selection rules

A
  1. The Spin Selection Rule
  2. The Orbital (Laporte) Selection Rule
  3. The Parity Rule
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11
Q

What does it mean when a transition is allowed or not

A
  1. If a transition is allowed, it has a high probability of occurring and e will be large
  2. If a transition is forbidden by a selection rule, it has a low probability of occuring and e will be small
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12
Q

State the Spin Selection Rule

A
  1. DS = 0 for spin allowed transitions
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13
Q

Describe the Spin Selection Rule

A
  1. The spin of an electron cannot change during an electronic transition
  2. Sum of all the metal d electron spin Ems when added together must be the same before and after the electronic transition
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14
Q

State the Laporte (orbital) Selection Rule

A
  1. Dl= +/-1 for orbital allowed transitions
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15
Q

Describe the Laporte (orbital) selection Rule

A
  1. l= secondary/orbital quantum number
  2. l= 0 –> S, l=1 –> p, l=2 –> d
  3. s p transitions are allowed
  4. p d transitions are allowed
  5. dd transitions are not allowed
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16
Q

State the Parity Rule

A
  1. In a molecule/ion with a centre of symmetry, a transition must involve parity change
17
Q

Describe the Parity Rule

A
  1. g–>u = parity allowed

2. g–>g = parity forbidden

18
Q

Describe parity rule for octahedral complexes

A
  1. g in symmetry labels for orbital sets (t2g, eg)

2. so transitions are parity forbidden

19
Q

Describe parity rule for tetrahedral complexes

A
  1. no ‘g’ in symmetry label of orbital sets

2. Crystal field transitions- parity allowed

20
Q

Describe colour in [Mn(H2O)6]2+

A
  1. Spin forbidden
  2. orbital forbidden- d–>d
  3. parity forbidden- octahedral so g–>g
  4. Breaks all 3 rules so colour is very weak
21
Q

Describe colour in [Ti(H2O)6]3+

A
  1. Spin allowed
  2. orbital forbidden- d–>d
  3. parity forbidden- octahedral so g–>g
  4. Breaks 2 rules so colour is weak
22
Q

Describe colour in [CoCl4]2-

A
  1. Spin allowed
  2. orbital forbidden- d–>d
  3. parity allowed- tetrahedral
  4. Breaks all 1 rules so colour is stronger
23
Q

Describe colour in [TiCl6]2-

A
  1. Spin allowed
  2. orbital allowed- p–>d (from ligand)
  3. parity allowed- electrons from ligand- not CFT
  4. Breaks NO rules so colour is very strong
24
Q

What type of transfer occurs in [TiCl6]2-

A
  1. Ligand to metal charge transfer (LMCT)
25
Q

Summarise results of rules

A
  1. All d-d transitions are forbidden (orbital selection rule)
  2. Tetrahedral complexes have more intense colours than octahedral ones (parity selection rules)
  3. Colours arising from electrons that move from ligand orbitals onto the metal are most intense (fully allowed)- LMCT bands
26
Q

How can you diagnose geometry using Molar extinction coefficients

A
  1. Octahedral, nearly octahedral and square-planar complexes: emax values in 1 or 2 figures
  2. Tetrahedral complexes: emax values in the hundreds
  3. Charge-transfer bands (MLCT or LMCT): emax values range in the thousands
27
Q

Why do d-d transitions occur

A
  1. Octahedral complexes can undergo asymmetrical vibrations which destroy the centre of inversion
  2. Complexes may not be perfectly octahedral- no real inversion centre
  3. Orbitals can mix with each other- means transitions may not be 100% pure d-d in nature
  4. A spin forbidden transition has more impact on reducing the colour intensity (emax) than an orbital forbidden transition
28
Q

Describe how [Co(en)2F2]+ emax varies with different structures

A
  1. Cis complex has a larger emax than trans complex
  2. Due to partial breaking of parity rule in cis complex
  3. CFT does not recognise the cis-complex as breaking the parity rule- not sophisticated enough

A CH20147 (10 decks)

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