Physical Methods in Biological Inorganic Chemistry Flashcards
What is Spectroscopy?
Anything where we have plotted energy vs transmission
What is the method behind X-ray absorption Spectroscopy (XAS)?
The absorption of X-rays can excite the core 2p electrons of a metal
What type of spectroscopy graph is produced when the core 2p electrons of a metal are excited in XAS
What affect does higher nuclear charge have on the edge?
Hence what can we deduct from the edge?
- The higher nuclear charge the higher the edge
- The edge position can tell us which metal we have in the protein
- What the oxidation state is - the higher the oxidation state the higher the edge
What is the Pre-edge?
What does the intensity of the pre-edge tell us?
- Appears before the edge
- Absorbtion of X-rays can excite the core 2p electrons into the empty d-orbitals
- The intensity of the pre-edge tell us about covalency - the more intense the pre-edge intensty, the more metal-ligand mixing or covalency there is (only p→d and s→d allowed)
What is the Extended X-ray Absorption Fine Structure (EXAFS) and why does it occur?
- When electrons are ejected from the metal centre it creates waves
- The waves can extend out and have the possibility to bump into some neighbouring ligands (with electron density associated with them)
- The ligand act as a mirror an sends the wave back
- When we have a wave going in one direction and a wave going in the other direction, they interfere with one another called constructive/deconstructive interference, resulting in the EXAFS
What does the EXAFS inform us of?
- We can use the interference pattern to analyse the number off ligand donor atoms we have around a metal centre (coordination number)
- It can also tell us the distance from the ligands to the metal
- Sometimes even what the ligand atom is
What is the benefit of X-ray Absorption Spectroscopy over X-ray Crystallography?
- You do not need any crystals for X-ray Absorption Spectroscopy
- Also can target metal atom coordination sphere exculsively
- Resolution is also good (0.03Å)
What are some disadvantages of X-ray Absorption Spectroscopy?
- Cannot tell the difference say between oxygen and nitrogen as they effectively mirrror as good as one another during back reflection
- There is also a ±25% error in predicting the coordination number - not always very helpful
- Only get an average coordination sphere - i.e. if you have the same metal centre with 4x2 difference ligands, it will create a hybrid of both
What is Electron Paramagnetic Resonance Spectroscopy?
- A technique which enable you to look at paramagnetic centres (centres containing 1+ unpaired electrons)
- Unpaired electrons behave like little bar magnets which when placed in a magnetic field (B₀) and the field is increased in size, the energy difference between the up and down electrons are pointing relative to the magnetic field increases
Electrons which are parallel to the magnetic field go….
Electrons which are antiparallel to the magnetic field go…
The difference between these two electrons is given the value?
- Electron which are parallel to the magnetic field go down in energy
- Electrons which are parallel to the magnetic field go up in energy
- gμbB₀
What do we look to solve in gμbB₀
(the difference in energy between the parallel and antiparallel electrons)
μb - is a constant for Bohr magnetism
B₀ - is the known magnetic field
so we look to work out g
Tells us what the electrons are doing and where they live in the metal - d-orbitals and crystal field
What is the difference between NMR and Electron paramagnetic resonance spectroscopy (EPR)
- NMR uses Radiowaves while EPR uses Microwaves (X-band)
- In contrast to NMR the frequence of EPR is kept fixed and the magnetic field is increased untill the seperation between the up and down spins matches the energy of the microwaves - this results in the signal
From the signal of EPR, what is recorded?
A derivative of the absorption is usually obtained
What is one big similarity between NMR and EPR?
- The EPR signal can be split by hyperfine coupling, A, through magnetic interactions with the transition metal nucleus
- E.g. Cu - (I=1.5) 2I+1 = 4 lines can be observed in the EPR spectrum of Cu(II) compounds
- The coupling constant A will inform on covalency
Why would we use Raman Spectroscopy over IR for metal centre analysis as a form of vibrational spectroscopy?
- Infra red vibrational spectra of proteins are extremely complex because all chromophores contribute
- Resonance Raman Spectroscopy can be used to probe selectively the ligand vibrations around the metal centre
What is the process behind raman spectroscopy?
- Raman involves shining a lazer light and collect data at 90°
- Then the distance between the light that gets emitted leaving the sample in the higher vibration level vs the light that comes straight out - results in a raman shift
- Raman leaves the molecule in a vibrationally excited state (this is a different mechanism to IR however
What does the intensity of Raman Spectra depend on?
- The intensity of Raman spectra depends on vₒ
- As vₒ approaches the frequency allowed for UV/vis transition in metal centre, the modes that are vibronically active in the electron transition become enhanced
How can LMCT be measured in a Raman spectrum?
- Example: [Fe(Scys)₄] centres - if the UV/vis transition results from S→Fe LMCT and the Raman experiment is performed at the energy of this LMCT transition, the the Fe-S bond stretches should be enhanced in the RR spectrum
- We can use the Resonance Raman to work out if bond is LMCT or d-d (if is LMCT we will see Raman metal-sulfur vibrations and d-d is weak/no Raman m-s vibrations)
Fill the molecular orbital diagram with the correct number of electrons to calculate the bond order and rationalise the Raman stretching frequenies for O₂
Paramagnetic or Diamagnetic?
O = 2s² 2p⁴
O₂ = 8(½) + 4(-½) = 2
bond order = 2
(paramagnetic - unpaired p electrons)
Raman is decreasing because the bond order (strenght) is decreasing - use raman to work out nature of O₂ unit
Fill the molecular orbital diagram with the correct number of electrons to calculate the bond order and rationalise the Raman stretching frequenies for O₂⁻
Paramagnetic or Diamagnetic?
O⁻ = 2s² 2p⁵
O₂⁻ = 8(½) + 5(-½) = 1.5
Bond order = 1.5
Paramagnetic - unparied electrons
Raman is decreasing because the bond order (strenght) is decreasing - use raman to work out nature of O₂ unit
Fill the molecular orbital diagram with the correct number of electrons to calculate the bond order and rationalise the Raman stretching frequenies for O₂
O⁻ = 2s² 2p⁵
O₂²⁻ = 8(½) + 6(-½) = 1
Bond order = 1
Diamagnetic - no unpaired electrons
Raman is decreasing because the bond order (strenght) is decreasing - use raman to work out nature of O₂ unit
