Electronic Spectroscopy Flashcards
What electronic transitions are allowed? 3 selection rules
Spin selection rule: Upon excitation ΔS = 0. Spin multiplicity must not
change.
Laporte rule: Transitions between states of the same symmetry (with
respect to inversion) are forbidden! (eg g-g not allowed. Td complexes are brighter - less restrictions.
Angular momentum rule: The change in total angular momentum can be ΔL=0, ±1 but L=0↔L=0 transitions are
forbidden.
LMCT and MLCT - orbitals, metal, ligands, where in the spectrum.
LMCT:
-metal-based orbital gains an electron - metal is reduced
-High oxidation state metal – low-lying empty orbitals
-Ligands will have lone pairs at relatively high energy
-Often absorb in visible region.
MLCT:
-Ligand-based orbital gains an electron during the transition – metal is oxidised.
-Low oxidation state metal
-Ligands will have low-lying empty orbitals (often p* orbitals)
-Often absorb in visible region.
Charges transfers are highly sensitive to solvent polarity - solvatochromic - red and blue shift
Red shift - increased wavelength - decreased wavenumber - polar solvents
Blue - decreased wavelength - increased wavenumber - apolar solvents
n = principal QN.
l = orbital angular momentum QN.
ml = magnetic QN.
ms = spin QN.
n = 1, 2, 3, 4 etc
l = 0, 1, 2, 3, (n-1) etc. (s, p, d, f etc.)
ml = -l ≤ 0 ≤ +l
ms = -½ or +½
L = total orbital angular momentum QN
S = total spin QN
ML and MS describe L and S for microstates.
ML = ML = +L, +(L-1)…. -L
MS = +S, +(S-1)…. -S
A state is a collection of degenerate microstates. State is classified by a term symbol 2S+1 L.
Number of microstates in each state?
The number of microstates in each state is
defined by (2S+1)(2L+1).
Determination of ground state from term symbols - Hund’s rules (2 points)
1) Term with the greatest spin multiplicity lies at
the lowest energy (triplet E < singlet E)
2) For a given multiplicity, the greater the value of
L for a term, the lower the energy
Racah parameters - ABC - what they summarise and describe.
They summarise how the energy of a Term is affected
by interelectronic repulsions
A - Average of the total interelectron repulsion
B, C - Relate to the repulsion energies between individual d-electrons
A - always cancels out. C ≈ 4B. C also only occurs in excited states which have different multiplicity than the ground state.
Such excitations would be formally forbidden.
Racah parameter B
Spectroscopy is used to measure B.
B increases as the oxidation state increases. B increases as the ion gets smaller (The size of the ion has a greater impact on electronic repulsion than the number of electrons.)
Components of three electrode cell: working electrode, counter electrode, reference electrode and supporting electrolyte
1) A working electrode – we will control the potential of this
electrode to inject electrons into or remove electrons from the dissolved analyte and monitor the current.
2) A counter electrode – as electrons are injected or removed a
current will develop, the counter electrode completes the
circuit and allows current to flow.
3) A reference electrode – a stable, self contained redox couple
which the voltage of the working electrode is measured with
respect to.
4) A supporting electrolyte dissolved in a suitable solvent – the
electrolyte allows the solvent to conduct.
5) Electrodes are wired to potentiostat.
Mass transport on the electrode:
1. Diffusion - Concentration controlled
2. Convection - Mechanically controlled
3. Migration - Electrostatically controlled
- Diffusion - Diffusion is driven entropically and serves to minimise variations in the
concentration distribution throughout a system - Convection - Movement induced by mechanical
force acting on the solution is
convection. Natural convection (eg.
thermal gradients) and forced convection (eg. stirring) exist. - Migration - Migration arises from the
electrostatic interactions between
the electric field generated at the
electrode surface and polarised or charged analytes in solution.
Cyclic voltammetry - peak-to-peak separation, current peak heights, Randles Sevcik equation.
𝐼𝑝 ∝ 𝜐1/2 (scan rate square dependence - surface of the electrode), 𝐸𝑝 𝑖𝑠 𝑖𝑛𝑑𝑒𝑝𝑒𝑛𝑑𝑒𝑛𝑡 𝑜𝑓 𝜐
Confirm is process is reversible - change scan rate
5 information bits from isotropic EPR spectrum (high symmetry, eg tetrahedral or octahedral)
i) g-value (parallels chemical shift in NMR)
ii) hyperfine coupling pattern (number of peaks = 2nI+1) - number and types of magnetic nuclei with unpaired electrons.
iii) hyperfine coupling
constants (A) and the g-values from spacing of the lines and the center of gravity of spectrum. Relates to electron spin density distribution.
iv) integrated intensity is proportional to concentration of radicals.
v) linewidth is related to the rate of rotation and rate of fluxional processes.
Anisotropic EPR spectrum
In less symmetrical species, consider g-factor to be a symmetry tensor, with three principle g-factors gxx, gyy and gzz, that are averaged out to give a single isotropic g-factor.
System with axial symmetry - g|| and g⊥.
