3: Magnetism and Electronic Spectroscopy Flashcards
What are the magnetic properties of an ion?
are a combination of spin and orbital angular momenta
First row transition metals: magnetic properties
Orbital angular momentum removed or quenched
by interaction of the ligands with the partially occupied d-orbitals
mag moment mostly corresponds to spin only formula
PICTURE -mu SO
can measure and then work out with theory to determine the metal and etc
In lanthanides- why can we no longer assume Orb ang. momentum is quenched.
The 4f orbitals are core like and shielded from the ligand field by the 5s and 5p electrons
orbital angular momentum occurs when electrons moving around d orbitals, creating a magnetic moment- usually quenched because w ligands- in directional orbitals w ligands - dont exist in isolation but in molecular orbital
in lanthanides- molecular orbital non existant- ionic must consider
Lanthanides and actinides: bonding A recap
F orbitals
Lan: The 4f orbitals are core like
Act: The 5f orbitals have greater radial extension
Bonding
Lan: Bonding is ionic in nature
Act: Bonding will have a degree of covalency
Extra
Lan: in the 3+ ion, 5s/p orbitals effectively screen the ligand based orbitals from the 4f
Act: Relativistic effects play a role, early more TM like, late more Lan like
Application of Lanthanides in modern societry
- broadly used in materials for technology due to magnetic properties and electrical spectroscopy
- found is strong permenant magnets- numerous pplications eg. electric cars, wind turbines and headphones
- display technologies and lasers rely on the defined emission spectroscopy of long-lived excited states of lanthanide ions
Microstates
show the number of possible electronic configurations and their energies
Magnetism of the molecules usually come from the ground state
spectroscopy comes from the fact that the excited states can be populated and can produce absorbtion or emission spectra
- population of these states can also arise from ligands producing fluorescence
represented by term symbol
Coordination chemistry simple, electronic structure complex
ligand doesnt influence ions therefore electronic structure stays pretty much same in diff
LS (Russel-Saunders) coupling: theory
Appropriate when the spin-orbit coupling is weak w respect to e-e repulsion
Assumes summation of the spin angular momentua, s, to give total spin S SIGMA s= S (total spin angular mom.
Assumes summation of the orbital angular momenta, l to give orbital angular momentum (total orb ang mom)
From these values we can define a new term J - the total angular momentum of the system given bt the coupling of L and S that can take values L+S, L+S-1, L+S-2…..L-S
for Ln3+ ions, spin orbit coupling is large (1000 cm-1)n comparison to ligand field effects (100 cm-1)
This results in only the ground state J-state being populated
What is the formula for a term symbol
the lande formula
(2S+1)LJ
L =0+
Symbol=S,P, D,F,G,H,I,K… alphabetical
How to determine the ground state term symbol
Ground state term symbol may be determined using Hund’s Rules
Rule 1: The Ground term always has the largest value of S (maximum multiplicity). Electrons in the same orbital repel (pairing energy is minimised by having maximum number of unpaired electrons
Rule 2: If more that one term has the same value of S, the one with highest value of L lies lowest in energy
Rule 3: For a shell less than half filled: J as low as possible
For a shell half filled or more: J as high as possible
Examples look over
2x pictures
What is the magnetism of Ln3+
-paramagnetic
-arises from presence of unpaired 4f e’s
-magnetic moments are little affected by the ligand environment
spin-orbit coupling splits the atomic/ionic term into a series characterized by their j values
SO coupling is usually so large that the excited levels are inaccesible and the magnetic moment is determined by the ground state of the free ion
-the magnetic moment of a J-state is expressed by the Lande formula
What is the lande formula?
Picture
How accurate is the Lande formula?
Excellents agreement with experimental observations
Eu3+ and Sm3+ exceptions
these cases have excited states close in energy to the ground state to be thermally accessible (Boltzmann distribution- excited states are closer than KbT)
this can lead to the contribution to magnetic moment
seen in microstate diagram
Mu(obs) decreases as T decreases therefore
Graph of experimental, spin only and LS
Picture
What are the magnetic properties of the actinides
-complex
-SO coupling is strong, but ligand field effects are of comparable magnitude to spin orbit coupling
-J is no longer a good quantum number and each j state is split by the ligand field
-Experimental values of magnetic moment vary with temperature and in general lower for the corresponding lanthanides
(L is at least partially quenched)
spin only and lande formula inadequate
What is NdFeB
- one of the strongeset known permenant magnetic
- Nd minor componant- aligns all of the magnetic moments of the Iron atoms to produced an extremely strong bulk magnetism
What are the electronic spectra of TM’s like
intense colours
broad d-d bands
-allows ligand metal charge transfer and MLCT
-dominated by ligand field effects
Theory
d-d transitions are forbidden by the laporte selection rules
gain intensity and width by molecular vibrations/vibronic coupling
What is vibronic coupling
molecular vibrations around the metal atom temporarily reduces the symmetry around the metal atom and causes one or more d-orbitals to acquire some p-character allowing the transition to acquire some p-> d character.
What are the electronic spectra of lanthanides like?
Sharp peaks/transitions, less intense colours and the spectra being independant of the environment of the ion
lathanide ions undergo transitions from their ground state to excited state by absorbtion of energy , majority of which involve f-f transitions and are thus laport forbidden (f->f = u-> u)
In lanthanides the 4f orbitals are radially contracted and do not interact strongly with surrounding ligands. Hence crystal/ligand field effects and vibronic coupling in lanthanide complexes are insignificant
What causes strong intensity electronic spectra of some lanthanides
SMI2, CE3+ and TB3+
these are due to allowede 4f to 5d transitions - allow half filled stability or 0 filled etc
Why do Ln3+ complexes fluoresce after UV excitation
1) UV light can promote an electron in ligand to an excited singlet state
(vib relax)
2)non-rad ISC to an excited ligand triplet state can occur
(vib relax)
3) further non- rad ISC to and excited state Ln3+ state
4) relaxation to the ground state by f-f fluoresence
What Ln’s are favourable to fluoresence
Eu and Tb - these have excited states below typical triplet state of ligands
used in cathode ray TV sets
What are electronic spectra of actinides like?
Early An
Increased 5f interaction increased vibronic coupling
bands due to f-f transitions are broader and more intense (10x)
Late An
sharp and low intensity
more closely resemble Ln3+
colours are correspondingly paler
due to stabilisation of 5f as Zeff increases
