Skill 1: Determining a Ground State Term Symbol

Question 1

What principle is the Russell-Saunders Coupling Scheme based on

Answer 1

1. 4f orbitals have limited radial extension and are core-like
2. This means they have little role in bonding - the orbital angular momentum is not quenched
3. The magnetic properties of the Ln3+ ions are very well described from the coupling of spin and orbital angular momenta

Question 2

What is S in the Russell-Saunders Coupling Scheme

Answer 2

1. Assumes that spins of the electrons couple
2. Total spin angular momentum
3. sum of (s) = total S

Question 3

What is L in the Russell-Saunders Coupling Scheme

Answer 3

1. Orbital momenta also couple
2. Total orbital angular momentum
3. Sum of (l) = total L

Question 4

What is J in the Russell-Saunders Coupling Scheme

Answer 4

1. Total angular momentum of the system given by the coupling of S and L
2. i.e. S + L = J or (L- S) etc

Question 5

What is spin orbit coupling compared to ligand field effects and what is consequence

Answer 5

1. Typically large (1000cm^-1) in comparison to ligand field effects (100 cm^-1)
2. Means only the ground J-state is populated
3. Magnetism is essentially independent of environment

Question 6

What is Term symbol

Answer 6

1. (2S+1)L(J)

Question 7

What are the rules that determine ground state symbols

Answer 7

1. Hunds Rules

Question 8

What is Hund’s first rule

Answer 8

1. The ground term always has the largest value of S (maximum multiplicity)
2. Have the max number of unpaired electrons

Question 9

What is Hunds second rule

Answer 9

1. If more than one term has the same value of S, the one with the highest value of L lies lowest in energy

Question 10

What is Hund’s third rule

Answer 10

1. For a shell less than half-filled, J is as low as possible - L-S
2. For a shell more than half-filled J is as high as possible - L+S

Question 11

How do you find S

Answer 11

1. Each unpaired e- = 1/2
2. Count the spins
3. Only unpaired electrons though
4. Then the first part of Term symbol in superscript is 2S+1

Question 12

How do you find L

Answer 12

1. Add the l values
2. e.g. for f orbitals +3,+2,+1,0,-1,-2,-3
3. Highest value of L is required
4. Fill e- from +3 first
5. If paired, count the l number twice
6. Then the L is assigned a letter

Question 13

What letters are assigned to what numbers of L

Answer 13

1. 0=S
2. 1=P
3. 2= D
   4.3=F
4. 4=G
5. 5=H
6. 6= I
7. 7=K

Question 14

How do you find J

Answer 14

1. Less than half filled, J=L-S
2. More than half filled J= L+S
3. subscript after letter

Question 15

Can J be negative

Answer 15

1. NO

Question 16

Describe transitions of lanthanides

Answer 16

1. Electrons in lanthanide atoms and complexes may be excited from their ground states by absorption of energy
2. Majority of transitions in Ln3+ ions occur from the ground state to excited 4f levels (f-f transitions)
3. Orbitally forbidden by the Laporte selection rule

Question 17

How are transition metals relaxed

Answer 17

1. Relaxed by vibronic coupling

Question 18

Are crystal field effects/ vibronic coupling important for lanthanide 3+ complexes

Answer 18

1. More radially contracted 4f orbitals do not interact strongly with surrounding ligands
2. Crystal/ligand field effects/vibronic coupling lanthanide 3+ complexes insignificant

Question 19

What are the consequences for Ln3+ absorption spectra

Answer 19

1. The colours of Ln3+ compounds are usually less intense (pale) than those associated with d-d transitions in d-block compounds
2. The f-f absorption bands are very sharp and weak (fingerprinting of Ln3+). [d-d] transitions in transition metal compounds are also orbitally forbidden, but gain intensity from and are broadened by the effects of molecular vibrations in distorting the crystal field
3. Optical spectra are virtually independent of environment and similar spectra in are obtained in the gas/solution/solid phases (sharp lines like typical gas atom spectra)

Question 20

Describe the spectra of f block compared to d-block

Answer 20

1. Sharper and weaker than d-block

Question 21

What are exceptions for spectra of f-block

Answer 21

1. Ce3+ and Tb3+ - more intense electronic absorption bands in the UV
2. Sm2+

Question 22

Why do Ce3+ and Tb3+ have more intense electronic absorption bands in the UV

Answer 22

1. Not f-f transitions but [Xe]4fn –> [Xe]4fn-15d1 promotions
2. Not orbitally forbidden as f-d transitions
3. Occur because 4fn-1 for Ce3+ = 0 = empty sub-shell : Tb3+ = 7 half-filled - instead promoted to d-orbital

Question 23

The colours of divalent Ln2+ compounds in spectra

Answer 23

1. The colours of divalent Ln2+ compounds tend to be very much more intense
2. [Xe]4f6 => [Xe]4f5 5d1
3. For example, SmI2 is an intense purple colour in the solid state.
   4.This is due to the reduced charge which means that a (completely allowed) 4f to 5d transition ([Xe]4f6 –> [Xe]4f55d1) now lies in the visible party of the electromagnetic spectrum.

Question 24

Describe fluorescence of Ln3+ complexes

Answer 24

1. Many Ln3+ complexes fluoresce after UV excitation - f-f transitions (emission not absorption)
2. Due to the deep seated nature of the 4f orbitals, this has to happen via initial excitation of a coordinated ligand which behaves like an antenna
3. The UV light promotes an electron to an excited ligand (antenna) singlet state
4. Non-radiative intersystem crossing (ISC) to an excited ligand triplet state
5. Further ISC to an excited state of the Ln3+ ion
6. Relaxation to the ground state by f-f fluorescence

Question 25

What are the two most useful ions for fluorescence

Answer 25

1. Tb3+ and Eu3+ are the two most useful ions
2. Fluoresce with green and red respectively

Question 26

What are uses of Ln’s fluorescence

Answer 26

1. Such rare earth phosphors are employed in cathode ray television sets, the screen of which is made up of tiny clusters of red, green and blue phosphor dots.
2. The red phosphor is typically Eu3+ (in Y2O2), while Tb3+ (in La2O2S) has been employed as the green emitter.
3. Lanthanide coordination complexes are currently attracting attention for the generation of colour in flat panel displays, as luminescent sensors in chemical analysis and (for ions such as Yb3+) as infra-red emitters for medical diagnostics.

Question 27

What are two other uses of lanthanides fluorescence

Answer 27

1. Banknote anti counterfeiting
2. Lanthanide lasers

Question 28

Describes how lanthanides are used in anticountfeitting

Answer 28

1. Eu3+ gives very narrow, single wavelength emission
2. Can be used to spot counterfeit banknotes.

Question 29

Describe how lanthanides are used in lasers

Answer 29

1. Single wavelength emission can be used to create lasers which emit narrow wavelength, coherent light
2. Nd:YAG (neodymium:yttrium aluminium garnet) most well-known class.
3. Consist of yttrium aluminium garnet doped with ~1% Nd3+ ions of the formula Ndx:Y3-xAl5O12.
4. Pumping at ~800 nm occupies high energy levels which decay to metastable 4F3/2
5. Stimulated emission from 4F3/2–> 4I11/2 states emits at 1064 nm (infra-red)

Question 30

What is Nd:YAG laser

Answer 30

1. Nd doped in YAG
2. Reduces the cost

Question 31

Why is Uj value likely to be correct for both aqueous and gas-phase Tm3+ ions.

Answer 31

1. Likely to be similar due to large s-o coupling, so only ground state term occupied and lack of 4f interaction with the environment meaning the magnetism is environment independent.

Question 32

A student is intending to develop a new lanthanide compound which can provide a yellow light emission under UV irradiation. Which lanthanide centre should they use and what characteristics should they consider for possible ligands?

Answer 32

1. Dy shows a yellow emission and as lanthanide emissions are independent of environment this should be the correct lanthanide to use.
2. The ligand should possess a system capable of absorbing light such as an extended π-system
3. the ligand should possess a long-lived triplet excited state which is higher in energy than the emissive excited state of dysprosium.