electronic spectra of complexes - radchem.nevada.eduradchem.nevada.edu/classes/chem431/lectures/lect...
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13-1
Electronic Spectra of Complexes
• Interpret electronic spectra of coordination compounds❧ Correlate with bonding❧ Orbital filling and electronic transitions
➠ Electron-electron repulsion❧ Application of MO theory
• Spectroscopic terms• Complex transitions
❧ Ligand field❧ Charge transfer❧ Selection rules
13-2
Electronic Spectra• Cr(NH3)6
3+
❧ d3
❧ Weak low energy transition➠ Spin forbidden
❧ 2 stronger transitions➠ Spin allowed
* t2g and egtransitions
➘ Lower energy to higher energy
❧ CT at higher energy➠ Ligand to metal
transition
13-3
Spectroscopic terms• Electronic designation
❧ Configuration does not provide all information➠ Does not contain angular or spin configuration
* 2p2
➘ p x, y, or z; 1, 0, -1;ml➘ Spin pair or unpaired; -1/2, +1/2; ms➘ Describes mircostate➘ Energy of different microstate can vary due to
interelectronic repulsion➘ Same energy levels are called terms
❧ 3d and 4f➠ Electron spin important in energy, then orbital
* Total spin S and total orbital L➘ Russell-Saunders coupling
❧ In heavier atoms (4d, 5d and 5f)➠ spin-orbital coupling
* Total angular momentum j➘ j-j coupling
13-4
Spectroscopic terms• For 2 electrons
❧ S=s1+s2, s1+s2-1, …| s1-s2 |❧ L=l1+l2, l1+l2-1, … | l1-l2 |
➠ S or L cannot be negative• d2
❧ s=1/2➠ S=1, 0
❧ l=2➠ L=4, 3, 2, 1, 0
• d3
❧ Combine l3 and s3 with L and S from 2 electron system• Angular momentum
❧ ML➠ L, L-1,,-L
* 2L+1❧ MS
➠ S, S-1,,,-S* 2S+1
• For a given microstate, ML and Ms is sum of each electron state❧ (0+, -1-); MS=0 and ML=-1
13-5
Spectroscopic terms
• L=S, P, D, F, G…..• S=2S+1
❧ Written as SL➠ Term symbol
• 3P❧ L=1, S=1
➠ Triplet state• What is the term symbol for s1p1
❧ s=0, p=1, L=1=P, S=0 or 1➠ 1P and 3P
• For p1d1
❧ L=1+2, F; S=0 or 1➠ 1F and 3F
13-6
Term symbols• d2 configuration
❧ Pauli principle and Hund’s rules limit configurations
❧ Largest ML is both electrons in l=2, spin paired➠ L=4, S=0, 1G
* 1x9 states❧ L=3, S=1,0,-1 3F
➠ 3x7 states❧ L=2, S=0, 1D
➠ 1x5 states❧ L=1, S=1,0,-1, 3P❧ L=0, 1S
13-7
Term symbols• Lowest energy identified by Hund’s rules
❧ Lowest energy with parallel spin➠ Triple state
* 3F and 3P states➘ Highest MS value most stable
➠ Greater L, lower energy* 3F
➘ Highest L with highest MS state❧ Order
➠ Predicted; 3F<3P<1G<1D<1S➠ From spectroscopy; 3F<1D<3P<1G<1S
• Ground state term for Cr3+
❧ 3d3
➠ 3 electrons with same spin, S=3/2; MS=4➠ Each electron in different orbital
* L=2+1+0=3; F; 4F• Ground state of Mn2+
❧ 3d5
➠ 5 electrons with same spin, 5/2, MS=6❧ All must occupy different orbital
➠ L=2+1+0-1-2=0* 6S
13-8
Term • Racah parameter
❧ Electron-electron repulsion
❧ Parameter based on combination of 3 terms➠ A, B, and C
❧ All positive➠ 1S=A+14B+7C➠ 1G=A+4B+2C➠ 1D=A-3B+2C➠ 3P=A+7B➠ 3F=A-8B
13-9
Ligand Field Transitions• Cr(NH3)6
3+
❧ Oh configuration, d3
➠ t2g3
❧ Transition➠ t2g
2eg1 t2g
3
* Near 400 nm➘ Two peaks
❧ Molecular term symbols based on multiplicity and orbital
➠ 4T2g4A2g (higher
energy) and 4T1g4A2g
* Superscript denotes S=3/2
* Orbitals from character tables
• Weak and strong fields❧ Ignore electron repulsion in
strong field❧ Ligands of increasing field
strength effect terms
13-10
Ligand field
• Tanabe-Sugano diagrams❧ Symmetry and
ligand field strength➠ Energy and
Racah term B❧ States of same
symmetry avoid crossing
❧ Permits evaluation of orbitals➠ At 0 energy
lowest term
13-11
Charge transfer bands• High energy absorbance
❧ Energy greater than d-dtransition➠ Electron moves between
orbitals* Metal to ligand* Ligand to metal
➠ Sensitive to solvent• LMCT
❧ High oxidation state metal ion❧ Lone pair ligand donor
• MLCT❧ Low lying pi, aromatic❧ Low oxidation state metal
➠ High d orbital energy
13-12
Selection rules• Allowed and forbidden transitions
❧ ∆S=0 is allowed➠ Otherwise spin forbidden
* More likely in heavier atoms➘ 4d and 5d
❧ Transitions must involve electronic dipole change to absorb light➠ d transition become more allowed with
asymmetric vibrations➠ f sharp due to shielding
13-13
Transition metals
13-14
Actinide transitions
400 500 600 700 8000
1
2
3
4
5
Abs
orba
nce
Wavelength (nm)
Normal Heavy Light
Pu4+ (489 nm)
Pu6+(835 nm)
Figure 2: UV-vis spectra of organic phases for 13M HNO3 system
13-15
Energy diagram
13-16
Excitation• Rates of absorption are high (fs)
❧ Fluorescence emission has a longer lifetime (ns)• Relaxation
❧ Vibration❧ Internal conversion
➠ Overlap vibration levels❧ External conversion
➠ Quenching with solvent❧ Intersystem crossing
➠ Spin flip• Quantum Yield
❧ Ratio of molecules that luminence to total excited molecules
❧ Mainly n->π∗ or π ->π∗
13-17
Fluorescence
• Increase fluorescence❧ Aromatic groups❧ Rigidity
• decrease fluorescence❧ Temperature
increase❧ Heavy atoms in
solvent❧ Dissolved O2
• pH can change species• Self absorption at high
concentration
13-18
Emission and Excitation
13-19
Circular Dichroism
• Chiral complexes❧ Different
interactions for polarized photons➠ Plot difference
in absorbance for polarized light
13-20
Electron Paramagnetic resonance
B0=0 B0≠0
me=+1/2
Energy levels split in the presence of external field
∆E=geβeB0=hν
me=-1/2
13-21
Electron-Nuclear Hyperfine Interaction
me=+1/2
me=-1/2
B0=0 B0≠0
mI
-1
me=+1/2
+10
+1
-10
me=-1/2
Hyperfine interaction
• Electron-nuclear interaction further split the energy levels• Transitions are allowed between Dme=1, DmI=0.• For an I=1 nucleus, 3 allowed transitions.
13-22
EPR spectrum
Absorbancespectrum
1st derivativespectrum
mI=0 mI=+1mI=-1
Magnetic field
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