Download Selection Rules for electronic transitions

Chem 341
Coordination Chemistry – Electronic Spectra
Chapter 11: pp 409-418
HW: 1 2 3c 4c 5a 6 7 8 9 10
Goals: Use Beers Law to calculate Abs; convert wavelength
into wavenumbers; Derive ground state term symbols
Upcoming:
11/18, 11/21: Ch. 11. Electronic transitions in metal complexes
11/23 no class
11/28, 11/30: Acc. Chem. Res. 2003, 36, 876-887
Photochemistry for solar energy
12/2: Exam III
UMass Amherst

Created by Karsten Theis
Selection Rules for electronic transitions
 Laporte selection: transitions between states of
same parity are not allowed (g->u or u->g are
OK)
 Spin selection: transitions involving a change in
spin multiplicity are not allowed (4A2  4T1 ok;
4A  1T not OK)
2
1
 Charge transfers are often very intense
        ...   
2
Which is the ground state for an octahedral d2 ion,
with weak-field ligands?
3
        ...   
Which is the ground state for an octahedral d2 ion,
with weak-field ligands?
 t2g2
3T
1g
(?)
 t2geg
3T
2g
 t2g2
1T (?)
2g
 t2g2
1A
(?)
1g(?)
        ...   
4
Which electronic transition is most intense?
 a) A  B
 b) A  C
 c) A  D

3T
1g
 3T2g

3T
1g
 1T2g

3T
1g
 1A1g
        ...   
5
Which electronic transition is most intense?
 a) A  B

3T
1g
 3T2g
 But note the g g
parity! (not
intense)
        ...   
6
Calculate Ground state Term Symbols
 Describe angular momentum (spin and orbital)
using LS coupling (Russell-Suanders)
 Write out electron config. Find L and S
MS = ms
 ML = ml
 ML = 0, ±1, ±2…±L
MS = 0, ±1, ±2…±S
 The ground state has maximal spin mulitpliticity!
 The ground state has highest S (Hunds rule)
 Tiebreaker; state with highest L
        ...   
7
Easy way for ground state term symbols
 To find maximal spin multiplicity (2S+1) and highest
orbital angular momentum (L):
 Write ml values for the valence subshell(s)
 Eg: d4 ion
 s1p1 ion
        ...   
8
All Terms: go through the microstates! Ex: 15
microstates for p2 ion (eg: C atom)
        ...   
9
Reducing microstates to the Free Atom States
(AKA Terms)
 Break down microstates into
rectangular groups, to figure out L.
 (2S+1)L for spin multiplicity
 Ground state has maximum
multiplicity
        ...   
10
Beers Law
 A = lc l (pathlength); c (mol/L);  (M-1 cm-1)
 A = log (Io/I)
 E = h = hc/= hcത




h = 6.626 x 10-34 Js
c = 2.998 x 108 m s-1
 (frequency, s-1)
ത (wavenumber, cm-1)
        ...   
11
        ...   
12
450 550 650
        ...   
13
        ...   
14
        ...   
15
        ...   
16