selection rules for electronic transitions · 2011-11-21 · 7 calculate ground state term symbols...
TRANSCRIPT
UMass Amherst Created by Karsten Theis
Chem 341
Coordination Chemistry – Electronic SpectraChapter 11: pp 409-418HW: 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 complexes11/23 no class11/28, 11/30: Acc. Chem. Res. 2003, 36, 876-887
Photochemistry for solar energy12/2: Exam III
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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; 4A2 1T1 not OK)
Charge transfers are often very intense
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Which is the ground state for an octahedral d2 ion, with weak-field ligands?
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Which is the ground state for an octahedral d2 ion, with weak-field ligands?
t2g2 3T1g (?)
t2geg3T2g (?)
t2g2 1T2g(?)
t2g2 1A1g(?)
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Which electronic transition is most intense?
a) A B b) A C c) A D
3T1g 3T2g
3T1g 1T2g
3T1g 1A1g
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Which electronic transition is most intense?
a) A B
3T1g 3T2g
But note the g g parity! (not intense)
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Calculate Ground state Term Symbols Describe angular momentum (spin and orbital)
using LS coupling (Russell-Suanders) Write out electron config. Find L and S
ML = ml MS = ms
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
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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
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All Terms: go through the microstates! Ex: 15 microstates for p2 ion (eg: C atom)
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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
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Beers Law A = lc l (pathlength); c (mol/L); (M-1 cm-1) A = log (Io/I) E = h = hc/= hc
h = 6.626 x 10-34 Js c = 2.998 x 108 m s-1
(frequency, s-1) (wavenumber, cm-1)
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450 550 650
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