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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/= hcത 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