How to identify types of transition in experimental spectra
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4 How to identify types of transition in experimental spectra 1. intensity 2. Band width 3. polarization Intensities are governed by how well the selection rules can be applied to the molecule under investigation. No change in spin multiplicity between ground and excited state (Spin selection rule). Broken via spin-orbit coupling. Transitions must involve change in dipole moment of molecule, for atomic spectra this means 5 = ±1 (Laporte selection rule, s Û p, p Û d, but d Û/ d etc. ) broken via covalency effects, charge transfer, and by vibronic coupling. 20
5 Typical Electronic Intensities Molar Absorptivity Type Examples s/f & L/f high spin d 5 ionic complexes 1 10 s/f & L/f tetrahedral d 5 covalent complexes s/a & L/f s/f & L/f s/a & L/f s/a & L/f octahedral ionic complexes 5d x complexes octahedral complexes with organic ligands; square planar complexes tetrahedral complexes; square planar complexes with organic ligands s/a & L/a Some MLCT in complexes with unsaturated ligands s/a & L/f non-centrosymmetrtic complexes with covalent ligands s/a & L/a Many CT transitions; symmetry-allowed transitions in organic molecules 21
6 Band Widths 1. Absorption bands can appear broad because they include several unresolved components. These might result from low-symmetry ligand fields, spin-orbit coupling (both f which split the terms into more levels, often closely spaced, or from unresolved vibrational fine structure. 2. Metal-ligand vibrations will result in -values that are time-dependent. Franck-Condon effects lead to a range of excitation energies. Since all spin-allowed (and some spin-forbidden) transitions will result in M L bond-lengths that are different for the ground and excited states, the energies of these states will not run parallel in the Orgel or Tanabe-Sugano diagrams, and the resulting bands will be broad as shown by the above diagram. Some spin-forbidden transitions do not incur bond-length changes, and the corresponding absorption bands are narrow. 22
7 In general the energies of transitions that correspond to excitations of electrons from t 2g to e g levels (or corresponding inter-configurational transitions in other symmetries) will always be sensitive to the value of, and the resulting absorption bands will be broad. This is the case for all spin-allowed and some spin-forbidden transitions. Other spin-forbidden transitions correspond to intraconfigurational transitions and will be independent of. Example of [Mn(H 2 O) 6 ] 2+ Note narrow bands for transitions to 4 A 1g and 4 E g (G). These should appear at the same energy as 4 G 6 S in atomic spectrum of Mn 2+. First clear indication of the nephelauxetic affect (reduction of B and C parameters in the complex ion.) Nephelauxetic Series (in terms of ligand donor atom): F > O > N > Cl > Br > I > S > Se > As...decreasing value of B
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9 Spin-allowed d-d transitions gain intensity by mixing d and p character. This occurs easily in non centrosymmetric complexes, e.g. both sp 3 and d 3 s hybridization yields a tetrahedral set of orbitals, so any linear combination of these will also. In centrosymmetric (and non centrosymmetric) symmetries, vibronic coupling For an octahedral complex ML 6 there are 15 normal modes of vibration. We can show them to be A 1g + E g + 2T 1u + T 2g + T 2u Notice that two of these have u symmetry, and are shown below 25
10 Except at the midpoint of these vibrations, the molecule has no center of symmetry, and the bonding must involve some d-p mixing. Take the example of Co III complexes (t 2g 6 or 1 A 1g ) Two possible transitions, to T 1g at ~ cm -1 (corresponds to ) T 2g at ~ cm -1 Recalling that the dipole operator (x,y,z) transforms as T 1u in O h, the 1 T 1g 1 A 1g and 1 T 2g 1 A 1g transitions can only be observed if T 1g T 1u A 1g (or T 2g T 1u A 1g ) contains a representation with the symmetry of a normal mode of vibration of the complex. This clearly occurs with the T 1u vibration shown above. 26
11 For mixed ligand complexes the effective symmetry for spectroscopic purposes depends upon the positions of the ligands in the Spectrochemical Series. (+ve e % ) I < Br < S 2 < N 3 < F < OH < O 2 < H 2 O < NH 3 < en < NO 2 < CN < CO (-ve e % ) For CoA 5 B: if A and B are well separated in the spectrochemical series the T 1 A 1 band is split since T 1 Ú A 2 +E in an axial complex. Identification of cis and trans isomers of CoA 4 B cis isomer has no center of symmetry - bands should be more intense than for trans isomer 4. Splitting of the T 1 state is greater in the trans isomer Why? Use the Angular Overlap Model, ignoring % effects Refer to Table of Scaling Factors for e ) and assume that e ) (B) > e ) (A) trans isomer with B-ligands in positions 1 and 6 cis isomer with B-ligands in positions 2 and 3 27
12 E(z 2 (x 2 y 2 )) = 2e B 2e A e B e A for the trans isomer and for the cis isomer In practice the cis isomer shows no band splitting. 28
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