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6.1 Predict the vanadium 2p XPS spectrum of the oxide VO.
The vanadium oxidation state in VO is solely V2+. We have seen that there is a gradual decrease in the vanadium 2p electron binding energies going from V2O5 (3d0) to V2O3 (3d2) which reflects the increased shielding of the 2p electron as the number of the 3d electrons increases. V2+ should continue this trend so a binding energy of ~515 eV would be expected and a fairly sharp peak (as only a single oxidation state is present).
6.2 Would the 4f electrons in Hf(BH4)4 be expected to be ionised below 20 eV and, therefore, be observable in its UPS spectrum?
The 4f orbitals fall in energy with increasing atomic number much faster than the 5f and become core-like as soon at the end of the lanthanide block is reached. Their energy will, therefore, be significantly lower than the ionisation energies of 6s and 5d electrons that are seen from Hf(BH4)4 in the range 11-20 eV. Therefore, ionisation of 4f electrons would not be expected to be seen below 20 eV; in fact the ionisation energy of 4f electrons occurs at around 25 eV. See Inorg. Chem. 2005, 44, 7781-7793.
6.3 The following data give the energy (eV) of the main pre-edge feature in Mn K-edge XAS as a function of oxidation state.
Mn(II) 6540.6, Mn(III) 6541.0, Mn(IV) 6541.5, Mn(V) 6542.1, Mn(VI) 6542.5, Mn(VII) 6543.8.
An oxide containing manganese showed a pre-edge feature in its XAS spectrum consisting of peaks at 6540.6 eV (intensity 1) and 6540.9 (intensity 2). Explain the observed variation in the energy of the pre-edge feature and propose a formula for the manganese oxide.
The experimental data can be reconciled with the mixed oxide Mn3O4, which can be more fully represented as the mixed valence Mn(II)(Mn(III)2O4. The signal at 6540.6 eV falls within the range for Mn(II) and that at 6540.9 for Mn(III).
6.4. Mn3+ can be doped into YInO3 to produce a bright blue pigment in which the Mn3+ ion partly replaces In3+ on a trigonal bipyramidal site rather than yttrium on a site with cubic symmetry. Would the location of Mn3+ be evident in the Mn K-edge XAS data?
Yes. The cubic symmetry site will have eight nearest neighbour oxygen atoms and at a fairly long distance. Five coordinate, trigonal pyramidal geometries typically divide into two distance sets with shorter Mn-O (equatorial) than Mn-O (axial). These are all parameters that control the form of the Mn K-edge XAS data; with a trigonal bipyramidal site showing two shells at slightly different distances of around 2.00Å – and whose intensity would be in the ratio 2:3. The cubic environment would show a much longer distance (~2.4 Å) and a single shell.
6.5 The equilibrium between the dimer of the palladium complex (a) and its monomer (b) in Figure 6.15 is controlled by the level of dilution in the solvent NMP (N-methyl-2-pyrrolidone) with the monomer being favoured at high dilutions. What would be the main change observed in the FT of the EXAFS spectrum as a solution of the monomer is concentrated?
The dimer has an intense peak in its Pd EXAFS spectrum due to the neighbouring metal at 3.00 Å. This peak will be absent in the monomer. Thus as the solution becomes more concentrated a peak will appear at 3.00 Å (this peak will be relatively intense due to the strong backscattering from the high atomic number Pd) representing the formation of a dimer; this peak will become more intense as the solution becomes more concentrated. As the remainder of the ligand environment (nearest neighbour carbon, phosphorus and oxygen atoms) is very similar in the monomer and dimer the rest of the EXAFS spectrum would remain similar in form.
6.6 Why would the copper and zinc atom percentages in this analysis have a higher error than those for tin and selenium?
Because these are neighbouring elements on the periodic table their X-ray emission peaks overlap in the EDX spectrum. This means that accurate determination of the individual peak intensities will be more difficult and lead to higher errors on determined atom percentages. In addition X-rays emitted by the lighter copper and zinc atoms can be absorbed by the heaver tin and selenium and while this sample self-absorption can be corrected for with appropriate standard mixtures it can lead to higher errors in the analysis.