Absorption spectra, crystal field splitting

From the color (absorption spectrum) of a complex ion, it is sometimes possible to deduce the value of AOJ the crystal field splitting energy. The situation is particularly simple in 22Ti3+, which contains only one 3d electron. Consider, for example, the Ti(H20)63+ ion, which has an intense purple color. This ion absorbs at 510 nm, in the green region. The... 

The effect of crystal field splitting is easily seen by studying the absorption spectrum of [Ti(H20)6]3+ because the Ti3+ ion has a single electron in the 3d orbitals. In the octahedral field produced by the six water molecules, the 3d orbitals are split in energy as shown in Figure 17.3. The only transition possible is promotion of the electron from an orbital in the t2g set to one in the eg set. This transition... 

The absorption spectrum of Cp3Nd(NCCH3) in a single crystal form at liquid nitrogen temperatures showed a truncated crystal field splitting pattern could be derived. The parameters of an empirical Hamiltonian were fitted [41] with the energies of 41 levels with a root mean square deviation of 26 cm1. 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

FIGURE 22.19 (aj The process of photon absorption and (bj a graph of the absorption spectrum of [Ti(H20j,j]. The energy of the incoming photon is equal to the crystal field splitting. The maximum absorption peak in the visible region occurs at 498 nm. 

Lanthanide impurity-ion spectra consist of a series of sharp lines that appear in groups of closely spaced sublevels that correspond to transitions between crystal-field split free-ion levels. The simplest absorption spectra occur at very low temperatures ( 4K), at which only the lowest Stark level is populated, in general. As the temperature is raised, transitions originating from thermally accessible excited levels are possible, thus complicating the spectrum. In fluorescence spectra, transitions arise at low temperature only from the lowest lying sublevel of the excited free-ion level. At higher temperatures, other transitions become possible. 

T FIGURE 23.30 shows what happens to crystal-field splitting when the ligand is varied in a series of chromium(III) complexes. Because the Cr atom has an [Ar]3d 4s electron configuration, Cr has the configuration [Ar]3d and therefore is a d ion. Consistent with Hund s rule, the three 3d electrons occupy the t2 set of orbitals, with one electron in each orbital and all the spins the same. (Section 6.8) As the crystal field exerted by the six ligands increases, A increases. Because the absorption spectrum is related to this energy separation, these complexes vary in color. 

Figure 3-8. Infrared absorption spectrum of highly crystalline polyethylene where crystal field splitting is shown in the 1450 and 720 cm range due to crystal field splitting (crystaUinity bands).

Fig. 34.3. The chain-dotted line gives the reflection spectrum of YlAhOo- The dashed line indicates the reflection spectrum of Y2.9Ceo Al50 2- The solid line gives the excitation spectrum of the Ce luminescence of Y2 9Ceo lAfiOo the relative quantum yield q, of the luminescence is plotted as a function of the wavelength of the exciting radiation. The Ce absorption bands correspond to the various 4f-5d transitions. The distance between them in the spectrum is equal to the crystal-field splitting of the 5d level (from Blasse and Bril, 1970).

The titanium(III) cases d ) are examples in which there is a direct correspondence between the frequencies absorbed and the crystal field splitting energies. More or less direct correspondence is also observed in high-spin d, high-spin d, and d cases. More commonly, however, it is not that simple. Some examples of more complicated spectra are shown in Figure 4.17. For example, look at the absorption spectrum for the case, that is, the [V(H20)6]33" complex ion. It... 

The absorption spectrum of the complex ion [Rh(NH3)6] has maximum absorbance at 295 nm. Calculate the crystal field splitting energy (in kJ/tnol) for this ion. 

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