Octahedral symmetry, absorption

So far, we have discussed the crystalline field acting on the ion A due to an octahedral environment of six B ligand ions. In many optically ion activated crystals, such as Ti +rAlaOj, the local symmetry of the active ion A is slightly distorted from the perfect octahedral symmetry Oh symmetry). This distortion can be considered as a perturbation of the main octahedral field. In general, this perturbation lifts the orbital degeneracy of the tag and eg levels and then produces additional structure in the tag eg absorption/emission bands. 

Narrow absorption lines can be observed in the liquid state only when the relaxation processes do not produce small values for Tv Thus the resonance of Ti(H20)6+ is not seen because the octahedral symmetry gives orbital states close to the ground state and this in turn leads to short 7Vs and hence broad absorption lines. When fluoride ions are added to produce a complex of low symmetry (23) in which there are no excited states close to the ground state, the ESR spectrum is observed as a narrow line. 

Some Molv complexes of the type MoCl4-2L, including L = Cl, show rather intense bands at 2OOO0 and 26000 cm"1 which have tentatively been assigned as 37 2g<-3rlg and 3T g(P) -3T g in octahedral symmetry. The Mo(CN)84 ion shows rising absorption from 18000 (s 1) to 38000 cm-1 (e 1000) with structure which has been assigned to the four transitions between the cf-orbitals split by the dodecahedral stereochemistry.142... 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

In order to explain these results we consider at present that the CrF 3 complex has a perfect octahedral symmetry, without spin-orbit and phonon interactions. A trigonal splitting of the levels by the distorted octahedral symmetry is neglected because the low-temperature ground-state absorption spectra do not show any signs for an appreciable splitting [8]. 

Infrared absorptions for KrF AuFs and 2KrF AuFs have been reported. They suggest reduction of the octahedral symmetry of the AuFs group to Ciy of C, probably caused by fluorine bridging. The oxidation state -1-5 of gold was confirmed by Mossbauer spectra (100). FXe-----FAuFs and XejFJAuFg are obtain from AuFj and XeF. ... 

For a configuration, the ground state in octahedral symmetry is a Eg term and the excited state is a "T2g term. On distortion to >4/, geometry, these terms split, as shown in Figure 11-10. In an octahedral complex, we would expect excitation from the Eg state to the state and a single absorption band. Distortion of the complex to >4 , geometry splits the "Tig level into two levels, the Eg and the B2g. Excitation can now occur from the ground state (now the Big state) to the Ajg, the Eg, or the B2g (the... 

Corma et al. have recently used EXAFS and XANES to assist in the characterisation of Ti-MCM-41 structures [61]. Figure 22 shows Ti XANES spectra for the Ti-MCM-41 material after calcination to remove the surfactant template and exposure to air (i.e.hydrated), and following subsequent dehydration. The dehydrated sample shows an intense preedge peak whose position and intensity are consistent with the presence of tetrahedrally coordinated Ti +. This peak is reduced in intensity and slightly shifted in the hydrated sample there are also some changes in the absorption edge profile. Corma et al. attribute these changes to a transition to distorted octahedral symmetry on hydration. 

The UV-vis spectra of the trinuclear aza- and nitro-capped aminoethanethiol cobalt sarcophaginates are very similar to those for the initial [Co(Co(aet)3)2] cation and intermediate hexaimine complex. The absorption band at 18 180 cm- is due to the ig— d-d transition for a cobalt(III) ion in octahedral symmetry, and another one at 23 260 cm- was observed. These broad asymmetric bands have more than one contributing component, consistent with a deviation from octahedral symmetry and with an overlap of both terminal C0N3S3 and central CoSe chromophore transitions. The UV bands for these complexes are much more intense than the visible ones and were attributed to CT from ligand (sulphur atoms) to terminal and central cobalt(III) ions. The band at 28 570 cm- was assigned to the CT transition from the bridged sulphur to cobalt ion, because this one is not observed in the spectrum of the mononuclear complex [129]. 

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