LMCT transition

Consider first blue sapphire Al203 Ti(III), Fe(III) (Fig. 2). In the absence of Fe(III) the absorption spectrum is easy to interpret. The weak band with a maximum at about 500 nm is due to the t2 —> e crystal-field transition on Ti(III) (3d ), the strong band at 2<280nm is due to a Ti(III)-0( — II) LMCT transition. The absorption band in the region around 700 nm in the case of the codoped crystal cannot be due to Fe(III). It has been ascribed to MMCT, i.e. to a transition within an iron-titanium pair ... 

An interesting case is the optical absorption of M(II)-doped MgTi205 [33]. The spectra of interest are given in Fig. 3. The undoped MgTi205 shows a strong optical absorption which starts at about 320 nm. This is due to the 0( - II)-Ti(IV) LMCT transition. The spectra of MgTijOj doped with Mn(II), Fe(II), Co(II) and Ni(II) show considerable additional absorption in the visible. Only Co(II) and Ni(II) are expected to show spin-allowed crystal-field transitions in this spectral range [14]. These are in fact observed (see Fig. 3) ... 

These structures should, in principle, show LMCT transitions at two different positions. Except for TS-1, data representing these angles for other titanosilicates are not available. Such data would be useful in determining the influence of the Ti-O-Si angle on the ease of hydrolysis of the Ti-O-Si bond, which is crucially important for the stability and, hence, utility of the material in catalytic applications. 

When photoluminescence spectra were recorded for a Ti(OSi(CH3)3)4 model compound, upon excitation at 250 nm only one emission band was detected (at 500 nm), which was assigned to a perfect closed Ti(OSi)4 site. The excitation of these species is considered to be a LMCT transition, 02 Ti4+ —<> (0-Ti3+), and the emission is described as a radiative decay process from the charge transfer state to the ground state, O Ti3+ — 02 Ti4+. Soult et al. (94) also observed an emission band at 499 nm, which they attributed to the presence of a long-lived phosphorescent excited state. The emission band at 430 nm of TS-1 was tentatively assigned to a defective open Ti(OSi)3(OH) site (49). 

Ligand-to-metal charge transfer (LMCT) transitions between the bonding ligand-centred MOs and antibonding metal-centred MOs. Such transitions are found where a ligand is easily oxidised and the metal is easily reduced. 

In turning to cases where strong, specific interactions with the solvent are expected, the picture can change considerably and it is no longer obvious that dielectric continuum theory provides a reasonable basis for calculating x It is apparent that dielectric continuum theory can not be used to account for solvent induced variations in AE but, as mentioned earlier, there is hope that a combination of dielectric continuum theory and the use of empirically determined solvent parameters can provide a framework for understanding solvent effects. The importance of specific solvent effects shows up dramatically for MLCT or LMCT transitions in complexes such as shown below (21) ... 

LMCT) transition indicates that PtCll is sorbed within Gn-OH dendrimers. The spectroscopic data also indicate that the nature of the interaction between the dendrimer and Cu or Pt ions is quite different. As discussed earlier, Cu + interacts with particular tertiary amine groups by complexation, but PtCl undergoes a slow ligand-exchange reaction, which is consistent with previous observations for other Pt + complexes [119]. The absorbance at 250 nm is proportional to the number of PtCl ions in the dendrimer over the range 0-60 (G4-OH(Pb+)n, n = 0 - 60), which indicates that it is possible to control the G4-OH/Pb+ ratio. 

The complex Os04(4-dimethylaminopyridine) has been synthesized. It shows an absorption at 473 nm which is assigned to LMCT transition. Excitation of this band in ethanol leads to a reduction of Os to Os and oxidation of ethanol to ethanal with a quantum yield of 0.1 at 436 nm. 

The absorption spectra of blue copper proteins typically include one major peak and two other peaks of varying size in the range 10,000-30,000 cm-1 (164-166). MCD spectroscopy has proved useful in assigning these peaks. The electronic excitations of the active site can be classed as either d—>d or LMCT transitions. The d- fd transitions will involve excited states where the electron hole remains on the Cu atom while the LMCT transitions will move the hole to the ligands, in particular the sulfur atoms of the Met and Cys groups. Thus the d- d transitions would be expected to be more strongly influenced by spin-orbit coupling and this should be reflected in the relative size of the Cj/Dj ratios of the bands in their MCD spectra. 

The theoretical assignments of the first two peaks are the same as those suggested by Helton et al. (172). A number of transitions were found to contribute to the higher energy part of the spectrum between 24,000 and 30,000 cm-1. The negative MCD at the high end of the spectrum is assigned to another LMCT transition from the cysteine sulfur. 

The reduction of DMSO catalyzed by molybdenum is an important step in the process of anaerobic respiration carried out by a number of bacteria (169). Much like sulfite oxidase, early MCD studies of DMSO reductase were complicated by the presence of heme iron (173). The discovery of two enzymes that do not include an iron center led to the measurement of MCD spectra of Rhodobacter sphaeroides DMSO reductase that could be assigned exclusively in terms of transitions of the Mo site (Fig. 10b) (174). The six major peaks are assigned as LMCT transitions from the three highest energy occupied orbitals to the two lowest unoccupied orbitals (174). 

The spectra of the complexes LmM"+l R, containing a metal-carbon -bond, usually consist of several distinguishable bands the major bands with high extinction coefficients in the UV region are LMCT bands, followed sometimes by bands with mixed characters and the d-d bands in the visible region with low extinction coefficients. The location of the maximum of the LMCT transition is naturally strongly affected by the nature of the substituents R and by the redox potential of the central M(ra + 1) ions. 

The permanganate ion, MnO, meets the criteria set forth in the preceding paragraph Manganese is in a formal oxidation state of + 7 and combined with four oxide ions. The molecular orbital diagram for tetrahedral complexes in Fig. 11.52 allows us to identify possible LMCT transitions. In any tetrahedral complex, the four... 

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