Field transitions

Figure 2. Partial 100 MHz P.M.R. Spectrum of 3,4,6-tri-O-acetyl-v-glucal (1) measured for a chloroform -d solution (A normal spectrum of the Hi and H2 resonances respectively (B) frequency sweep spin-decoupled spectrum of the Hi and H2 resonances, with a strong decoupling field centred on the Hs resonance (C), as in (B) above, but with an additional weak radiofrequency field centred on the high field transition of the H2 resonance (D), as in (B) above, but with a weak radiofreauency field centred on the low field 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) ... 

The Fe(II) complex of this ligand shows crossover behavior both in solution and in the solid state. The complex has a distinct green color derived from the ligand field transition 2=620 nm) of the low-spin form of... 

As usual in the field, "transition state" is also loosely used as equivalent to "reactive intermediate."... 

In general, the plume flow field is divided into the near-field, transition, and far-held regions [5]. The near-held region which is shown in Fig. 29.1 consists of a nearly inviscid jet core dominated by strong wave structures and a thin... 

These three types of clusters all involve one electron orbital. They provide a basis for the description of the d-d (i. e. ligand field) transitions and the ligand to metal charge transfer transitions which are responsible for most of the UV-visible spectra and opti-... 

The charge transfer transitions - involving Fe -0 or Fe"-Fe" - are mainly responsible for absorption of visible light. They produce an absorption band centered in the near UV, one side of which (the absorption edge) extends into the visible region. This intense absorption is overlain by bands due to the ligand field transitions (between 550-900 nm) and double excitation processes at ca. 450 nm. The d-d transitions contribute more to the colour of iron oxides than would be expected, owing to the interactions between the Fe -Fe pairs. 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

The important feature of magnetic insulators is that, being nonmetallic, they have a band gap and possess unpaired electrons. They show crystal-field transitions due to the presence of open-shell (d") ions. Mott proposed that electron repulsion can be responsible for the breakdown of the normal band properties of transition-metal... 

Applications to Small Molecules. - 2.2.1 Intensity of Half-field Transition or Resolved Dipolar Splitting. In the pH range 8-10 the dominant species in solutions of Cu(II) and 2,2 -bipyridine (bpy) is [Cu(bpy)(OH)]22+.30 Well-resolved EPR spectra were obtained. The value of the zero-field splitting parameter D and the relative intensity of the half-field transition gave r = 3.4 A. 

The X-ray diffraction crystal structure of another copper dimer showed a distance between ligand planes of 2.91 to 3.31 A, an intramolecular Cu-Cu distance of 6.01 A and an intermolecular Cu-Cu distance of 10.209 A.32 Calculation of the Cu-Cu distance based on the relative intensity of the half-field transition gave r = 5.88 A. 

Applications to Biological Samples. - Methods of distance measurements were compared for four doubly spin-labelled derivatives of human carbonic anhydrase.53 The distances between the spin labels were obtained from continuous wave spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of the line-shape broadening, and computer simulation of line-shape changes. For variants with interspin distances greater than 18 A, the DEER method also was used. For each variant, at least two methods were applicable and reasonable agreement between distances obtained by different methods was obtained. The useful distance ranges for the techniques employed at X-band with natural isotope abundance spin labels were estimated to be half-field transition (5-10 A), line-shape simulation (up to 15 A), Fourier deconvolution (8 - 20 A), and four-pulse DEER (> 18 A).53... 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

For the dimer (18), at 200 K, the ESR powder spectrum shows an intense central signal and a strong half-field absorption.427 Below 200 K, well-resolved hyperfine structure appears. The hyperfine constant for the half-field transition, 84 G, is approximately half of the Az value for the monomers. From single crystal ESR,427 the exchange integral -410 cm"1,... 

Figure 6. Camphausen coke 525°C. (a) section observed in bright field (b) in dark field transitions between more or less fine granular zones X 8520...

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

Consider first of all the so-called crystal field transitions these involve moving the b electron to the e, bi, and Iai M.O. s, which are of course essentially the 3d metal orbitals, resulting in E(I), Bi, and Ai exdted states, respectively. To a good approximation dectron repulsion... 

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