Transfer band

The long-wave absorption most likely results from a chaige-transfer band See Coleman, L. E. 7. Org. Chem. 1956, 21, 1193. 

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

Quinolizinium iodide, 3,4-dihydro-dehydrogenation, 2, 547 Quinolizinium iodide, 2-methylthio-synthesis, 2, 544 Quinolizinium ions, 2, 525-578 aza analogues, 2, 525-578 charge transfer bands, 2, 527 MO calculations, 2, 527 nomenclature, 2, 526 structure, 2, 3 UV spectra, 2, 19, 526-527 Quinolizinium ions, hydroxydihydro-reactions, 2, 549... 

Donor solvent Formation constant X(20°C)/1 mol- -AH / kJ moC Charge-transfer band W/nm fmax A VI/cm- 2 ... 

It is possible to observe spin-allowed, d d bands in the visible region of the. spectra of low-spin cobalt(lll) complexes because of the small value of 0Dq, (A), which is required to induce spin-pairing in the cobalt(lll) ion. This means that the low-spin configuration occurs in complexes with ligands which do not cause the low -energy charge transfer bands whieh so often dominate the spectra of low-spin complexes. 

It is relevant to note at this point that, because the metal ions are isoelcctronic, the spectra of low-spin Fe complexes might be expected to be similar to those of low-spin Co ". However, Fe" requires a much stronger crystal field to effect spin-pairing and the ligands which provide such a field also give rise to low-energy charge-transfer bands which almost always obscure the d-d bands. Nevertheless, the spectrum of the pale-yellow [Fe(CN)f,] shows a shoulder at... 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

It is the n- n type of excitation which leads to significant fluorescence, the n-> z type producing only a weak fluorescence. The electronic transitions corresponding to charge-transfer bands also lead to strong fluorescence. 

The mixed-valence ion has an intervalence charge transfer band at 1562nm not present in the spectra of the +4 and +6 ions. Similar ions have been isolated with other bridging ligands, the choice of which has a big effect on the position and intensity of the charge-transfer band (e.g. L = bipy, 830 nm). 

Since the energy of the transfer band is determined by the difference between the donor ionization potential and the acceptor electron affinity, this fact points to the increase of the PCS ionization potential with decreasing conjugation efficiency. Therefore, the location of the transfer band of the molecular complexes of an acceptor and various PCSs can serve as a criterion for the conjugation efficiency in the latter. In Refs.267 - 272) the data for a number of molecular complexes are given, and the comparison with the electrical properties of the complexes is made. 

Thus, irradiation in the transfer band results in practically the whole molecular complex being found in the complete transfer state. The course of the curve corresponding to the increasing ESR signal (Fig. 20) is typical of a number of PCSs and is well described by the relationship AI = where a is constant. 

We have already pointed out that the reduction in conjugation efficiency in PCSs is followed by a short-wave shift of the CTC transfer band. This accounts for the fact that poly(schiff base)s and polyazines having conjugated sections separated by oxygen and sulfur atoms are characterized by a short-wave shift of the transfer band of CTC with all acceptors compared to the respective polymers having no interruption of the conjugated chain. This shift may reach 20-50 nm. 

In the UV spectral range complexation with 18-crown-6 causes a hypsochromic shift of the band with the longest wavelength in various solvents (Bartsch et al., 1976 Hashida and Matsui 1980). Gokel and Cram (1973) reported that complexation with binaphtho-20-crown-6 (11.2) produces a yellow to red color. This phenomenon is very likely to be due to a charge-transfer band between a naphthalene ring as donor (7i-base) and the arenediazonium ion as acceptor (7i-acid). 

In our opinion the lower frequency band found by Korzeniowski et al. (1977 b, 1981) in 1 1 mixtures of host and guest at wavenumbers which are almost identical ( 5 cm-1) with that of the free diazonium ion may be a charge-transfer band. 

Figure 4-4. a) Spectrum of [CoC J and b) Spectrum showing intensity stealing from a charge-transfer band at higher energy. 

The second example in Fig. 4-4 shows how a (spin-allowed or spin-forbidden) band lying close to a charge transfer band may acquire unusually high intensity. We shall discuss charge-transfer bands more in Chapter 6. For the moment, we note that they involve transitions between metal d orbitals and ligands, are often fully allowed and hence intense. On occasion, the symmetry of a charge transfer state... 

Finally, we must remember that just as a d-d spectrum is not properly described at the strong-field limit - that is, without recognition of interelectron repulsion and the Coulomb operator - neither is a full account of the energies or number of charge-transfer bands provided by the present discussion. Just as a configuration... 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

Delocalization band Pseudoexcitation band Transfer band... 

With the power of the donors and acceptors, changes occur in the important frontier orbital interactions (Scheme 2) and in the mechanism of chemical reactions. The continuous change forms a mechanistic spectrum composed of the delocalization band to pseudoexcitation band to the electron transfer band. 

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