Spectra charge transfer

Charge-transfer absorptions in solutions of halogens are described in Chapter 6. In these cases, a strong interaction between a donor solvent and a halogen molecule, X2, leads to the formation of a complex in which an excited state (primarily of X2 character) can accept electrons from a HOMO (primarily of solvent character) on absorption of light of suitable energy  

The absorption band, known as a charge-transfer band, can be very intense it is responsible for the vivid colors of some of the halogens in donor solvents. 

Uncoordinated metal Octahedral complex Ligand sigma orbitals 

MLCT results in oxidation of the metal a MLCT excitation of an iron(III) complex would give an iron(IV) excited state. MLCT most commonly occurs with ligands having empty tt orbitals, such as CO, CN , SCN , bipyridine, and dithiocarbamate (S2CNR2 ). 

In complexes such as Cr(CO)5 which have both o--donor and rr-acceptor orbitals, both types of charge transfer are possible. It is not easy to determine the type of charge transfer in a given coordination compound. Many ligands give highly colored complexes that have a series of overlapping absorption bands in the ultraviolet part of the spectrum as well as the visible. In such cases, the d-d transitions may be completely overwhelmed and essentially impossible to observe. 

For each of the following configurations, construct a microstate table and reduce the table to its constituent free-ion terms. Identify the lowest-energy term for each. 

Class 2 Ligand to metal (reduction) charge-transfer transitions (LMCT) 

These are a common type of transition, in which a ligand electron is transferred to a metal orbital and the charge separation within the complex thereby reduced. The process can lead to a permanent transfer of charge so that the compound involved is photolytically changed. This offers not only a method of measuring the intensity of a light beam (actinometry) but in recent years has been the subject of much research in an attempt to couple such a process into the reaction 

Class 3—Metal to ligand (oxidation) charge-transfer transitions (MLCT) 

In this type of transition a metal electron is transferred to an orbital largely located on the ligands. It is therefore, in a sense, the opposite to type of charge-transfer transition described in the prevous section. So, it corresponds to an oxidation of the metal and reduction of the ligand. An example is provided by the [M(H20)6] ions of the first transition series, for which Dainton has shown that the energy of the first charge-transfer band is linearly related to the redox potential of the system 

Much of what has been said about ligand metal charge-transfer bands applies to this class also—they, too, can be used as a basis for actinometry it would be attractive to combine both types in a catalysed photolytic decomposition of water. 

Typical for perylene and many other n-systems is the relatively high energy of the highest occupied molecular orbital (HOMO). Typical for the bromine molecule, on the other hand, is the low energy of the lowest unoccupied molecular orbital (LUMO). Therefore, in the experiment of Akamatu et al., a Mulliken charge transfer complex is formed between perylene and bromine, where perylene is the electron donor (D) and bromine is the acceptor (A). 

In the band model, the semiconductivity property is interpreted as a gap at the Fermi level. The delocalized (one-electron) energy bands are used to obtain a theoretical gap. There are two problems with this interpretation. The first is that the energy splitting between the HOMO and the LUMO is not excitation energy in the Hartree-Fock method, but a difference between the ionization energy and electron 

The small value of AG for ET and conductivity in a Mulliken charge transfer complex is because the complex gains energy after the electron has been transferred by simple Coulomb attraction. The donating power of D may be measured by its 

FIGURE 18.6 Optimized structure (PM3) for the charge transfer complex between tetra-methylbenzene (TMB) and tetracyanoethylene (TCNE). 

Concerning solvent reorganization, it is important to remember that polarizability corrections, included via the refractive index, is a part of the electronic energy. The dependence on the refractive index can be expressed as in the Weller model or in other ways. Kuriyama, Ogilby, and Mikkelsen have shown that the dependence alternatively can be written in a dipolar form. 

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For examples of EDA complexes that do not show charge-transfer spectra, see Bentley, M.D. Dewar, M.J.S. Tetrahedron Lett., 1967, 5043. 

The donor-acceptor formation can be considered by transfer of electrons from the donor to the acceptor. In principle one can assume donor-acceptor interaction from A (donor) to B (acceptor) or alternatively, since B (A) has also occupied (unoccupied) orbitals, the opposite charge transfer, from B to A. Such a view refers to mutual electron transfer and has been commonly estabUshed for the analysis of charge transfer spectra of n-complexes [12]. A classical example for a donor-acceptor complex, 2, involving a cationic phosphorus species has been reported by Parry et al. [13]. It is considered that the triaminophosphines act as donor as well as an acceptor towards the phosphenium cation. While 2 refers to a P-donor, M-donors are in general more common, as for example amines, 3a, pyridines, 3b, or the very nucleophilic dimethylaminopyridine (DMAP) [ 14], 3c. It is even a strong donor towards phosphorus trichloride [15]. 

Charge-Transfer Spectra and Molecular Orbital Calculations. 

Although numerous investigations of the electronic spectra of metal sandwich complexes have been carried out, few reliable assignments of the charge-transfer spectra have as yet been made. Thus, only for a few d5 and d6 species (Fe(Cp)2+, Fe(Cp)2, and Co(Cp)2+) have the Laporte-allowed bands been systematically studied (48), although some speculative identifications have been given for Co(Cp)2 and Ni(Cp)2 (88), and apart from the metallocene series hardly any assignments have been attempted. 

Fig. 1 Charge-transfer absorption spectra of enol silyl ethers complexes with re-acceptors. (a) Spectral changes accompanying the incremental additions of cyclohexanone enol silyl ether [2] to chloranil in dichloromethane. Inset Benesi-Hildebrand plot, (b) Charge-transfer absorption spectra of chloranil complexes showing the red shift in the absorption maxima with decreasing IP of the enol silyl ethers, (c) Comparative charge-transfer spectra of EDA complexes of a-tetralone enol silyl ether [6] showing the red shift in the absorption maxima with increasing EAs of the acceptors tetracyanoben-zene (TCNB), 2,6-dichlorobenzoquinone (DCBQ), chloranil (CA), and tetracyanoqui-nodimethane (TCNQ). Reproduced with permission from Ref. 37.

Fig. 16 Charge-transfer spectra of (a) MeOPyN02+ with various aromatic donors (as indicated) and (b) hexamethylbenzene with various TV-nitropyridinium acceptors (as indicated) in acetonitrile. Reproduced with permission from Ref. 235a.

We have not as yet however treated the charge-transfer data available for complexes of the 5 d series. For these latter species though the effective spin-orbit coupling constants are often of the order of 3 kK. or more, as compared with only about 1 kK. for Ad systems, and smaller values still for the 3d elements. Consequently, as for the d—d transitions it is often necessary explicitly to consider relativistic effects in the interpretation of charge-transfer spectra, and in particular to make allowance for the changes in spin-orbit contributions which may accompany a given di transition. In fact one of us has shown (18) that these changes are... 

The solvent effects on charge-transfer spectra between Me3Sn-NCS and I2 was investigated. Onsager s theory of dielectrics was used to estimate the stabilisation energy of excited states257. 

The micellar surface appears to be less polar than water, based largely on shifts in fluorescence or charge-transfer spectra (Section 1). Although it may not be reasonable to apply bulk solvent parameters such as Z or dielectric constant to submicroscopic species such as micelles, the spectral and kinetic evidence are self-consistent. An additional point is that these reactions have... 

Charge-transfer spectra represent one of the most important classes of spectra for analytical chemistry since the molar absorptivities tend to be very large. Charge-transfer can occur in substances, usually complexes that have one moiety that can be an electron donor and another that can be an electron acceptor. Both the donor and acceptor must have a small difference in their energy levels so that the electron can be readily transferred from the donor to the acceptor orbitals and back again. One example is the well-known, deep-red color of the iron (III) thiocyanate ion. The process appears to be... 

A somewhat related treatment of the charge transfer spectra in some chloro-cuprates(II), using the E2 model, was performed by Day and Jorgensen (98). 

Fig. 13 (A) Absorption spectra of the uncomplexed donor and acceptors. (B) Comparative charge-transfer spectra of...

Thus, MO theory is generally applied to interpretation of the so-called charge transfer spectra. However, for a great variety of centers in solids, crystalline field theory suffiees to provide at least a qnalitative interpretation of spectra. 

Dibenzothiophene acts as a 7r-electron donor and readily forms complexes with known electron acceptors. In such cases the electronic spectrum of a solution of the two compounds shows a new absorption band, usually in the visible region. The order of donor strengths of several o,o -bridged biphenyls has been estimated from their respective charge-transfer spectra and found to be carbazole > fluorene > dibenzothiophene >dibenzofuran. Dibenzothiophene forms complexes with tetracy-anoethylene, various polynitro derivatives of fluorenone, > naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, and tetra-methylmic acid. ... 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

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