Benzene-iodine complex, charge-transfer

Bhowrmk, B.B. (1971) Solvent effect on the charge transfer intensity of benzene-iodine complex. Spectrochim. Acta, Part A, 27 A, 321—327. 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

The first example of a donor-acceptor molecular complex was noted in 1949 by Bensei and Hildebrand [137] in their studies involving charge transfer complexes between benzene and molecular iodine. Subsequently such complexes were studied by Mulliken [138] and now more recently have been used by Stoddart et al. [16,139] in designing novel self-assembling systems. 

Fig. 2. Orientation principle in the benzene-iodine charge-transfer complex (a) S and P Oi (b) jS and = 0.

Microscopic Solvation and Femtochemistry of Charge-Transfer Reactions The Problem of Benzene(s)-Iodine Binary Complexes and Their Solvent Structures, P. Y. Cheng, D. Zhong, and A. H. Zewail, Chem. Phys. Leu. 242, 368 (1995). 

Direct excitation of electron-transfer states may yield surprising results. This is the case with the bimolecular benzene iodine charge-transfer complex. In solutions this system is the prototypical case of charge transfer as reported by Mulliken [262]. The characteristic 280 nm absorption band of the benzene-iodine system is distinct from any absorption features of neat iodine or benzene. It has been identified as being due to a promotion of the HOMO benzene n electron to a a LUMO orbital on iodine resulting in benzene iodine electron transfer. 

Although iodine has been most extensively studied, chlorine and bromine show similar behavior. For a given group of donors, the frequency of the intense charge-transfer absorption band in the ultraviolet is dependent upon the ionization potential of the donor solvent molecule, and electronic charge can be transferred either from a -electron system as in benzene or from lone-pairs as in ethers or amines. Charge-transfer spectra and complexes are of importance elsewhere in chemistry. 

The charge-transfer complexes formed by iodine with ethane- and benzene-thiols, diphenyl sulphide, and diphenyl disulphide have been investigated in cyclohexane at 25 °C. Spectroscopic data for the 1 1 complexes and the... 

Processes involving a single-electron transfer (SET) step and cation-radical intermediates can occur in the reactions of X - or X -iodanes with electron-rich organic substrates in polar, non-nucleophilic solvents. Kita and coworkers first found that the reactions of p-substituted phenol ethers 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of some nucleophiles in fluoroalcohol solvents afford products of nucleophilic aromatic substitution 31 via a SET mechanism (Scheme 1.5) [212,213]. On the basis of detailed UV and ESR spectroscopic measurements, it was confirmed that this process involves the generation of cation-radicals 30 produced by SET oxidation through the charge-transfer complex of phenyl ethers with the hypervalent iodine reagent [213,214],... 

The effect of the former can be seen well, for instance, from a comparison of the reactions between iodine and tetraalkyl lead in benzene and in carbon tetrachloride [Pi 68]. In spite of the facts that both solvents are apolar and have small relative permittivities, and neither of them solvates the tetraalkyl lead molecule, the reaction takes place at a rate 15-20 times higher in benzene than in carbon tetrachloride solution. The explanation of the phenomenon is that with benzene iodine forms a charge-transfer complex of relatively high stability this causes a greater polarization of the iodine molecule than does the very weak interaction between iodine and carbon tetrachloride. 

Charge-transfer complex reactions. Iodine + benzene... 

Interaction between iodine and benzene molecules in low-dielectric solvents gives rise to a complex with a strong absorption in the ultraviolet. Spectroscopic studies show that absorption of ultraviolet light produces an excited charge-transfer state, which may be represented by The spectra, structures and thermodynamics of such clusters have been much... 

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