Benzene iodine complexes

The iT-values show clearly that the mesitylene-iodine complex is more stable than the benzene-iodine complex. The ratio of the T-values of the two complexes corresponds roughly to the ratio of the. K -values obtained from an evaluation of vapour pressure measurements (Table 9). It must, however, be borne in mind that these measurements were carried out at very different temperatures, so that this comparison is only a qualitative one. 

The yield of labeled benzene can be enhanced by the presence of the benzene-iodine complex during irradiation. Pozdeev et al. (1962a) ascribe this effect to stabilization of a highly excited intermediate which results in increased probability of hydrogen loss from the intermediate. Phase-effect studies on cyclohexane, cyclohexene, cyclohexadiene and benzene by Pozdeev et al. (1962b) lend further support to the excited-... 

The application of these techrriques is corrsiderably erthanced by the irttroduction of sarrrple-seeded supersonic jets. Gas-phase spectra are obtained at terrtperatures close to the absolrrte zero arrd the problem of Boltzmaim congestion is effectively overcome. Besides making the artalysis of previously hopelessly congested spectra tractable it has revealed a new farrrily of weakly-botmd van der Waals dimers or clusters. Some of the analyses are limited to general conclusions, as e.g. the distinction between end-on and sideways-on orientation of diatomic iodine in a benzene-iodine complex. Such data are not included in the present compilation. Other analyses, however, yield acciuate intemuclear distances as in the benzene-rate gas complexes. 

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

The last condition, the orientation principle, is illustrated for the benzene-iodine and ethylene-platinum complexes. It is seen that the orientation depicted in Fig. 2a leads to a positive overlap integral and... 

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

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. 

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. 

Figure 7.14 Formation and dissociation of the excited benzene-iodine charge-transfer complex. Top Transient of free iodine atoms following the excitation of the Bz-l2 complex to its charge-transfer state. (The full line is a single exponential convoluted with the response time of the detector system.) Bottom The four panels (a), (b), (c) and (d) illustrates a possible series of structural changes see text...

Weak donor-acceptor complexes, such as benzene/iodine (Bz/I ), are... 

Lang, F.T. and Strong, R.L. (1965) Gas-phase molecular complexes. The diethyl ether-iodine and benzene-iodine charge-transfer complexes. /. Am. Chem. Soc., 87, 2345—2349. 

In 1991, Arduengo isolated one of the first NHC-Group 17 element adducts. The team found that NHCs cleanly reacted with iodopentafluoro-benzene to form hypervalent iodine complex 254 (Scheme 5.37). According to the NMR data, complex 254 was fluxional in solution, and in equilibrium with the free carbene and iodopentafluorobenzene. After several hours at room temperature, solutions of 254 were shown to undergo cleavage of the I-Cph bond, generating pentafluorobenzene and the 2-iodoimidazolium ion 255. 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

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 charge-tranter concept of Mulliken was introduced to account for a type of molecular complex formation in which a new electronic absorption band, attributable to neither of the isolated interactants, is observed. The iodine (solute)— benzene (solvent) system studied by Benesi and Hildebrand shows such behavior. Let D represent an interactant capable of functioning as an electron donor and A an interactant that can serve as an electron acceptor. The ground state of the 1 1 complex of D and A is described by the wave function i [Pg.394]

Because Me3SiI (TIS) 17 is relatively expensive and very sensitive to light, air, and humidity, it is usually prepared in situ from TCS 14 and Nal in acetonitrile [1-6], although other solvents such as CH2CI2, DMF, benzene, or hexane have also been used [5, 6] (Scheme 12.1). It is assumed that TIS 17 forms, in situ, with MeCN, a (T-complex 1733 [2, 3-6], yet Me3SiI 17 can also be prepared by treatment of hex-amethyldisilane 857 with iodine in organic solvents [4-6]. The chemistry of TIS 17 has been reviewed [4—6]. 

Fig. 6 Proposed structures for iodine-benzene complex a resting, b axial, c oblique (turned 30° around center of mass), d above carbon, e above bond, f resting on bond, g displaced resting, and h T-shaped [79]...

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. 

Such reactions are also possible in vitro, as several mild oxidizing agents are at hand nowadays. Thus, the Dess-Martin periodinane (DMP) [50] has been proven to be a versatile and powerful reagent for the mild oxidation of alcohols to the corresponding carbonyl compounds. In this way, a series of new iodine(V)-mediated reactions has been developed which go far beyond simple alcohol oxidation [51], Ni-colaou and coworkers have developed an effective DM P-mediated domino polycy-clization reaction for converting simple aryl amides, urethanes and ureas to complex phenoxazine-containing polycycles. For example, reaction of the o-hydroxy anilide 7-101 with DMP (2 equiv.) in refluxing benzene under exposure to air led to polycycle 7-103 via 7-102 in a yield of 35 % (Scheme 7.28) [52]. 

---