Iodine molecular structure

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

With iodine, the imidazol-2-ylidenes (IV) form stable adducts (Scheme 8.22), in which the carbene clearly acts as a basic a donor, just like a tertiary phosphine. Interestingly, the molecular structure of this adduct may be considered as an isolated transition state that models the nucleophilic attack of the carbene on the iodine molecule. 

The blue PVA-Iodine complex was first found by Herrmann et al [5] who first synthesized PVA, and by Staudinger et aL [6] as early as 1927. Since then, the complex has attracted the interest of many researchers, and many studies have been done from different points of view. Some studied the complex formed in PVA solutions, while others studied the complex formed in bulk (film and fibers) PVA. Some concentrated their attentions on the effects of the PVA molecular structure on the complex formation, i.e. the 1,2 glycol hetero structure content of the chain backbone [10-12], the stereoregularity [10,11,13-15], the degree of saponification [16-18] and the chemical modification such as formalization [18-20]. 

An amylose solution is colorless. The iodine solution is reddish-brown. Yet when you combine these two solutions, you observe an intense blue color. What changes in molecular structures give this coloration ... 

The nature and magnitude of the solvent-solute interaction depend on the molecular structures of the species. However, it should be apparent that this type of interaction provides a way to assess the interaction between a solute and the solvent. This is an extremely important area of chemistry with regard to understanding the role of the solvent as it relates to effects on solubility, equilibria, spectra, and rates of reactions. As a result, several numerical scales have been devised to correlate the effects of solvent interactions, some of which are based on solvatochromic effects. However, in most cases complex dyes have been utilized as the probe solutes, but it is interesting to note that iodine also exhibits solvatochromism. 

The central iodine atom has five pairs around it, requiring a trigonal bipyra-midal arrangement. Several possible arrangements of the lone pairs are shown in Fig. 13.20. Note that structures (a) and (b) have lone pairs at 90 degrees, whereas in (c) all lone pairs are at 120 degrees. Thus structure (c) is preferred. The resulting molecular structure for I3 is linear. 

Charge-transfer (CT) molecular complexes of some pyrazole donors (pyrazole, 4-methylpyrazole, 3-methylpyrazole, and 3,5-dimethylpyrazole) with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) and tetracyanoethylene (TCNE) as 7t-electron acceptors have been studied in methylene chloride at 25 °C by UV spectroscopy <2002SAA1895>. CT complexes of some pyrazoles with iodine as a cr-electron acceptor and with DDQ, TCNE, and chloranil (CHL) as 7t-electron acceptors were also investigated spectroscopically <2003SPL357>. In both reports, the spectral characteristics and stability constants of the CT complexes formed were discussed in terms of the nature of donor and acceptor molecular structure, as well as in relation to solvent polarity. The thermodynamic parameters (A/7, AG, and AS) associated with CT complex formation were also examined. 

The iodide complexes are somewhat less stable, and well-characterized examples exist only for thorium, protactinium, and uranium. The thorium and uranium derivatives can be coveniently prepared by the reaction of iodine and metal, while protactinium tetraiodide is generated by reduction of Pals. The molecular structure of Thl4 has been examined the metal lies within a distorted square antiprism of iodide ions. 

In general, little information on the absorption spectra of iodine complexes of glycogens of known molecular structure is available, and attempts to relate spectra with details of fine structure appear to be premature. 

The dimeric mono(cyclooctatetraenyl)lanthanide chlorides [(COT)Ln(/r-Cl)(THF)2]2 are long known and still represent the most useful precursors in (COT)Ln chemistry. A recently reported alternative preparation of the Sm derivative involves the reaction of samarium metal with COT in THF in the presence of a small amount of I IgCL. The molecular structure of [(COT)Sm(/i-CI)(TT 11 )2]2 has been determined.805,806 Iodo-(cyclooctatetraenyl)lanthanide iodides of the type (COT)Lnl(TIIF) (Ln = La, Ce, Pr, n = 3 Ln = Nd, n = 2 Ln = Sm, n l) are readily accessible in a one-pot reaction of metallic lanthanides with COT in the presence of an equimolar amount of iodine. Bromo- and chloro-bridged binuclear complexes of samarium, [(COT)Sm(/.t-X)(THF )2]2 (X = Br, Cl), were also prepared by the reaction of samarium metal with COT in the presence of 1,2-dibromoethane or Ph3PCl2, respectively.807 Alternatively, the iodo complexes (COT)LnI(THF)3 (Ln = Nd, Sm) can be synthesized directly from the lanthanide triiodides and K2COT. The molecular structure of (COT)Ndl(THF)3 has been determined by X-ray diffraction.808 A clean preparation of the monomeric half-sandwich complex (GOT)TmI(THF)2 involves treatment of Tml2 with equimolar amounts of COT in THF at room temperature (Scheme 227). The product was isolated as red crystals in 75% yield.628... 

This way of viewing molecular structure and chemical reactions bridged the two opposing theories of Berzelius and Dumas. Its success was extraordinary, as is well known For instance, hydrolysis of methyliodide was explained by dissociation of water in ions H+ and OH, followed by replacement of iodine (negative) by the OH ions ... 

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