Selenium chains

Selenium exists in the form of a cyclic octomer, which can rearrange to long, polymeric chains. As can be seen from 7.5, the selenium chain is very similar to that of sulfur. The bond angles are 104°, essentially the same as those in polymeric sulfur, and rotational states are also thought to occur at the same angles, 90°.52-54 However, the skeletal... 

Selenium. - Nitrogen-matrix IR spectra were reported for Na2Sen clusters. Bands were consistent with selenium chains capped at each end by sodium.655 Raman spectroscopy was used to characterise hexagonal selenium nanowires.656 DFT calculations gave vibrational wavenumbers associated with Cl impurities in amorphous selenium.657... 

The largest number of selenium atoms in covalent selenium chain compounds isolated so far appears to be three, and even so, the representative compounds known are relatively few in number. In the case of tellurium, the maximum number is two. Rbeinboldt (191) has recently reviewed preparative methods for di- and triselenides, ditellurides, and compounds containing mixed sulfur-selenium and sulfur-tellurium chains. 

The three lowest members of the corresponding selenium series are known. Cyanogen triselenide, or selenium diselenocyanate Se(SeCN)2, was prepared by Verneuil 219) in 1886, by treatment of potassium selenocy-anate with oxidizing agents the yellow crystals melt at 132° and are quite stable in the dark. Three analogs containing mixed sulfur-selenium chains have been described, namely diselenium dithiocyanate Se2(SCN)2 and the compound SeS(SCN)2 by Baroni 16) in 1936, and selenium dithiocyanate Se(SCN)2 by Ohlberg and vander Meulen 180) in 1953. 

Similarly, Zhan and Li [851] synthesized and characterized by Raman spectroscopy Mn(II)-l,10-phenanthroline complexes in Y-type zeolites. Selenium chains and rings confined in voids of various zeolites (zeolite A, zeolite X, mor-denite, chabazite, and cancrinite) were studied by Raman spectroscopy in the range 300-100 cm in the contribution of Poborchii [852]. 

Among the metalloids, iodine can be introduced in appreciable amounts (up to 17 at.%) into glassy arsenic selenide and it also forms bonds that are essentially covalent in nature. The iodine atoms are incorporated in selenium chains and rings forming bond terminations of the type. .. Se-I. The Se-Se and I-Se bonds have comparable ionization energies [4]. For this reason iodine does not have an appreciable effect on the electrical conductivity of arsenic selenide. It merely eliminates the blocking of carriers by the chains and rings of excess selenium and thereby renders continuous conduction possible [5]. The formation of chain terminations when iodine is introduced into arsenic selenide reduces the chemical stability of the latter. 

Secular determinant, 11 for solids, 238 Selenium chains. 245 Self-consistent-lield iteration, 117 Semibullvalene, 193 SF., 269. 270 SFs. 260 Sh .259... 

Diselenium dichloride acts as a solvent for selenium. Similarly disulphur dichloride is a solvent for sulphur and also many other covalent compounds, such as iodine. S Clj attacks rubber in such a way that sulphur atoms are introduced into the polymer chains of the rubber, so hardening it. This product is known as vulcanised rubber. The structure of these dichlorides is given below ... 

Tellurium is diamagnetic below its melting point. Its intrinsic electrical resistivity at room temperature is about 0.25 ohmcm, when the current is parallel to the i -axis, and decreases with increasing temperature and pressure. The element forms a continuous range of isomorphous solutions with selenium, consisting, in the soHd state, of chains of randomly alternating Se and Te atoms. 

Weak intramolecular interactions between sulfur or selenium and nitrogen are a recurrent phenomenon in large biomolecules. They may occur in the same residue or between neighbours of a peptide chain. The formation of four- or five-membered rings of the types 15.1 and 15.2, respectively, is most common. A feature that is unique to proteins is the participation of sulfur atoms in bifurcated N S N contacts. 

Figure 16.1 Structures of various allotropes of selenium and the structure of crystalline tellurium (a) the Seg unit in a- fi- and y-red selenium (b) the helical Se chain along the c-axis in hexagonal grey selenium (c) the similar helical chain in crystalline tellurium shown in perspective and (d) projection of the tellurium structure on a plane perpendicular to the c-axis.

The isomer of isoproterenol in which both aromatic hydroxyl groups are situated meta to the side chain also exhibits bron-chiodilating activity. Oxidation of 3,5-dimethoxyacetophenone by means of selenium dioxide affords the glyoxal derivative (15). Treatment of the aldehyde with isopropylamine in the presence of... 

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