Se-Se Bonds Research Articles

Insertion and 2+2 Dismutation Reactions Including P-P, Te-Te and Se-Se Bonds

Abstract Reactions of diphosphanes and cyclophosphanes with selenium, tellurium and organic ditellurides have been studied.

An ab initio comparison of dichalcogen hydrides

A theoretical study of HSH, HSeH, HTeH, HSSH, HSeSH, HSeSeH, HTeSH, HTeSeH, and HTeTeH was carried out by ab initio molecular orbital methods employing minimal Gaussian basis sets MINI-1 and MINI-1* of Huzinaga and his group. Both basis sets yield accurate estimates on the equilibrium geometries of monochalcogen hydrides. In the case of dichalcogen hydrides, however, the inclusion of d-polarization functions for sulfur, selenium, and tellurium greatly improve the accuracy of the geometry prediction. The unpolarized MINI-1 basis sets yield essentially correct orbital energies and therefore suffice for the comparative study on the electronic structures in similar molecules. The results with both basis sets imply close similarities in the electronic structures of SS, SeS, and SeSe bonds with more marked differences in bonds containing tellurium as a consequence of notably smaller orbital energy of the 5 s- orbital of tellurium as compared to the corresponding orbitals in sulfur and selenium. The barriers to internal rotation about the chalcogen—chalcogen bond in all dichalcogen hydrides are similar. The cis- and trans-barrier heights are ca. 23 and 14 kJ mol −1, respectively. The relative stabilities of different hydrides are discussed.

Conductivity and Structure of Crystalline Carbon Diselenide

Abstract Crystalline poly(carbon diselende) is a new synthetic metal. The room-temperature resistivity of the compaction sample is 0.02–0.05 Ωcm. The thermoelectric power is as small as 13 μV/K at room temperature. Metallic sample gave sharp X-ray powder pattern. Poly(carbon diselenide) becomes less conductive when crystalline imperfection increases . EXAFS spectra showed the existence of Se-Se bonds. The coordination number of Se in metallic poly(carbon diselenide) is almost. 2.0, indicating the, infinite chain of Se atoms.

A theoretical investigation of the sulfur—selenium bond

A theoretical ab initio study of HSH, HSeH, HOOH, HOSH, HSSH, HSSeH, HSeSeH, HSSSH, HSeSSH, HSSeSH, HSeSeSH, HSeSSeH and HSeSeSeH has been performed using minimal Gaussian basis sets. Full geometry optimization has been carried out for HSH and HSeH. For the four-atom molecules only the chalcogen —chalcogen bond length and the dihedral angle have been optimized while other bond parameters are held fixed at experimental values. The geometries of the five-atom molecules have not been optimized. The calculated bond parameters of HSH and HSeH are in good agreement with the experimental values. The optimum XY (X, Y  S, Se) bond lengths are somewhat larger than the values estimated from IR and microwave data. The electronic structures of the molecules indicate that the SS, SeS and SeSe bonds are very similar. The total energy change calculated for the interconversion reaction between these bonds is small, as indicated by experimental studies. This may well explain why the rearrangement reactions between various Se n S 8— n molecules are possible both in solution and in the molten state. A possible mechanism for the rearrangement is discussed.

Calculated optical and photoemission properties of GeSe2

The electronic properties of Ge${\mathrm{Se}}_{2}$ are analyzed by carrying out calculations for the hightemperature crystalline structure (48 atoms/cell) and the Phillips raft model for the glass phase. The theoretical results provide a possible explanation for many of the salient features in the optical and photoemission spectra of this material. The formation of Se-Se bonds in the raft structure dominates the changes from the crystalline density of states. Characteristic defect states in the band gaps associated with the Se dimers are predicted.

Neutron amorphography

An outline is given of the topological aspects of random network structures and a classification scheme is introduced, based on an underlying topological network. The difficulty of determining the topology of real materials is discussed and it is argued that useful information can frequently be obtained from a consideration of the corresponding crystalline materials. As an example neutron scattering data on amorphous Ge and the AX 2 tetrahedral network glasses are discussed in the light of the ideas put forward. It is concluded that the structure of amorphous Ge is best described in terms of a random network, with more network distortion than has been incorporated into the best random network models produced to date. The structure of vitreous GeSe 2 contains edge sharing GeSe 4 tetrahedra, but the number of Se-Se bonds present must be very small. There is strong evidence for significant covalency in the ZnCl bonds in vitreous ZnCl 2 as evidenced by the fact that molecular dynamics simulations based on purely ionic potentials fail to predict the relative arrangement of the Zn 2+ ions. The structure of vitreous ZnCl 2 may be envisaged as a random close packed array of Cl − ions with Zn 2+ ions in tetrahedral holes arranged so that the resulting ZnCl 4 2− tetrahedra form a corner linked AX 2 random network.

A theoretical investigation of the sulfur-selenium bond

A theoretical ab initio study of HSH, HSeH, HOOH, HOSH, HSSH, HSSeH, HSeSeH, HSSSH, HSeSSH, HSSeSH, HSeSeSH, HSeSSeH and HSeSeSeH has been performed using minimal Gaussian basis sets. Full geometry optimization has been carried out for HSH and HSeH. For the four-atom molecules only the chalcogen—chalcogen bond length and the dihedral angle have been optimized while other bond parameters are held fixed at experimental values. The geometries of the five-atom molecules have not been optimized. The calculated bond parameters of HSH and HSeH are in good agreement with the experimental values. The optimum XY (X, Y = S, Se) bond lengths are somewhat larger than the values estimated from IR and microwave data. The electronic structures of the molecules indicate that the SS, SeS and SeSe bonds are very similar. The total energy change calculated for the interconversion reaction between these bonds is small, as indicated by experimental studies. This may well explain why the rearrangement reactions between various Se n S 8− n molecules are possible both in solution and in the molten state. A possible mechanism for the rearrangement is discussed.

Electrical and optical properties of n-type semiconducting chalcogenide glasses in the system Ge-Bi-Se

The glass-forming region was determined for the system Ge-Bi-Se; 13 at.% Bi was found to be incorporated at its maximum content into glasses, the Ge content of which was 20 at.%. Electrical resistivity, thermoelectric power, and optical absorption coefficient were measured mainly on a series of Ge20BixSe80−x glasses (x=0 –13). The resistivity decreased by about four orders of magnitude between x=9 and 10, and remained almost constant for x?10. The thermoelectric measurement showed the change of conduction type from p to n, accompanied by the above-mentioned abrupt decrease of resistivity. In n-type glasses, hopping conduction in the tail of localized states was proposed in parallel with conduction in extended states. The optical band gap was very slightly changed with the incorporation of more than 2.5 at.% Bi, in contrast to the remarkable decrease in the activation energy for conduction between x=9 and 10. The discussion based on the concentration of covalent bonds formed in the glasses led to the conclusion that the formation of a fairly large number of Bi-Se bonds and the disappearance of Se-Se bonds near x=10 were closely related to the composition dependence of the electrical and optical properties of the glasses in the present system.

The crystal structures of α-AuSe and β-AuSe

α- and β-AuSe crystallize in the space group C2 m with a = 12.202(2) and 8.355(2) A ̊ , b = 3.690(7) and 3.663(1) A ̊ , c = 8.433(2) and 6.262(1) A ̊ , β = 103.15(2) and 106.03(2) ° , respectively. Both modifications of AuSe contain Au + and Au 3+. Au + is coordinated linearly to two Se atoms. Au 3+ is surrounded by four Se atoms in the form of a planar square. These essential bonds form infinite rods parallel to the monoclinic b axis in α-AuSe and waved sheets parallel to (100) in β-AuSe. Rods and sheets are connected by weak Au-Au and Se-Se bonds in α-AuSe and by weak Au-Se bonds in β-AuSe.

Structures of sulfur-selenium mixtures

Most mixed structures of sulfur and selenium are built up from 8-atomic ring molecules in essentially the same way as the pure elements. These ring modifications are all soluble in CS2. At all mixing ratios chain modifications may also occur. They are of the same kind as found with the pure elements, but the great strength of extended pure sulfur fibres is not retained in the mixed structures. One new structure type (C), and possibly a second one (D), were found, but not fully analysed. The question whether mixed bonds S-Se occur as readily in rings or chains as pure S-S and pure Se-Se bonds (each in their own kind of molecules) remains unsettled.