Synthesis of 1,1,3,3,5,5-Hexamethyl-7,7-diorganocyclotetrasiloxanes and Its Copolymers

This paper reports a method for the synthesis of 1,1,3,3,5,5-hexamethyl-7,7-diorganocyclotetrasiloxanes by the interaction of 1,5-disodiumoxyhexamethylsiloxane with dichlorodiorganosilanes such as methyl-, methylvinyl-, methylphenyl-, diphenyl- and diethyl dichlorosilanes. Depending on the reaction conditions, the preparative yield of the target cyclotetrasiloxanes is 55–75%. Along with mixed cyclotetrasiloxanes, the proposed method leads to the formation of polymers with regular alternation of diorganosylil and dimethylsylil units. For example, in the case of dichlorodiethylsilane, 70% content of linear poly(diethyl)dimethylsiloxanes with regular alternation of units can be achieved in the reaction product. Using 7,7-diethyl-1,1,3,3,5,5-hexamethylcyclotetrasiloxane as an example, the prospects of the mixed cycle in copolymer preparation in comparison with the copolymerization of octamethyl- and octaethylcyclotetrasiloxanes are shown.

The Crystal Structure of Tl2.36Sb5.98As4.59S17, the Lead-Free Endmember of the Chabournéite Homeotypic Group

ABSTRACT The crystal structure of Tl2.36Sb5.98As4.59S17, the lead-free endmember of the chabournéite homeotypic group, from the Tl-As-Sb-rich gold deposit at Vorontsovskoye (the Urals, Russia) was determined and refined to R(obs) 0.099 for 9340 unique observed X-ray reflections. The triclinic unit-cell parameters determined from single-crystal data are as follows: a = 8.63253(19) Å, b = 16.3055(7) Å, c = 21.8196(8) Å, α = 75.094 (3)°, β = 83.631(2)°, γ = 89.303 (2)°, V = 2949.18(18) Å3 (Z = 4), space group . The crystal structure is composed of (001) slabs based on PbS and SnS archetypes, arranged in regular alternation. All Sb(As) coordination polyhedra are (Sb,As)S3+2+(1 or 2) coordination pyramids, in the majority of cases with a mixed Sb-As occupancy in both slab types. Bond-length distributions were studied in detail. The zig-zag boundary between the slabs is composed of a repeating sequence of [100] Tl-Tl, Sb-Sb (1/3 substituted by As), Tl-Tl, and Tl-Sb columns. Thallium forms tricapped trigonal coordination prisms and (Sb,As) forms bicapped prisms. Differences compared to two related structures—parapierrotite and tsygankoite—are specified. Twinning of chabournéite is connected with the (imperfect) order-disorder character of the structure, which is connected with the configurations observed on slab boundaries. The structure refinement of the lead-free Tl-(Sb,As) chabournéite endmember presented in this paper is the best starting point for a restudy of all complexities of the chabournéite homeotypic group.

Membrane barrels are taller, fatter, inside-out soluble barrels

Up-and-down β-barrel topology exists in both the membrane and soluble environment. However, β-barrels are virtually the only topology that exist in the outer membrane. By comparing features of these structurally similar proteins, we can determine what features are particular to the environment rather than the fold. Here we compare structures of membrane β-barrels to soluble β-barrels and evaluate their relative size, shape, amino acid composition, hydrophobicity, and periodicity. We find that membrane β-barrels are generally larger than soluble β-barrels in with more strands per barrel and more amino acids per strand, making them wider and taller. We also find that membrane β-barrels are inside-out soluble β-barrels. The inward region of membrane β-barrels have similar hydrophobicity to the outward region of soluble β-barrels, and the outward region of membrane β-barrels has similar hydrophobicity to the inward region of the soluble β-barrels. Moreover, even though both types of β-barrel have been assumed to have strands with amino acids that alternate in direction and hydrophobicity, we find that the membrane β-barrels have more regular alternation than soluble β-barrels. These features give insight into how membrane barrels maintain their fold and function in the membrane.

Polymers with regular alternation of 1,7-bis[(dimethyl)silylmethylene]-m-carborane and diorganosiloxane units

New high-thermostable carboranylmethyl-containing diorganosiloxane polymers have been obtained by polycondensation of equimolar amounts of 1.7-bis[hydroxy(dimethyl)silylmethylene]-m-carborane with bis(dimethylamino)diorganosilanes. It was shown, that in the case of replacement of 24 to 33 mol. % of dimethylsiloxane units with methylphenylsiloxane or diphenylsiloxane units with bulk phenyl groups, only amorphous polymers are formed. The polymers are highly soluble in organic solvents and have high thermostability.

Кристаллографические особенности структуры alpha-фазы гафния и сплавов гафний--титан

The structure of a hafnium crystal undergoing β → α (bcc → hcp) polymorphic transformation upon gradual cooling and the structure of Hf_55Ti_45 and Hf_30Ti_70 alloys formed under various kinetic conditions of polymorphic transformation are studied. The structure of the α phase in cast hafnium is shown to consist of lath crystals grouped into packets. The misorientations between separate laths in a packet are less than 1°. The Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types. A structure of packet martensite is observed in the Hf–Ti alloys after quenching. Each packet includes laths of several crystallographic orientations. There is no regular alternation of differently orientated laths in the packet. The same set of α-phase orientations within an initial β-phase grain is observed independently of the cooling rate of the Hf–Ti alloys upon β → α polymorphic transformation. The misorientation of substructural elements within an α-phase crystal in the Hf–Ti alloys is ~5° for the cast state and ~2.2° after quenching.

Modular Crystal Chemistry of Thallium Sulfosalts

Complex sulfides of thallium with As, Sb, or Bi and with other cations (‘thallium sulfosalts’) are a large group of crystal structures with extreme variability. Incorporation of the large Tl+ cation in them is solved in several different ways: housing of Tl in columns of capped trigonal coordination prisms, which form separate walls in the structure (in different combinations with Pb and/or Sb), regular alternation of large Tl with small cations (As), presence of structural arrays of Tl coordination polyhedra paralleled by arrays of As coordination pyramids with a frequency ratio 1:2, omission derivatives with cavities for Tl accommodation and formation of layer structures with thallium concentrated into separate (inter)layers of different types. The first principle leads to a large family of sartorite homologues and rare lillianite homologues, as well as to the chabournéite group. The second one to the hutchinsonite family, omission derivatives form the routhierite and galkhaite groups, and the 1:2 periodicity ratio principle results in several outstanding structures from different groups. Layer structures consist of two-component and three-component layer combinations. Close cation-cation interactions are present but rare.

In defence of underlying representations: Latin rhotacism, French liaison, Romanian palatalization

The surface realization of a linguistic expression can often be predicted from the form of paradigmatically related items that are not contained within it: in Latin, the nominative singular of a noun can often be inferred from the genitive; in French, the final consonant of a prenominal masculine adjective in liaison can typically be predicted from the feminine; in Romanian, the plural form of a noun determines whether its stem will exhibit palatalization before the derivational suffix /-ist/. Such instances of phonological paradigmatic dependence without containment have been claimed to challenge cyclic models of the morphosyntax-phonology interface. In this article, however, they are shown to be established indirectly through the acquisition of underlying representations. This approach correctly predicts that phonological paradigmatic dependencies are never systematically extended to new items if they involve suppletive allomorphy rather than regular alternation, whilst those surface phonological properties of derivatives that are under strict phonotactic control evade paradigmatic dependence on the inflectional forms of their bases. Theories relying on surface-to-surface computation fail to recover these empirical predictions because they are inherently nonmodular, positing generalizations that promiscuously mix phonological, morphosyntactic, and lexical information. Underlying representations, therefore, remain indispensable as a means of establishing a necessary modular demarcation between regular phonology and suppletive allomorphy.

Multidimensional structural variation in the cyanotrichite family of merotypes: camerolaite-3b-F\bar 1

A new superstructure of the mineral camerolaite, Cu6Al3(OH)18(H2O)2[Sb(OH)6](SO4), has been refined in space groupP\bar 1 with unit-cell parametersaP= 7.7660 (16),bP= 8.759 (4),cP= 11.306 (2) Å, αP= 108.67 (4), βP= 83.41 (3), γP= 126.64 (2)°,V= 581.6 (3) Å3andZ= 1, withR1= 0.0951 (all data). This is the first refined example of a cyanotrichite-group mineral in which long-range order of interlayer anions produces a superstructure alongb, although diffuse scattering has previously been reported that corresponds to short-range order. Though the structure shares with other members of the cyanotrichite group a structural unit in which ribbons of edge-sharing Cu and Al octahedra form layers || (001), the superstructure arises from regular alternation of [SO4] and [Sb(OH)6] polyhedra along rods that lie between the layers, and phase coupling between rods that maximizes the distance between [SO4] groups of adjacent rods alongaandcdirections. This arrangement suggests an alternative nearly orthogonal choice of unit cell inF\bar 1 with parametersaF= 12.473 (26),bF= 8.759 (4),cF= 21.476 (7) Å, αF= 85.94 (4), βF= 95.91 (5), γF= 92.34 (9)°, which is recommended for comparing this structural variety of camerolaite with other members of the group.