Si Angle Research Articles

Comparison of radiation and quenching rate effects on the structure of a sodium borosilicate glass

The effects of quenching rate and irradiation on the structure of a sodium borosilicate glass were compared using 29Si, 11B, and 23Na nuclear magnetic resonance and Raman spectroscopy. Quenching rate ranging from 0.1 to 3×104Kmin−1 was studied. Various irradiation conditions were performed, i.e. gold-ion irradiation in a multi-energy mode (from 1 to 6.75MeV), and Kr and Xe ion irradiations with energy of 74 and 92MeV, respectively. In pile irradiation with thermal neutron flux was performed as well, to study the effect of alpha radiation from the nuclear reaction 10B(n,α)7Li.Both irradiation and high quenching rate induce similar local order modification of the glass structure, mainly a decrease of the mean boron coordination and an increase of Q3 units. Nevertheless, the variations observed under irradiation are more pronounced than the ones induced by the quenching rate. Moreover, some important modifications of the glass medium range order, i.e. the emergence of the D2 band associated to three members silica rings and a modification of the Si–O–Si angle distribution were only noticed after irradiation. These results suggest that the irradiated structure is certainly not exactly the one obtained by a rapidly quenched equilibrated melt, but rather a more disordered structure that was weakly relaxed during the very rapid quenching phase following the energy deposition step.Raman spectroscopy showed a similar irradiated structure whereas the glass evolutions were controlled by the electronic energy loss in the ion track formation regime for Kr-ion irradiation or by the nuclear energy loss for Au and OSIRIS irradiation. The similar irradiated structure despite different irradiation routes, suggests that the final structural state of this sodium borosilicate glass is mainly controlled by the glass reconstruction after the energy deposition step.

Density functional theory study of the impact of tin doping on oxygen diffusion in Czochralski silicon

AbstractThe impact of tin (Sn) doping on interstitial oxygen (Oi) diffusion in Czochralski silicon is investigated by density functional theory (DFT) calculations. Two aspects are taken into consideration: (i) Sn induced lattice strain and lattice expansion; (ii) direct and vacancy‐mediated interaction between Sn and Oi atoms. The calculations show that the interstitial oxygen migration energy barrier increases with increasing lattice constant which grows with the concentration of Sn. Structure analysis indicates that the geometric parameters such as the bond length and the angle of SiOSi configuration also increase with the lattice constant, which is considered to be the origin of the increased energy barrier for oxygen diffusion. SnO pair formation is found to be energetically unfavorable, while the SnVO complexes have a large binding energy. Sn is however able to trap Oi atoms by stimulating vacancy generation around it, which increases the oxygen diffusion energy barrier as well. It is thus predicted that oxygen diffusion in silicon with a high concentration of Sn atoms will be retarded.

Synthesis and Coordination of the Cagelike Siloxane Compounds E2{(SiMe2)2O}3 (E = As, Sb, Bi)

AbstractThe cage compounds [E2{(SiMe2)2O}3] for E = P, As, Sb, and Bi (1, 4, 5, and 6, respectively) are readily accessible in moderate yields by the reaction of Na3E (for E = P, As, Sb, Bi) or [Li(dme)EH2] (for E = P, As; dme = dimethoxyethane) with 1,3‐dichloro‐1,1,3,3‐tetramethyldisiloxane. These compounds were isolated and characterized by X‐ray crystallographic analysis. The major structural parameters such as the E–Si bond length and the Si–E–Si angles are discussed and compared with those of similar compounds. The small Si–E–Si angles in these compounds are attributable to steric and electronic reasons. Further, the R3M–E[(SiMe2)2O]3E–MR3 (M = Al, E = As, Sb; M = Ga, E = Sb) Lewis acid–base adducts were formed as well as the W(CO)5 adduct of compound 1 and a heteroleptic Na[Al(ORF)4] (RF = C(CF3)3) adduct with 4 to investigate the coordination properties of the cage compounds. The structural changes caused by coordination follow the Haaland model, which gives qualitative information on the adduct stability. The assumptions made by the Haaland model are valid for a versatile set of Lewis acids. To investigate coordination on the inside of the cage, a reaction of 4 with Li[Al(ORF)4] was performed to yield a new dimeric ligand of the constitution As4[(SiMe2)2O]6.

Flux Synthesis and Structural and Spectroscopic Characterization of a Cobalt Europium Trisilicate

AbstractA trisilicate with a new structure type, CoEu2Si3O10, was obtained from molybdate–carbonate mixtures in Pt crucibles by using a high‐temperature flux‐growth method. The structural characterization of CoEu2Si3O10 was done by evaluation of single‐crystal X‐ray diffraction data collected at three different temperatures (105, 293, and 370 K). The light violet crystals are monoclinic, space group P21/n with a = 6.5620(13), b = 6.7207(13), c = 16.986(3) Å, β = 90.06(3)°, V = 749.1(3) Å3, and R(F) = 2.49 %, at room temperature. The new crystal structure type features layers parallel to (001) containing Si3O10 groups and Co2O10 dimers. The Si–Si–Si angle of the Si3O10 unit is 115.7°. Raman spectroscopy and a study of the photoluminescence properties of the Eu3+ ions in CoEu2Si3O10 were undertaken.

Structure analysis on synthetic emerald crystals

Single crystals of emerald synthesized by means of the flux method were adopted for crystallographic analyses. Emerald crystals with a wide range of Cr3+-doping content up to 3.16 wt% Cr2O3 were examined by X-ray single crystal diffraction refinement method. The crystal structures of the emerald crystals were refined to R1 (all data) of 0.019–0.024 and wR2 (all data) of 0.061–0.073. When Cr3+ substitutes for Al3+, the main adjustment takes place in the Al-octahedron and Be-tetrahedron. The effect of substitution of Cr3+ for Al3+ in the beryl structure results in progressively lengthening of the Al–O distance, while the length of the other bonds remains nearly unchanged. The substitution of Cr3+ for Al3+ may have caused the expansion of a axis, while keeping the c axis unchanged in the emerald lattice. As a consequence, the Al–O–Si and Al–O–Be bonding angles are found to decrease, while the angle of Si–O–Be increases as the Al–O distance increases during the Cr replacement.

Intermolecular β-Hydrogen Abstraction in Ytterbium, Calcium, and Potassium Tris(dimethylsilyl)methyl Compounds

A series of organometallic compounds containing the tris(dimethylsilyl)methyl ligand are described. The potassium carbanions KC(SiHMe2)3 and {KC(SiHMe2)3TMEDA}2 are synthesized by deprotonation of the hydrocarbon HC(SiHMe2)3 with potassium benzyl. {KC(SiHMe2)3TMEDA}2 crystallizes as a dimer with two types of three-center–two-electron K–H–Si interactions: side-on coordination of SiH (∠K–H–Si = 102(2)°) and more obtuse K–H–Si structures (∠K–H–Si ≈ 150°). The divalent calcium and ytterbium compounds M{C(SiHMe2)3}2L (M = Ca, Yb; L = 2THF, TMEDA) are prepared from MI2 and 2 equiv of KC(SiHMe2)3. Low 1JSiH coupling constants in the NMR spectra, low-energy νSiH bands in the IR spectra, and short M–Si distances and small M–C–Si angles in the crystal structures suggest β-agostic interactions on each C(SiHMe2)3 ligand. The IR assignments of M{C(SiHMe2)3}2L (L = 2THF, TMEDA) are supported by DFT calculations. The compounds M{C(SiHMe2)3}2L react with 1 or 2 equiv of B(C6F5)3 to give the 1,3-disilacyclobutane {Me2SiC(SiHMe2)2}2 and MC(SiHMe2)3HB(C6F5)3L or M{HB(C6F5)3}2L, respectively. In addition, M{C(SiHMe2)3}2L compounds react with BPh3 to give β-H abstracted products. The compounds M{C(SiHMe2)3}2THF2 react with SiMe3I to yield Me3SiH and disilacyclobutane as the products of β-H abstraction, while M{C(SiHMe2)3}2TMEDA and Me3SiI form a mixture of Me3SiH and the alkylation product Me3SiC(SiHMe2)3 in a 1:3 ratio.

Study on internal short-range order microstructure characteristic of SiO2 thin film

SiO2 is one of important low refractive index materials, and SiO2 films are prepared by both ion-beam sputtering (IBS) and electron-beam evaporating (EB) technology. Dielectric constants of SiO2 films are calculated by infrared spectrum inversion technique in a wavenumber range from 400 cm-1 to 1500 cm-1. Through analyzing dielectric energy loss function, the oscillation frequency and the Si–O–Si angle of two types of SiO2 films are obtained in the transverse optics and longitudinal optics oscillating mode. The research results indicate that the attended modes of SiO4 are main coesite-like structure, three-plane folding ring structure, and keatite-like structure in the range of short-range order for EB-SiO2 films, but the attended modes of SiO4 are main coesite-like structure, three-plane folding ring structure, four-plane folding ring structure, and keatite-like structure in the range of short-range order for IBS-SiO2 films.

Raman and IR investigation of silica nanoparticles structure

In the present investigation we report experimental data regarding the Raman and Infra-Red (IR) absorption activities of commercial silica nanoparticles. We compared the data of the nanoparticles with the ones acquired in the same experimental conditions for commercial bulk silica. By this comparison we highlighted that the variability of the spectral features of the matrix related Raman and IR bands in the nanoparticles is above the one observed for the bulk systems before any treatments. Furthermore, by studying nanoparticles with different sizes (diameters from 40 to 7nm) and applying the shell-model we can suggest that the core network of the nanoparticles is close to the one encountered in the bulk materials. By contrast for the surface shell, having a thickness of about 1nm, as suggested by previous investigations, the structure is strongly modified. In fact, from Raman and IR data it appears that the Si–O–Si angle has a lower main value than those encountered in bulk systems and that the ring statistic is shifted towards lower member rings.

Redetermination of clinobaryl-ite, BaBe(2)Si(2)O(7).

Clinobaryl­ite, ideally BaBe2Si2O7 (chemical name barium diberyllium disilicate), is a sorosilicate mineral and dimorphic with baryl­ite. It belongs to a group of compounds characterized by the general formula BaM 2+ 2Si2O7, with M 2+ = Be, Mg, Fe, Mn, Zn, Co, or Cu, among which the Be-, Fe-, and Cu-members have been found in nature. The crystal structure of clinobaryl­ite has been re-examined in this study based on single-crystal X-ray diffraction data collected from a natural sample from the type locality (Khibiny Massif, Kola Peninsula, Russia). The structure of clinobaryl­ite can be considered as a framework of BeO4 and SiO4 tetra­hedra, with one of the O atoms coordinated to two Be and one Si, one coordinated to two Si, and two O atoms coordinated to one Si and one Be atom. The BeO4 tetra­hedra share corners, forming chains parallel to the c axis, which are inter­linked by the Si2O7 units oriented parallel to the a axis. The Ba2+ cations (site symmetry m..) are in the framework channels and are coordinated by eleven O atoms in form of an irregular polyhedron. The Si—Obr (bridging O atom, at site symmetry m..) bond length, the Si—Onbr (non-bridging O atoms) bond lengths, and the Si—O—Si angle within the Si2O7 unit are in marked contrast to the corresponding values determined in the previous study [Krivovichev et al. (2004 ▶). N. Jb. Miner. Mh. pp. 373–384].

The microstructure, optical and magnetic property of nitrogen implanted SiO2

The microstructure, optical property and magnetism of nitrogen ion-implanted SiO2 were studied. The nitrogen incorporation into SiO2 was confirmed by means of x-ray photoelectron spectra and a Fourier-transform infrared absorption spectrum. The bond angle of Si–O–Si is decreased by nitrogen implantation. The absorption in the ultraviolet region is enhanced, which may be due to the band gap narrowing effect. Room-temperature ferromagnetic-like behavior is obtained in the nitrogen-implanted sample with a nominal dose of 1 × 1018 ions cm−2.

Synthesis and deprotonation of (phenylselenolatodimethylsilyl)bis(trimethylsilyl) methane: The crystal structure of (Me3Si)2(Me2SiBr)CH

The moisture-sensitive compound [{bromodimethylsilyl}bis(trimethylsilyl)methane], [(Me3Si)2(Me2SiBr)CH] 1 has been synthesized by the bromination of the hydride [(Me3Si)2(Me2SiH)CH] in CCl4 in good yield. Compound 1 has been characterized by multinuclear NMR and a single crystal X-ray diffraction studies. The average Si–C and BrMe2Si–C distances are, respectively, 1.894(2) and 1.886(2) Å. The Si–C–Si angle is 114.98(13)°. Treatment of 1 with NaSePh (obtained from the sodium triethylborohydride reduction of diphenyldiselenide) in benzene at room temperature resulted in the isolation of light yellow oil, [{phenylselenatodimethylsilyl}bis(trimethylsilyl)methane], [(Me3Si)2(Me2SiSePh)CH] 2. Compound 2 has been metallated with methyllithium as well as lithium diisopropylamide (LDA) in THF to give [(Me3Si)2(Me2SiSePh)CLi·2THF] 3. Compounds 2 and 3 have been characterized by elemental analysis, mass and multinuclear NMR studies.

Hezuolinite, (Sr,REE)4Zr(Ti,Fe3+,Fe2+)2Ti2O8(Si2O7)2, a new mineral species of the chevkinite group from Saima alkaline complex, Liaoning Province, NE China

Hezuolinite is a new mineral species occurring as an accessory mineral in the Saima alkaline complex in Liaoning Province, NE China. The mineral occurs in a green-aegirine nepheline syenite as non-metamict grains and is associated with microcline, nepheline, aegirine, biotite, eudialyte, rinkite and titanite. The anhedral crystals are several hundred micrometres in size. The mineral is black with dark-brown streak. It is translucent with a resinous luster. Cleavage or parting was not observed. It is brittle with a conchoidal fracture. The hardness is VHN 100 683–964 kg mm −2 (5.5–6 on Mohs’ scale). The measured density is 4.28 g cm −3 , and the calculated value is 4.30 g cm −3 from the empirical chemical formula. Optically, hezuolinite is biaxial negative, with n > 1.8 and 2V = 75°. The dispersion is strong with r > v . The mineral shows a strong pleochroism, from pale brown to dark brown. The average of twenty-five analyses leads to the empirical formula (Sr 2.15 Ce 0.55 La 0.49 Ca 0.49 Na 0.12 Nd 0.09 Pr 0.03 Th 0.03 Sm 0.01 Eu 0.01 ) ∑3.98 (Zr 0.82 Fe 2+ 0.14 Hf 0.02 Mn 0.01 ) ∑1.00 (Ti 1.38 Fe 3+ 0.36 Fe 2+ 0.14 Al 0.04 Nb 0.02 ) ∑1.94 Ti 2 O 8 (Si 2.01 O 7 ) 2 , on the basis of O = 22. The simplified formula is (Sr, REE ) 4 Zr(Ti,Fe 3+ ,Fe 2+ ) 2 Ti 2 O 8 (Si 2 O 7 ) 2 , ideally Sr 2 REE 2 Zr(Fe 2+ )Ti 3 O 8 (Si 2 O 7 ) 2 . The five strongest reflections in the X-ray powder-diffraction pattern [ d in A (I)( hkl )] are: 2.98(100)(−204), 3.02(90)(−313), 1.96(90)(024), 2.18(40)(−422), and 2.84(70)(020). The structure was solved and refined in space group C 2/ m , with a = 13.973(3), b = 5.6984(11), c = 11.988(2) A, β = 114.10(1) °, V = 871.3(3) A 3 , Z = 2, to R1 = 0.037, wR2 = 0.097. The structure of hezuolinite is similar to rengeite. The changes of Sr-O, Zr-O and Ti(1)-O distances confirm partial substitutions of REE for Sr, and Fe for Ti. The sites of O(1), O(2) and O(3) of the hezuolinite structure ( C 2/ m ) are split into O(1) and O(1′), O(2) and O(2′), O(3) and O(3′) sites in the rengeite structure ( P 2 1 / a ), respectively. The Si(1)-O(7)-Si(2) angle in hezuolinite is 175.8°, and is significantly less bent than in rengeite (169.7°). The observed systematic extinctions are in agreement with space group C 2/ m . Hezuolinite is a member of the perrierite-subgroup and can be considered as a chemical intermediate between rengeite and perrierite.

1,1,4,4-Tetra-tert-butyl-1,4-dichloro-2,2,3,3-tetraphenyltetrasilane

The title compound, C40H56Cl2Si4, was synthesized by the coupling of 1,1-di-tert-butyl-1,2-dichloro-2,2-diphenyl­disilane with lithium. The asymmetric unit contains one half-mol­ecule, which is completed by an inversion centre. In the mol­ecule, the tetra­silane skeleton adopts a perfect anti conformation and the Si—Si bonds [2.4355 (5) and 2.4328 (7) Å] are longer than the standard Si—Si bond length (2.34 Å). The Si—Si—Si angle [116.09 (2)°] is larger than the tetra­hedral bond angle (109.5°). These long bond lengths and the wide angle are favorable for reducing the steric hindrance among the tert-butyl and the phenyl groups. The dihedral angle between the phenyl rings in the asymmetric unit is 37.36 (8)°.

The Nature of Hydrogen Bonding Involving the Siloxane Group

Variation of the Si–O–Si angle in siloxane compounds is a way to tune their basicity from highly hydrophobic systems at linear geometry to hydrophilic systems at small angles. This has great potential in the design of new siloxane materials with properties distinct from those of known silicones. We investigate hydrogen bonds with the siloxane linkage as an acceptor in a large range of Si–O–Si angles for the two hydrogen-bonded complexes disiloxane⋯silanol [(H3Si)2O⋯HOSiH3] and disiloxane⋯water [(H3Si)2O⋯HOH] with free disiloxane [H3SiOSiH3] as reference in a quantum-mechanical ab-initio study. Geometry, electron density, and the electron localizability indicator provide several complementary indicators of hydrogen bonding which show how Si–O–Si angle variation affects the nature and strength of these unusual hydrogen bonds.

Structural elucidation of β-(Y,Sc)2Si2O7: combined use of89Y MAS NMR and powder diffraction

Although the structures of pure Sc2Si2O7and β-Y2Si2O7have been described in the literature using theC2/mspace group,29Si magic angle spinning (MAS) NMR measurements of the intermediate members of the Sc2Si2O7–β-Y2Si2O7system indicate a lowering of the symmetry to theC2 space group. Indeed, these compositions exhibit a unique Si crystallographic site and an Si—O—Si angle lower than 180°, incompatible with theC2/mspace group.C2 is the only possible alternative. Space groupCmcan be discarded with regard to its two different Si sites per unit cell. Moreover,89Y MAS NMR data have revealed the existence of two different Y sites in the structure of the intermediate members of the Sc2Si2O7–β-Y2Si2O7system, confirming the lowering of the symmetry to theC2 space group. The viability of theC2 model has therefore been tested and confirmed by refinement of synchrotron and neutron powder diffraction data for the different members of the system. The structural evolutions across the Sc2Si2O7–β-Y2Si2O7system are discussed.

Raman, Brillouin, and nuclear magnetic resonance spectroscopic studies on shocked borosilicate glass

Using Brillouin and Raman scattering and NMR techniques, we have investigated the elastic and structural properties of four post-shocked specimens of borosilicate glass, recovered from peak pressures of 19.8, 31.3, 40.3, and 49.1 GPa. The Raman spectra of shock-wave compressed borosilicate glass for peak pressures of 19.8 and 31.3 GPa show two new peaks at 606 cm−1 and near 1600 cm−1, while a peak at ∼923 cm−1 disappears in these glasses following shock-loading. The mode at 606 cm−1 is correlated with four-membered rings, composed of one BO4 and three SiO4 tetrahedra (a reedmergneritelike configuration). Modes near ∼1600 cm−1 are of uncertain origin, while that at 923 cm−1 may associated with silica tetrahedra with two nonbridging oxygens, although standard models of this type of glass suggest that total nonbridging oxygen contents should be low. The Raman spectra for the shocked samples at 40.3 and 49.1 GPa are similar to that of the unshocked sample, suggesting that much of the irreversible density and structural changes are recoverable following adiabatic decompression and thermal relaxation. This reversibility for the highest pressure shocked samples is in accord with the Brillouin results, which show an increase in the product of sound velocity and index of refraction at pressures up to 20 GPa. The Raman band initially at 450 cm−1, which corresponds to the bending vibration mode of the Si–O–Si, Si–O–B (with primarily six-membered rings in the network) reaches a maximum frequency of 470 cm−1 and narrowing at a peak shock pressure of 31.3 GPa, and then also decreases to its initial values for samples shocked at 40.3 and 49.1 GPa. This shift toward higher frequency under shock-wave compression indicates the average Si–O–Si, Si-O-B angles decrease with pressure. The narrowing of this band suggests a narrower distribution of Si–O–Si angles in the shocked samples for peak pressures of 19.8 and 31.3 GPa. 11B NMR spectra for all four shocked glasses are similar, and indicate ratios of BO3 to BO4 that are not greatly changed from the starting material. However, changes in peak shapes suggest significant changes in the connectivity of the B and Si components of the network, with more silicon neighbors surrounding BO4 tetrahedra in the shocked glasses, and a modest increase in the number of nonring related BO3 groups following shock-loading. Thus, the irreversible effects of shock-loading appear to be to generate smaller rings of tetrahedra (hence decreasing the average T–O–T bond angle), and to increase the average number of neighbors of Si around boron tetrahedra.

Polymerization of aqueous silica in H 2O–K 2O solutions at 25–200 °C and 1 bar to 20 kbar

Understanding the polymerization of aqueous silica is important for modeling fluid–rock interactions at high pressure and temperature. However, it remains unclear how changes in SiO 2 concentration, alkalinity, pressure, and temperature affect silica polymerization and depolymerization. We studied silica speciation in alkaline K 2O–SiO 2–H 2O solutions at ambient conditions (0.05, 0.5 and 5 molal Si, 0.5–0.8 K 2O/SiO 2), as well as at elevated pressure and temperature in a diamond anvil cell (5 molal Si, K 2O/SiO 2 = 0.7). The controls on polymerization were assessed by comparing Raman spectroscopic results to ab initio calculations that focused on four neutral species: the monomer, the dimer, and the linear and ring trimers. In addition to a ~ 785 cm − 1 band associated with Si–OH symmetric stretching in the monomer, the calculations show that Si–O–Si stretching in the dimer and trimers yields peaks at ~ 600 cm − 1 . A broad 600 cm − 1 feature has been observed in previous studies at high pressure and temperature, and the broadness has been attributed to a soft Si–O–Si angle in the dimer and temperature; however, our calculations show that this is not alone sufficient to explain this feature, and that other species likely contribute. The breathing mode of the ring trimer yields an additional feature at ~ 500 cm − 1 , which may be representative of contributions from highly polymerized species to the low-frequency limb of the broad 600 cm − 1 feature. Individual Si–OH stretching and bending in all species gave features at ~ 825–875 cm − 1 and ~ 950–1050 cm − 1 . These bands overlap significantly with Si–O − stretching modes of deprotonated species, which are more abundant in the alkaline solutions studied experimentally. Measured Raman spectra at ambient conditions display broad peaks associated with monomers (735–800 cm − 1 ), oligomers (475–650 cm − 1 ) and deprotonated species (850–1125 cm − 1 ). Peak areas attributed to oligomers and deprotonated species increase linearly with total Si concentration, with a small dependence on the K 2O/SiO 2 ratio, indicating that Raman intensities are proportional to species concentrations. The Raman peak area of the monomer feature does not scale linearly with total SiO 2 concentration, however, and depends strongly on the K 2O/SiO 2 ratio. In broad agreement with NMR results, the Raman spectra show that increasing silica concentration promotes silica polymerization, and that increasing the K 2O/SiO 2 ratio or temperature depolymerizes aqueous silica. The Raman spectra show that increasing pressure also causes aqueous silica to depolymerize. The results provide a foundation for characterizing aqueous silica polymerization at crustal and upper mantle conditions.

Computerized Control and Operation of Rutherford Backscattering/Channeling for an in situ Ion Beam System and Its Application for Measurement of Si(001) and ZnO(001)

A computer-automated Rutherford backscattering/channeling (RBS/C) system is developed to provide in situ ion beam analysis of the accelerator-TEM system in Wuhan University. The basic system components are a PC equipped with a multichannel analyzer data acquisition board, motion control hardware including the Panmure stepping motor controller and integrated circuit modules, and a Labview programmed operating system with associated electronics. Single crystalline Si(001) and ZnO(001) implanted with Mn ions were characterized with this computerized setup. The crystalline quality χmin and channeling half angle of Si(001) were measured to be 4.65% and 0.52°, respectively, which are comparable to theoretical values 4.2% and 0.32°. The ion implantation induced damage depth profile derived from channeling and random spectrum is in reasonable agreement with the result calculated by the SRIM Monte-Carlo simulation code.

A computational study of the molecular and crystal structure and selected physical properties of octahydridosilasequioxane–(Si2O3H2)4. I. Electronic and structural aspects

The free molecule octahydridosilasequioxane, Si8O12H8, was computationally studied, as well as embedded in the unit cell. The point group of the free molecule is indeedOh, and its crystal symmetry is reduced toC3i, thus confirming the occurrence of two different types of Si−O−Si bond lengths found experimentally. The molecular orbitals of the free molecule show that some electron density occurs in the cubic cavity, thus contributing to the opening of the Si−O−Si angle. A study of the packing in the unit cell identifies a new type of packing scheme in which eight (partial) molecules participate: each apex H atom of one protruding Si−H bond of every molecule points to the corner of anequilateraltriangle having 2.631 Å sides. All hydrogen atoms in both the free molecule and in the solid state carry negative partial charges. The reason for this is also explored, as well as its consequences for the unique packing scheme.

ChemInform Abstract: Synthesis and Crystal Structure of the Crowded Disiloxane (((Me3Si)3C) Cl2Si)2O.

Abstract The disiloxane (TsiCl 2 Si) 2 O (Tsi = (Me 3 Si) 3 C) has been prepared from TsiLi and (Cl 3 Si) 2 O. An X-ray diffraction study has revealed extensive disorder in the crystal, and although the data favour a linear SiOSi arrangement, with fully staggered Tsi groups, an alternative interpretation involving a bent structure, with an SiOSi angle of 139.5(1)°, cannot be completely ruled out. It is suggested that crystals of the related disiloxane (TsiF 2 Si) 2 O, diffraction data for which were previously erroneously interpreted by others, show the same disorder, and that the two disiloxanes are isomorphous.