SiH Units Research Articles

An investigation into the Brønsted acidity of the perfluorinated alkoxy silanes {(F3C)3CO}3SiH and {(F6C5)3CO}2Si(Cl)H.

The perfluorinated alkoxy silanes {(F3C)3CO}3SiH (1) and {(F5C6)3CO}2Si(Cl)H (2) were prepared and fully characterized. Despite the high calculated Brønsted acidities, all attempts to deprotonate 1 and 2 to give the conjugate silanide ions failed due to the exceptionally short and strong Si-H bonds. In the solid state, the Si-H units are not involved in any intermolecular interactions, but instead the crystal packing consists of exceptionally short and strong F⋯F interactions. The cohesive energies are entirely comprised of London dispersion interactions, similarly as in the crystal structures of noble gases.

Polysiloxanes Grafted with Mono(alkenyl)Silsesquioxanes-Particular Concept for Their Connection.

Herein, a facile and efficient synthetic route to unique hybrid materials containing polysiloxanes and mono(alkyl)silsesquioxanes as their pendant modifiers (T8@PS) was demonstrated. The idea of this work was to apply the hydrosilylation reaction as a tool for the efficient and selective attachment of mono(alkenyl)substituted silsesquioxanes (differing in the alkenyl chain length, from -vinyl to -dec-9-enyl and types of inert groups iBu, Ph at the inorganic core) onto two polysiloxanes containing various amount of Si-H units. The synthetic protocol, determined and confirmed by FT-IR in situ and NMR analyses, was optimized to ensure complete Si-H consumption along with the avoidance of side-products. A series of 20 new compounds with high yields and complete β-addition selectivity was obtained and characterized by spectroscopic methods.

Pyridine Functionalized N-Heterocyclic Silane Complexes of Iridium and Rhodium–An Unexpected Change in Coordination

A new pyridine functionalized N-heterocyclic silane with ambident reactivity as a ligand has been synthesized and characterized by NMR spectroscopy (1H, 13C{1H}, 29Si), mass spectrometry, elemental analysis, and X-ray crystallography. This ligand reacts with iridium and rhodium cod precursors (cod = 1,5-cyclooctadiene) to yield two new complexes that possess divergent, and unexpected, binding properties. In particular, no oxidative addition occurs at the intraligand Si-H unit. With the iridium(I) center, the ligand acts as a tertiary amino-pyridine chelator, whereas coordination of rhodium(I) to the ligand occurs with arene π-complexation on an opposite side. The latter interaction yields an unprecedented, electronically induced, coordination change at the silicon center over a long spatial distance. The new iridium and rhodium compounds are of high interest as they provide two potentially different reactions sites in one complex and as these sorts of complexes are known to activate small molecules like d...

1H and 29Si NMR investigation of SinHn polysilanes with n≤60: A DFT study

Density functional theory (DFT) calculations were performed to investigate the electronic features of the most stable structures of SinHn polysilanes (n=4, 6, 8, 10, 12, 20, 24, 28, 30, 36, 50, and 60). To this aim, 29Si and 1H CS tensors as well as natural charge analyses are calculated for the optimized structures. 29Si CS parameters detect equivalent electronic environment for silicon atoms within SinHn polysilanes with n<20 while 29Si chemical shifts δiso of SinHn polysilanes with n≥20 are separated to few peaks. Seeking correlation between these peaks and local structures around silicon sites, Siα, Siβ, Siγ observed in these models shows that δiso(Siγ)<δiso(Siβ)<δiso(Siα). The silicon atoms of Si60H60, similar to Si20H20, are equivalent but unlike Si20H20 (with Siα sites) have Siγ local structure and present only one peak in chemical shift value of −154.5ppm. The similar values (26–28ppm) of 1H calculated chemical shieldings obtained for all the polysilanes means the same tendency of the silicon atoms on the surfaces of all cages for contribution to chemical bonding with hydrogen atoms. Although the total magnitude of charge transfer from silicon atoms on the surface of the cages to their chemically bonded hydrogen atoms increase in larger cages, charge transfer per SiH units remains constant in good agreement with the similarity of chemical shieldings for hydrogen atoms.

Slurry Phase Reaction of Elemental Silicon with Methanol in the Presence of Copper: Direct Synthesis of Trimethoxysilane

Slurry phase reaction of elemental silicon with methanol has been studied in the presence of copper using a small amount of cuprous chloride as an activator in DBT (dibenzyltoluene) at various temperatures from 200 <TEX>${^{\circ}C}$</TEX> to 320 <TEX>${^{\circ}C}$</TEX>. Trimethoxysilane (1a) with a Si-H unit was obtained as the major product and tetramethoxysilane (1b) as the minor product. The reaction worked well using a 0.5 wt % CuCl as an activator. The optimum temperature for this direct synthesis of 1a was 240 <TEX>${^{\circ}C}$</TEX>. Methoxysilanes were obtained in 95% yield with 81% selectivity to 1a from 85% conversion of elemental silicon.

Surface Reaction of Alkynes and Alkenes with H-Si(111): A Density Functional Theory Study

Recent experiments on the addition of alkene and alkyne molecules to H-terminated silicon surfaces have provided evidence for a surface chain reaction initiated at isolated Si dangling bonds and involving an intermediate carbon radical state, which, after abstraction of a hydrogen atom from a neighboring Si-H unit, transforms into a stable adsorbed species plus a new Si dangling bond. Using periodic density functional theory (DFT) calculations, together with an efficient method for determining reaction pathways, we have studied the initial steps of this chain reaction for a few different terminal alkynes and alkenes interacting with an isolated Si dangling bond on an otherwise H-saturated Si(111) surface. Calculated minimum energy pathways (MEPs) indicate that the chain mechanism is viable in the case of C(2)H(2), whereas for C(2)H(4) the stabilization of the intermediate state is so small and the barrier for H-abstraction so (relatively) large that the molecule is more likely to desorb than to form a stable adsorbed species. For phenylacetylene and styrene, stabilization of the intermediate state and decrease of the H-abstraction barrier take place. While a stable adsorbed species exists in both cases, the overall heat of adsorption is larger for the alkyne molecules.

In situ attenuated total reflection FTIR investigations of H2O, HSiCl3 and Co2(CO)8 on ZnSe in the range 600–4000 cm−1

A new attenuated total reflection (ATR)-IR set-up attached to an external port of a BOMEM DA3 FTIR instrument is used for in situ spectroscopy on ZnSe surfaces in the range 600–4000 cm−1. ATR-IR studies on silylating reactions had been restricted to the 1400–4000 cm−1 range because of the strong absorption bands corresponding to νas(Si-O-Si) and other limiting factors. Noteworthy spectra of growing very thin water films are observed. HSiCl3 is used to form polysiloxane films bearing Si-H units at the surface. Si-H bending at 802 and 820 cm−1, ν(SiCl3) at 596 cm−1, Si-O-Si modes at 1150 and 1067 cm−1, and bands at 910 and 880 cm−1 are shown to be useful to investigate the reaction. The Si-H groups at the surface are reacted with Co2(CO)8 to form (-O)3Si-Co(CO)4 complexes; this results in a decrease of the δ(Si-H) mode at 820 cm−1. The final spectrum compares well with that of [Co(CO)4(H7Si8O12)] in the range of the carbonyl vibrations.

Structural and electronic properties of trans-polysilene (SiH)x: Many-body perturbation theory versus density-functional methods.

The possibility of a Peierls-type lattice dimerization was investigated in trans-polysilene (SiH${)}_{\mathit{x}}$ by ab initio quantum-mechanical methods that provided results for polyacetylene previously in complete agreement with experiments. Electron correlation was treated in the fourth order of many-body perturbation theory (MBPT) as well as, for the purpose of comparison, by using different versions of density-functional theory (DFT). The classical electrostatic interaction in itself produced a dimerized ground state for (SiH${)}_{\mathit{x}}$ in the Hartree approximation. The bond alternation was somewhat increased by exchange at the Hartree-Fock (HF) level (\ensuremath{\Delta}${\mathit{r}}_{\mathrm{HF}}$=0.1392 \AA{}) and substantially reduced by electron correlation (\ensuremath{\Delta}${\mathit{r}}_{\mathrm{MP}4}$=0.0805 \AA{}). The HF dimerization energy of -40 meV per SiH unit was also diminished to -17 meV at the MP4 level. Gradient-corrected DFT potentials provided results for both the structural and energetic parameters in excellent agreement with those of the fourth-order MBPT. Though the HF value of the forbidden energy gap (4.04 eV) was reduced to 1.32 eV for correlated quasiparticles, (SiH${)}_{\mathit{x}}$ still remained an intrinsic semiconductor at all theoretical levels.

Vibrational properties of H/Si(111)-(1 × 1) surfaces: infrared absorption and electron energy loss spectroscopic studies

Flat, ideally H-terminated Si(111) surfaces can be chemically prepared with different degrees of homogeneity. The chemical and structural perfection of these surfaces has been investigated with infrared absorption and electron energy loss spectroscopies. The infrared lineshape of the Si-H stretch is clearly dependent on the extent of the perfect domains, and can be used as a probe of the structural perfection of the H/Si(111)-(1 × 1) surfaces. Best prepared surfaces contain domains, with an average size of 2 × 10 4 Si-H units. The samples contain no contaminant, at least to a 1% level, as inferred by electron energy loss spectra. The electron scattering mechanism is dominated by impact scattering, except for an intriguing loss, located at 795 cm -1. Chemical origin of this mode is discussed, but does not appear to be well supported. A strong anharmonic coupling (or Fermi resonance) between a silicon surface phonon and the Si-H bending mode is invoked.

Controlling Benzylic and Anomeric Functionality and Stereochemistry: Methodology and Syntheses Utilising Intramolecular Ionic Hydrogenation

Intramolecular hydride transfer from a hydroxyl-tethered SiH unit to a stabilised carbonium ion is shown to be a useful method for stereochemical control in benzylic and carbohydrate systems, as exemplified by racemic and nonracemic syntheses of key intermediates for the pseudopterosin/secopseudopterosin family, and by the synthesis of 2-deoxy-C-glycosides. These syntheses illustrate and compare two alternative methods of carbonium ion generation: olefin protonation, and Lewis acid-promoted sulfone ionisation. Simple model systems are used to show that ring size restrictions to hydride transfer in a 5-membered ring may be overcome by the appropriate choice of reagent, permitting the reaction to be used for stereochemical control from a vicinal, hydroxyl-bearing carbon.

Linear and nonlinear polarizabilities of t r a n s-polysilane from a b i n i t i o oligomer calculations

Linear and nonlinear static electronic polarizabilities of the σ-conjugated polymer trans-polysilane (t-PSi) are obtained at the Hartree–Fock level by extrapolation of ab initio calculations on the oligomers H3Si–(SiH2)N–SiH3 (N odd). For the longitudinal components, in particular, a 6–31 G basis is shown to yield accurate values if the chains are long enough. No computational advantage is found in using effective core potentials. Chain lengths through N=13 are sufficient to give limiting infinite chain polarizabilities with relatively small uncertainty. The calculated longitudinal linear polarizability per SiH unit is about the same as the corresponding property in the prototype π-conjugated polymers polyacetylene (PA) and polydiacetylene (PDA), but the longitudinal second hyperpolarizability is significantly less in the σ-conjugated polymer. In t-PSi the conjugation length is about half that in either PA or PDA. Frequency-dependent vibrational distortion contributions are estimated in the doubly harmonic approximation and turn out to be important in trisilane and pentasilane even at optical frequencies. An analysis is developed which sheds light on the expected behavior for increasing chain lengths. Reasonable agreement with experiment is obtained for both the electronic and vibrational longitudinal second hyperpolarizabilities.

Thermal Stability of SiHn Configurations in Fz Silicon Single Crystals

ABSTRACTThermal stability of SiHn (n=1∼4) configurations in FZ silicon crystals grown in Ar/H2 has been investigated by means of infrared absorption spectroscopy. Infrared absorption peaks at 2210, 2192, 2123 and 1946 cm−1, which are due to SiH4, SiH3, SiH2 and SiH units in silicon lattice, has been observed. It is found that the concentration of SiH4 and SiH increase with the decrease in SiH2 and SiH3 concentration at 500°C, and vice versa at 600°C. Annealing results suggest thermally induced structural transformations of SiHn configurations. We propose a model of the transformations through the cleavage of adjacent Si-H bonds to form a Si-Si bond and a H2 molecule, as well as the reaction of a H2 molecule with a Si-Si bond.

A COMPLEX MODEL FOR THE HYDROGENATED VACANCY IN CRYSTALLINE SILICON

The electronic energies of hydrogenated silicon vacancy which has Td symmetry are calculated. Using the method of completely neglecting of differential overlap (CNDO), we find the probability of the formation of the hydride quartet, and the effect of interaction between four SiH units in the model. We conclude that the hydride quartet complex is a more plausible model for the hydrogenated vacancy in crystalline silicon than that with four SiH bonds unrelated one another.