A theoretical study of the effect of silicon substitution on hydrocarbon acidity

Ab initio molecular orbital calculations with double-zeta basis sets show the relative stabilities of three tautomers on the C₂SiH₄ energy hypersurface to be 3-silapropyne > 1-silaallene > 1-silapropyne. Comparison with literature values shows 1-silaallene to be more stable than 2-silaallene. Assuming deprotonation at carbon then the order of acidity is 1-silapropyne > 10-silaallene > 3-silapropyne > silaethane > silaethylene. For silaethylene and silaethane deprotonation occurs more easily at silicon than at carbon, while for both silapropynes and 1-silaallene carbon deprotonation is slightly favoured. The α-silyl group enhances the acidity of the adjacent methyl group and a silyl group in conjugation with a carboncarbon triple bond enhances the acidity of the alkynyl proton. The methyl, ethyl, and 2-silaethyl groups all weakly decrease the acidity of the alkynyl proton.     

The electron affinities of TCNE and TCNQ: the effect of silicon substitution

The cyanocarbons tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) are important electron acceptors used in organic electronic applications. A common approach to enhancing their performance is by structural modification with previous studies focusing on substituting the cyano ligands or annular moiety. In this work, we assess the effect of hypovalent substitution, swapping carbon for silicon, on the potential energy surfaces and adiabatic electron affinities (AEAs). Si-substitution generally enhances AEA, and in the case of TCNQ stabilizes an open-shell singlet diradical state. Such findings may find value in the design of new materials based on the cyanocarbon platform.

     

Photo-induced ligand substitution at a remote site via electron transfer in a porphyrin-appended rhenium carbonyl supermolecule

The photochemical and electrochemical properties of a Zn-porphyrin appended rhenium(I) tricarbonyl bipyridine 3-Me-pyridine complex have been investigated; visible-light sensitisation of electron transfer results in ligand substitution at a site remote from the chromophore.     

The substitution of germanium for silicon in AST-type zeolite

The substitution of silicon by germanium in the AST zeolite framework type, [Si_nGe_40−nO₈₀]*4(SDA⁺F⁻) expressed as unit cell content in its cubic F-centered symmetry, has been studied. Three different kinds of templates, dimethyldiethylammonium, dimethyldiisopropylammonium and isopropyltrimethylammonium cations, were used in the hydrothermal synthesis process in fluoride medium. The products were identified with XRD, MAS NMR, SEM and thermal analysis. The analysis of the X-ray powder diagrams shows that AST crystallizes in different space group symmetries depending on the nature of the SDA and the degree of Ge-substitution. The resonance signals of ¹⁹F in MAS NMR experiments for the pure Si- and Ge-end members are at −38.2 and −15 ppm, respectively, indicating that the F⁻-anion is located as co-template in the double-four-ring (D4R) of the tetrahedral framework. This is confirmed by Rietveld analysis of powder diffraction data of the pure Ge-end member. The peak splitting of the ¹⁹F NMR signal in pure GeO₂AST-type material is related to the displacement of F⁻ location inside the D4R. Two more distinct signals at −8 and −19 ppm, respectively, are observed for X-ray pure AST-samples of intermediate compositions and assigned to fluoride in D4R built of 4[GeO₄]- and 4[SiO₄]-tetrahedra (4Ge, 4Si) and to (2Ge, 6Si)-D4R, respectively. An ordered distribution of Ge in the AST-framework is proposed for cubic AST with compositions around Si/Ge=1.5–1 by correlating the intensities of ¹⁹F NMR signals and the results from chemical analysis. This model is further confirmed by the quantitative analyses of the corresponding ²⁹Si MAS NMR spectra.     

An ab-initio study of some homolytic substitution reactions of acyl radicals at silicon, germanium and tint

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in disilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/cc-pVDZ//MP2/cc-pVDZ), energy barriers (deltaE double dagger) of 77.2 and 81.9 kJ mol(-1) are calculated for the backside and frontside reactions respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (deltaE double dagger) of 53.7-84.2, and 55.0-89.7 kJ mol(-1) for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic substitution reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques.     

Internal energy requirements for remote site fragmentation

Attempts to induce remote site fragmentation by using low-energy collisional activation with a neutral target gas or with a surface (surface-induced dissociation) are reported. These data, in conjunction with knowledge of the internal energy deposition associated with these activation methods, provide information on the amount of internal energy needed to promote remote site fragmentation in selected compounds. The internal energy required for remote site fragmentation of lithiated oleyl alcohol was recently estimated to be 1.3–1.9 eV. Our experiments suggest that internal energies much greater than 2 eV are required to produce detectable remote site fragmentation in several different ions, including lithiated oleyl alcohol, n-dodecylbenzenesulfonate anion, stearylsulfate anion, stearate anion and protonated octadecylamine. The results also suggest that the internal energy required to record spectra which show remote site fragmentation is compound-dependent, i.e. strongly dependent on the nature of the ionic group.     

Size effects in reactive circular site interactions

The complete series solution for the reactant diffusion and reaction at two diffusion-controlled chemically reactive surface sites of radii a(1) and a(2), located in an inert plane an arbitrary center-to-center distance d apart, is presented. Rigorous, analytical forms are developed to calculate the site reaction rates in terms of the dimensionless intersite distance σ[=d/(a(1) + a(2))] and the site radius ratio γ(=a(1)∕/a(2)). Numerical simulation and approximate theoretical results from the recent literature are compared to the exact site reaction rates. While general agreement was noted over the ranges of γ and σ, significant errors in the Wilemski-Fixman-Weiss site rates were found at small γ and σ < 3.     

Synthesis of organosiloxanes with reactive groups at silicon atoms

The heterofunctional condensation of (dihydroxy)organodisiloxanes and -trisiloxanes with vinyl(hydro)organodichlorosilanes was studied with the purpose of preparing a,ω-dichloroorganosiloxanes with reactive Si-H and Si-CH=CH₂ groups. During the isolation, 1,9-dichloro-1,9-dihydropentamethyloctasiloxane disproportionates with the elimination of (dichloro)methylsilane to form heptamethylcyclotetrasiloxane. A series of cyclotetra-, cyclohexa-, and cyclooctasiloxanes with Si-H and Si-CH=CH₂ groups was obtained by the condensation of a,ω-dihydroxyorganosiloxanes with a,ω-(dichloro)divinyl(dihydro)organosiloxanes.     

Preparation of photovoltaic silicon by purification of metallurgical grade silicon with a reactive plasma process

The main methods of silicon purification involve physicochemical techniques of separation. They yield an ultrapure metal, but its price is not compatible with photovoltaic central power generation. The plasma zone melting process consists in the horizontal displacement of a melted zone under an argon-hydrogen-oxygen plasma. The melt zone is characterized by a vertical gradient of concentration and temperature which drains impurities toward the upper surface of the bar. Moreover, the displacement of the zone from the head to the tail of the bar ensures a horizontal drainage of impurities toward the tail. This technique has been used with success for purification of metallurgical grade silicon; for example, iron was reduced from 3500 ppm to 0.5 ppm at a displacement rate between 20 and 40 cm/h. The final impurity level was less than 1 ppm. The mass transfer was independent of the displacement rate at 40 cm/h, but dependent on the heat transfer rate. The segregation of the metals is caused by drainage toward the tail of the bar, while the plasma vaporizes impurities since the metallic and metalloidic impurities have high vapor pressures (e.g., S, P, C, O, Mn). Acid (HF-HNO₃) is used to dissolve the impurities between each passage of the zone. Boron elimination depends directly on the plasma oxygen concentration. Classical mathematic models of zone melting are not valid for plasma melting zone processes.     

Counter-ion effect in the nucleophilic substitution reactions at silicon: a G2M(+) level theoretical investigation

Three identity nucleophilic substitution reactions at tetracoordinated silicon atom with inversion and retention pathways: Nu + SiH₃Cl → Nu + SiH₃Cl[Nu = (1)Cl⁻, (2) LiCl, and (3) (LiCl)₂], are investigated using the G2M(+) theory. Our results show that changing the nucleophile can shift the mechanism (favorable pathway), stepwise from a single-well PES for reaction 1, via a double-well PES for reaction 2, to a triple-well PES for reaction 3, indicating the importance of steric and electronic effects on the S_N2@Si. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Isomorphous substitution of silicon by boron in layered sodium silicate hydrates

To investigate the influence of boron on the crystallization process and the substitution of silicon by boron in silicate layers, layered sodium silicate hydrates (Na-SH) have been synthesized hydrothermally from a boron-containing mixture. XRD measurements confirm the high crystallinity of the synthesized Na-SH.¹¹B MAS NMR measurements indicate that boron is incorporated luto the silicate layers instead of silicon atoms.     

An intramolecular nucleophilic substitution of oxetanes

The intramolecular attack of carbanions to oxetanes were examined and several three, five, and six membered carbocyclic compounds were synthesized.     

Mechanism of nucleophilic substitution at silicon: kinetic evidence on the slow formation of a pentacoordinate silicon intermediate

Kinetic studies of various coupling reactions between silicon derivatives (SiF, SiCl, SiOMe, SiH) and organometallic reagents (RMgX, RLi) were performed for systems involving retention and inversion of configuration. The results show that for both stereochemical courses, the rupture of the SiX bond is not the rate determining step. Results are in good agreement with the rate determining formation of a pentacoordinate silicon intermediate.     

Neutral and ionic dissociation patterns in hexacoordinate silicon chelates: a model nucleophilic substitution at pentacoordinate silicon

A model nucleophilic-displacement reaction coordinate at pentacoordinate silicon is demonstrated by neutral and ionic dissociation equilibria through a stable hexacoordinate complex.     

Diffusion-limited hyperbranched polymers with substitution effect

Highly branched structure has the essential influence on macromolecular property and functionality in physics and chemistry. In this work, we proposed a diffusion-limited reaction model with the consideration of macromolecular unit relaxations and substitution effect of monomers to study the structure of hyperbranched polymers prepared by slow monomer addition to a core molecule. The exponential relationship (R(g) ～ N(λ)) between the radius of gyration R(g) and the degree of polymerization N, was systematically analyzed at various branching degrees. It is shown that the effective exponent λ(eff) decreases at lower N and but increases toward that of diffusion-limited aggregation (DLA) clusters (λ(DLA) = 0.4) with the degree of polymerization increasing. The substitution effect of monomers in reaction strongly influences the evolution pathway of λ(eff). With the static light scattering technique, the fractal property of internal chains was further calculated. A general law about the radial distribution of the units of diffusion-limited hyperbranched polymers was found that, at smaller reactivity ratio k(12), the radial density of all monomer units D(A) declines from the center region to the peripheral layer revealing the dense core structure; however, at larger k(12), the density distribution shows a loose-dense-loose structure. These structural characteristics are helpful to deeply understand the property of hyperbranched polymers.