Silicon-containing Molecules Research Articles

Cybotactic behavior in the de Vries smectic-A* liquid-crystal structure formed by a silicon-containing molecule.

We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.

Electron-Deficiency Aromaticity in Silicon Nanoclusters.

Aromaticity in silicon-containing molecules has been a controversy for more than a century. Combining molecular dynamics simulations with ab initio calculations, we show here that it is possible to obtain aromatic-like behavior with pure hydrogenated silicon clusters without the need for multiple bonds. To this end, we exploit the natural tendency of silicon toward overcoordination to construct electron-deficient molecules with ring structures. Even without the incorporation of any protective bulky substituents the resulting structures are more stable than any other known hydrogenated silicon nanoparticles of this size and exhibit aromatic-like properties due to strong electron delocalization.

Design and Synthesis of Hydroxyapatite with Organic Modifiers for Application to Environmental Remediation

Composites of hydroxyapatite (HAP) and organic compounds have been prepared for biomedical applications. These same composites may be applicable to environmental remediation. The organic compounds used for modification include organic acids, calcium chelating agents, proteins, compounds capable of ring-opening polymerization, and hydrophobic silicon-containing molecules. The methods for modification that have been studied are co-precipitation and post-HAP particle formation. It is proposed that modified-HAP (mHAP) could have enhanced metal ion selectivities. The techniques used to synthesize mHAP are reviewed.

Copper, Silver, and Gold Complexes in Hydrosilylation Reactions

The reduction of diverse functional groups is an essential protocol in organic chemistry. Transition-metal catalysis has been successfully applied to the reduction of olefins, alkynes, and many carbonyl compounds via hydrogenation or hydrosilylation; the latter presenting several advantages over hydrogenation. Notably, hydrosilylation generally occurs under mild reaction conditions, and consequently over-reduced products are rarely detected. Moreover, the great majority of hydrosilanes employed in this reaction are easily handled, inexpensive, or both. A large number of multiple bonds can be involved in this context, and the hydrosilylation reaction can be regarded as a useful method for the synthesis of silicon-containing organic molecules or a convenient way of reducing organic compounds. Furthermore, the silyl group can also be retained as a protecting group, a strategy that can be of great usefulness in organic synthesis. Since the first Wilkinson's catalyst-mediated hydrosilylation of ketones in 1972, metals such as rhodium and iridium have attracted most of the attention in this area. A wide array of catalytic systems for hydrosilylation reactions is nowadays available, which has allowed for a great expansion of the synthetic scope of this transformation. After having been overlooked in the early years, group 11 metals (Cu, Ag, and Au), especially copper, have emerged as appealing alternatives for hydrosilylation. The use of a stabilized form of copper hydride, the hexameric [(Ph3P)CuH]6, by Stryker represented a breakthrough in copper-catalyzed reduction reactions. Nowadays, several copper-based catalytic systems compare well with a variety of reported rhodium-based catalysts, which generally suffer from the high cost of the catalyst. Tertiary phosphine ligands are the most widely used in these transformations. However, other families such as N-heterocyclic carbenes (NHCs) have shown promising activities. Compared with copper, little attention has been paid to silver- or gold-based catalysts. Silver salts have been considered inert towards hydrosilylation, and they are often employed as innocent anion exchange reagents for the in situ generation of cationic transition metal catalysts. Despite the rare reports available, they have already shown interesting reactivity profiles, for example, in the chemoselective reduction of aldehydes in the presence of ketones. Furthermore, 1,2-hydride delivery is favored over 1,4-reductions for alpha,beta-unsaturated carbonyl compounds, in contrast with most copper-based systems.

Chemical analysis by ultrahigh-resolution nuclear magnetic resonance in the Earth’s magnetic field

High-resolution NMR spectroscopy is a powerful tool for non-destructive structural investigations of matter1. Typically, expensive and immobile superconducting magnets are required for chemical analysis by high-resolution NMR spectroscopy. Here we present the feasibility of liquid-state proton (1H), lithium (7Li) and fluorine (19F) ultrahigh-resolution NMR spectroscopy2 in the Earth’s magnetic field. We show that in the Earth’s field the transverse relaxation time T2 of the 7Li nucleus is very sensitive to its mobility in solution. The J-coupling constants3 of silicon-containing (29Si) and fluorine-containing molecules are measured with just a single scan. The accuracy of the measured 1H–29Si and 1H–19F J-coupling constants is between a few millihertz up to 20 mHz. This is at least one order of magnitude better than the precision obtained with superconducting magnets. The high precision allows the discrimination of similar chemical structures of small molecules as well as of macromolecules.

Density functional theory calculation of 2p core-electron binding energies of Si, P, S, Cl, and Ar in gas-phase molecules

Density functional theory (DFT) calculations have been performed on the gas-phase 2p core-electron binding energies (CEBEs) of Si, P, S, Cl, and Ar in 145 cases using the following procedure: Δ E KS (scalar-ZORA + E xc)/TZP//HF/6-31G(d). Δ E KS is the difference in the total Kohn–Sham energies of the 2p-ionized cation and the neutral parent molecule calculated by DFT using different exchange-correlation functionals E xc with triple-zeta polarized basis set, at molecular geometry optimized by HF/6-31G(d), and relativistic effects have been estimated by scalar zeroth-order regular approximation. Among the 26 functionals tested, the form of E xc giving the best overall performance was found to be the combination of OPTX exchange and LYP correlation functionals. For that functional, the average absolute deviation (AAD) of the 145 calculated CEBEs from experiment is 0.26 eV. There are seven other exchange-correlation functionals that led to AADs of less than 0.30 eV. Some functionals give lower AADs than E xc = OPTX-LYP for some individual elements. In the case of Si, for example, the combination of either mPW91-PBE or Becke88-Perdew86 led to an AAD of only 0.10 eV for 56 silicon-containing molecules. Another example is the case of the argon atom, for which the choice of E xc = OPTX-Perdew86 yields a value for CEBE equal to the experimental value.

Extension of the PDDG/PM3 Semiempirical Molecular Orbital Method to Sulfur, Silicon, and Phosphorus

The PDDG/PM3 semiempirical molecular orbital method has been parameterized for molecules, ions, and complexes containing sulfur; the mean absolute error (MAE) for heats of formation, DeltaH(f), of 6.4 kcal/mol is 35 - 40 % smaller than for PM3, AM1, and MNDO/d. For completeness, parameterization was also carried out for silicon and phosphorous. For 144 silicon-containing molecules, the DeltaH(f) MAE for PDDG/PM3, PM3, and AM1 is 11 - 12 kcal/mol, while MNDO/d yields 9.4 kcal/mol. For the limited set of 43 phosphorus-containing molecules, MNDO/d also yields the best results followed by PDDG/PM3, AM1, and PM3. The benefits of the d-orbitals in MNDO/d for hypervalent compounds are apparent for silicon and phosphorous, while they are masked in the larger dataset for sulfur by large errors for branched compounds. Overall, for 1480 molecules, ions, and complexes containing the elements H, C, N, O, F, Si, P, S, Cl, Br, and I, the MAEs in kcal/mol for DeltaH(f) are 6.5 (PDDG/PM3), 8.7 (PM3), 10.3 (MNDO/d), 10.8 (AM1), and 19.8 (MNDO).

Bond-order correlation energies for small Si-containing molecules compared with ab initio results from low-order Møller–Plesset perturbation theory

The present study of small molecules containing silicon was motivated by (a) the considerable interest currently being shown in the kinetics and reactivity of such molecules, and (b) the biotechnological potential of silicon-derivate surfaces as substrates for the adsorption of, for instance, amino acids and proteins. Therefore, we have studied by (i) a semi-empirical approach and (ii) an ab initio procedure employing low-order Møller–Plesset perturbation theory, the molecular correlation energies of some neutral closed and open shell silicon-containing molecules in the series SiX n Y m . Procedure (i) is shown to have particular merit for the correlation of the ionic members studied in the above series, while the ab initio procedures employed come into their own for neutral species.

Surface micro-structuring of silicon by excimer-laser irradiation in reactive atmospheres

The formation mechanisms of cones and columns by pulsed-laser irradiation in reactive atmospheres were studied using scanning electron microscopy and profilometry. Deep etching takes place in SF6- and O2- rich atmospheres and consequently, silicon-containing molecules and clusters are released. Transport of silicon from the etched/ablated regions to the tip of columns and cones and to the side of the cones is required because both structures, columns and cones, protrude above the initial surface. The laser-induced micro-structure is influenced not only by the nature but also by the partial pressure of the reactive gas in the atmosphere. Irradiation in Ar following cone formation in SF6 produced no additional growth but rather melting and resolidification. Subsequent irradiation using again a SF6 atmosphere lead to cone restructuring and growth resumption. Thus the effects of etching plus re-deposition that produce column/cone formation and growth are clearly separated from the effects of just melting. On the other hand, irradiation continued in air after first performed in SF6 resulted in: (a) an intense etching of the cones and a tendency to transform them into columns; (b) growth of new columns on top of the existing cones and (c) filamentary nano-structures coating the sides of the columns and cones.

Silicate-siloxane fire retardant composites

The one-pot synthesis and characterization of a series of silicate-siloxane composites derived from vermiculite by reaction with hydroxy-terminated poly(dimethylsiloxane)s is described. Cone calorimetry has been used to obtain heat release rate measurements. The results show that even at the highest irradiance levels the samples had very long ignition times. Ignition resistance decreased as the hydrocarbon content of the composite increased. The results from the cone calorimeter are compared with those obtained by pyrolysis gas chromatography-mass spectrometry which showed that small volatile silicon-containing molecules are formed during pyrolysis.

Theoretical Study of the Heats of Formation of Small Silicon-Containing Compounds

Heats of formation for nine small silicon-containing molecules were obtained from large basis set ab initio calculations using coupled cluster theory with a perturbative treatment of triple excitations. After adjusting the atomization energies for the finite basis set truncation error, core/valence correlation, scalar relativistic, higher order correlation, and atomic spin−orbit effects, the theoretical and experimental 0 K values of ΔHf values were in good agreement. Using 106.6 kcal/mol as the heat of formation of silicon, we obtain ΔHf values of SiH = 87.7 ± 0.4 vs 89.5 ± 0.7 (expt); SiH2(1A1) = 64.1 ± 0.4 vs 65.5 ± 0.7 (expt); SiH2(3B1) = 85.4 ± 0.4 vs 86.5 ± 0.7 (expt); SiH3 = 47.3 ± 0.5 vs 47.7 ± 1.2 (expt); SiH4 = 8.7 ± 0.6 vs 9.5 ± 0.5 (expt); Si2 = 138.8 ± 0.4 vs 139.2 (expt); Si2H6 = 19.7 ± 0.5 vs 20.9 ± 0.3 (expt); SiF = −14.8 ± 0.4 vs −5.2 ± 3 (expt); SiF2 = −151.7 ± 0.5 vs −140.3 ± 3 (expt); and SiF4 = −384.5 ± 0.9 vs −384.9 ± 0.2 (expt). Based on the present work, we suggest a number of revisions in the interpretation of the experimental data. Although a revision in ΔHf°(Si) to 107.4 ± 0.6 kcal/mol at 0 K leads to improved agreement between theory and experiment for the SixHy compounds, it worsens agreement for SiF4. Given the remaining uncertainties in the theoretical approach, more definitive conclusions do not appear to be warranted.

Silicon microcolumn arrays grown by nanosecond pulsed-excimer laser irradiation

Arrays of high aspect ratio silicon microcolumns that protrude well above the initial surface have been formed by cumulative nanosecond pulsed-excimer laser irradiation of silicon. Microcolumn growth is strongly affected by the gas environment, being enhanced in air or other oxygen-containing ambient. It is proposed that microcolumn growth occurs through a combination of pulsed-laser melting of the tips of the columns and deposition of silicon from the intense flux of silicon-rich vapor produced by ablation of the surface regions between columns. The molten tips of the columns are strongly preferred sites for deposition, resulting in a very high axial growth rate. The growth process is conceptually similar to the vapor–liquid–solid method used to grow silicon whiskers. However, in the present case the pulsed-laser radiation fulfills two roles almost simultaneously, viz., providing the flux of silicon-containing molecules and melting the tips of the columns.

The influence of dust condensation on the molecular abundances of the carbon star circumstellar envelope

Abstract Calculations of chemical equilibrium in the circumstellar envelope of carbon star IRC+10216 are performed. In order to account for the amount of C and Si lost by gas phase due to C and SiC dust condensation the variations of concentration of C and Si in this phase were used and the problem of chemical equilibrium in the gas phase was solved. Our calculations lead to the conclusion that if C/O changes from 2 to 1.1 the abundances of such C-bearing molecules as HCN, CH4 and C2H2 change by about two orders and, if dust formation stops when C/O ≤ 1.1, the assumption of chemical equilibrium can explain the observable abundances of C4H2, CH4, HCN, and HC3N in the inner envelope of the star. The SiC dust formation in the envelope can explain the observable deficiency of some silicon-containing molecules, namely SiO and SiC2 when Si depletion is about 70%. The upper limit of SiC2 abundance given by Gensheimer et al. (1995) is in accordance with our equilibrium calculations. The calculated abundances of s...

Prediction of molecular Auger rates using a statistical model

Total Auger rates for some nitrogen- and silicon-containing molecules have been calculated by two different methods. Using a direct approach, the rates for individual transitions have been calculated and then totalled. Alternatively, the total rate has been determined using a statistical approach based upon knowledge of the Mulliken atomic orbital populations in the molecule and the atomic Auger subshell rates. There is good agreement between the two approaches. Total Auger rates based on the statistical approach enable a first estimate of some core-hole inherent lifetimes in molecules to be determined.

Substituent effects in silicon hydrides: implications for models of surface sites

To establish guidelines for using small silicon-containing molecules to model silicon surfaces, a series of substituted silanes have been studied using post-Hartree-Fock and density functional theories. The two theories differ in their detailed predictions, but similar trends are found with both methods. Si-Si and Si-H bond lengths vary slightly among the compounds studied. Mulliken charges on the substituted silicon atom are altered significantly, but Mulliken charges on the other atoms remain unaffected. The Si-Si bond energy decreases by about 0.5 kcal/mol with each silyl group that replaces hydrogen. Force constants change by a few percent

X-ray and Auger de-excitation processes in resonantly-excited and core-ionized molecules

Some recent developments in the understanding of molecular Auger and X-ray spectra are reviewed. Factors leading to complex molecular Auger spectra are discussed. The importance of correlation satellites in some spectra, such as for the SiF 4 molecule, are reported. The prediction of inner-shell photoelectron linewidths and the discrepancies with experiment for silicon-containing molecules are reported. The strong dependence of resonant X-ray emission spectra on photon excitation energy is discussed in the context of the CO 2 and N 2O molecules.

Nonclassical structures in silicon-containing molecules: The monobridged isomer of Si2H2

A monobridged isomer of the disilyne Si2H2, the planar bridged-2 form Si(H)SiH, has been detected through its millimeter- and submillimeter-wave rotational spectrum. The molecules are produced in a low pressure, low power plasma in a mixture of SiH4 and Ar cooled at liquid nitrogen temperature. From the analysis of a total of 148 lines, a set of accurate rotational constants have been derived. Using the geometry predicted by ab initio calculations (see the preceding paper), the spectrum of Si(D)SiD has been predicted, observed, and analyzed. From the two sets of rotational constants, a preliminary molecular structure has been derived by fixing the Si1–Si2–bridged H angle to the ab initio value of 52.5°: Si1–Si2=2.119 Å; Si2–bridged H=1.629 Å; Si2–terminal H=1.474 Å; Si1–Si2–terminal H=157.5°.

Synthesis and structure of silicon-containing cage molecules

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis and structure of silicon-containing cage moleculesRobert Damrauer, Joseph A. Hankin, and R. Curtis HaltiwangerCite this: Organometallics 1991, 10, 12, 3962–3964Publication Date (Print):December 1, 1991Publication History Published online1 May 2002Published inissue 1 December 1991https://doi.org/10.1021/om00058a008RIGHTS & PERMISSIONSArticle Views98Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (433 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Nonclassical double-bridged structure in silicon-containing molecules: Experimental evidence in Si2H2 from its submillimeter-wave spectrum.

The first spectroscopic evidence of a double-bridged silicon molecule is reported. The submillimeter-wave rotational spectrum of the disilyne ${\mathrm{Si}}_{2}$${\mathrm{H}}_{2}$ has been observed in a low-power silane plasma cooled at liquid-nitrogen temperature, with argon as buffer gas. These measurements led to the determination of the ${\mathit{r}}_{0}$ structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the $^{29}\mathrm{Si}$ and $^{30}\mathrm{Si}$ monosubstituted forms.

ChemInform Abstract: Laboratory Studies of Extraterrestrial Chemistry Initiated by Atomic Silicon Ions in the Gas Phase

Gas-phase chemistry initiated by atomic silicon ions occurs in a variety of extraterrestrial environments including diffuse interstellar clouds,1 dense interstellar clouds,2 and the atmospheres of certain stars.3 Reliable observational evidence for extraterrestrial molecules containing Si exists for SiO, SiS and SiC2 but is limited otherwise, in contrast to evidence which has been obtained for larger numbers of molecules containing less abundant elements. This lack of observational evidence is partly due to the lack of experimental spectral information for relevant silicon-containing molecules. Such information is difficult to obtain in the terrestrial laboratory. Also, one needs to know which silicon-containing molecules to search for. Suggestions have been made in the past,4 but the basis of these suggestions is somewhat obscure. Here we explore experimentally the kinds of silicon-containing molecules which may be produced via the chemistry initiated by atomic silicon ions in the gas phase.