Small-membered Rings Research Articles

Bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the molar CaO/Al2O3 = 1 join via molecular dynamics (MD) simulation

In order to understand how surface charge accumulation and other reactivity-related phenomena are controlled by the surface structure, it is necessary to have a detailed appreciation of the bulk glass structures. Here, we report a detailed MD study of the bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the tecto-silicate join.It is confirmed that the structures of these glasses contain small concentrations of over- and under-coordinated defect species, such as NBO, TBO and [5]-coordinated aluminum, AlC5, in contrast to the simple idea that these fully charge-compensated compositions, with Ca/Al2O3 ratios of unity, should have fully polymerized structures. The Si,Al distribution was found to be largely random (i.e. Lowenstein's rule is not obeyed) and no significant segregation of Ca to Al was observed.Interactions between TBO and AlC5 were manifest both in the presence of an additional small peak in the Al-O-Al BAD and in the formation of structural complexes involving small-membered rings, particularly 2-rings. These defect structural complexes contributed to the structural inhomogeneity of these glasses.

Amorphous silicene—a view from molecular dynamics simulation

Models of amorphous silicene (a-silicene) containing 104 atoms are obtained by cooling from the melt via molecular dynamics (MD) simulation. The evolution of various kinds of structural and thermodynamic behavior in models upon cooling from the melt is found, including total energy, radial distribution function (RDF), interatomic distance, coordination number, and ring and bond-angle distributions. We also show the buckling distribution and a 2D visualization of the atomic configurations. The diffraction pattern shows that a glass state is indeed formed in the system. The glass transition temperature of 2D silicon ( K) has a reasonable value compared to that of its 3D counterpart. Calculations show that although most atoms in a-silicene obtained at 300 K have a three-fold coordination and mainly evolve into six-fold rings, a-silicene also contains various structural defects including those not found in crystalline silicene (c-silicene) such as adatoms, clusters of small-membered rings, large-membered rings and local linear defects. The concentration of defects in a-silicene is much higher than that of the crystalline version. We find that buckling is not unique for all the atoms in the model. The strong distorted structure of a-silicene compared to that of the crystalline version may lead to physico-chemical properties, including the possibility of opening the band gap in the former compared to the zero band gap of the latter. Note that due to the fixed length being equal to buckling of 0.44 Å in the direction with the elastic reflection behavior boundary, our models are relevant for a-silicene formed in confinement between two planar simple hard walls.

Molecular dynamics simulation of amorphous SiO2thin films

Amorphous SiO 2 thin films have been studied via molecular dynamics (MD) simulations. Thin film models (with two free surfaces) have been obtained by cooling from the melt with reactive force field (ReaxFF) potential. Structural and dynamic properties of the thin films are analyzed via radial pair distribution functions (RPDFs), coordination number distributions, ring statistics and bond-angle distributions. Fraction and role of structural defects have been analyzed and discussed. We also show temperature dependence of various thermodynamic quantities of the system. We found that structure of interior of thin films is close to that of the bulk while surface region contains a large amount of structural defects including dangling bonds, undercoordinated sites, small membered rings. Small and large membered rings concentrate mainly in the surface region and their fraction has a tendency to decrease with decreasing temperature.

Size-dependent elasticity of amorphous silica nanowire: A molecular dynamics study

Size-dependent Young's modulus of amorphous silica nanowires was systematically studied using classical molecular dynamics simulations. Our study showed that different sample preparation methods (casting, cutting, and cutting-straining) can cause substantial difference in the elastic response of silica nanowires. An eigenstress model was developed to describe the size-dependent elasticity and proved to be applicable in silica nanowires with radius down to 2 nm. Coupling between the core softening and the surface stiffening dictates the overall elasticity of silica nanowires, whose structure is different from that of bulk silica glass mainly in the surface region where small membered rings and coordination defects reside.

Towards understanding of the germanate anomaly in europium–lead–germanate glasses

Glasses in the system xEu 2O 3·(100 − x)[7GeO 2·3PbO 2] with 0 ≤ x ≤ 25 mol% have been prepared from melt quenching method. Influence of europium ions on structural behavior in lead-germanate glasses has been investigated using infrared spectroscopy and DFT calculations. The structural changes have been analyzed with increasing rare earth concentration. The general trend in the intensity of IR spectra is to increase with Eu 2O 3 content in the 700–1200 cm − 1 region, whereas the intensity in the 400–600 cm − 1 is remained almost unaffected. This suggests that the glass network modification has taken place mainly in the germanate part whereas the lead part remained unmodified and its network consists mainly from the [PbO 3] and [PbO 4] structural units. Thus, the germanate network in europium–lead–germanate glasses exists mostly as the [GeO 4] tetrahedral units, the [GeO 6] octahedral structural units and with interconnected through Ge–O–Ge bridges in [GeO 4] structural units. These europium–lead–germanate systems exhibit a photosensitive effect which can be induced through laser exposures ( λ = 633 nm) directly on the bulk sample. Structural investigations by FTIR spectroscopy show that the presence of fivefold Ge as a possible transitional phase from four to sixfold Ge was necessary. At higher content of europium oxide, the anomaly behavior of the germanium is due to the formation of small membered rings of [GeO 4] and the apparition of non-bridging oxygens. The reversion to the formation of non-bridging oxygens may thus tend to become energetically more favorable as europium oxide content increases.

Raman characterization of planar waveguides fabricated by electron-beam irradiation of germanium-doped flame hydrolysis deposited silica

The structural changes responsible for the refractive index variations induced in Ge-doped flame-hydrolysis deposited (FHD) silica by electron-beam irradiation have been studied using confocal micro-Raman spectroscopy. Two structural mechanisms, an increase in the concentration of 4- and 3-membered rings in the silica structure and a reduction of the oxygen bridging bond angle between silica tetrahedra have been observed. The formation of small membered rings occurs along the penetration depth of the electrons in the layer, decreasing with depth. The reduction of the angle between silica tetrahedra only occurs on the surface of the material. The results are consistent with previous measurements of the compaction of the material and will determine the shape of the refractive index profile of the waveguides fabricated using this technique.

Structure and diffusion in amorphous aluminum silicate: a molecular dynamics computer simulation.

The amorphous aluminum silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100 K> or =T> or =2300 K as well as glass configurations that were obtained from cooling runs from T=2300 to 300 K with a cooling rate of about 10(12) K/s. Already at temperatures as high as 4000 K, most of the Al and Si atoms are fourfold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wave number q=0.5 A(-1). A comparison of experimental x-ray data with the results from the simulation shows good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.

Design and synthesis of inhibitors of inducible nitric oxide synthase. Discovery of a new chemical lead with potential for oral bioavailability

A series of 2-iminopiperidines fused to small-membered rings ( Tables 1 and 2) were synthesised and biologically evaluated using an in vitro human nitric oxide synthase (NOS) inhibition assay. Fused bicyclic compounds 5– 9 exhibited nearly the same potency as compound 1 in the hiNOS inhibition assay. Among these, the 1-methyl analogues 8 and 9 showed better isoform selectivity than their corresponding unsubstituted analogues 7 and 6, respectively. Compounds 5 and 6 were also evaluated by an in vivo NO accumulation assay in a mouse model. The discovery process of new chemical leads for an orally bioavailable inhibitor of human inducible NOS (iNOS) is reported. The structure–activity relationship (SAR) study and chemistry of these compounds are also reported.

Structure, energies, and vibrational properties of silica rings inSiO2glass

We have carried out ab initio molecular orbital calculations on four isomers of ${\mathrm{Si}}_{9}{\mathrm{O}}_{25}{\mathrm{H}}_{14}$ modeling the local structure of ${\mathrm{SiO}}_{2}$ glass at the Hartree-Fock level. These clusters consist of two-, three-, four-, five-, and/or six-membered silica rings. The strain energies of the two-, three-, and four-membered rings are estimated by comparing the total energies of the relevant isomers. The strain energy of the four-membered ring is estimated to be 0.02 eV, indicating that the configuration of the four-membered ring is almost fully relaxed. The strain energies of the two- and three-membered rings are calculated to be 1.85 and 0.26 eV, respectively, and these values are in good agreement with the previous results calculated for the continuous ${\mathrm{SiO}}_{2}$ network models based on a generalized-gradient approximation to density-functional theory [D. R. Hamann, Phys. Rev. B 55, 14 784 (1997)]. It has been shown that there exist bonding wave functions that are localized in these small membered rings. We then have performed frequency calculations for the clusters, and the vibrational modes associated with these silica rings are discussed.

The Pentamethylcyclopentadienyl Ligand at Phosphorus or Arsenic – Effect on Small-Membered Rings and Double–Bond Species

Abstract The versatility of the pentamethylcyclo-pentadienyl(Cp∗) ligand is based on its ability for kinetic stabilization and on its potential application as a leaving group1,2,3. Here we report new results from the chemistry of diphosphenes and arsaphosphenes as well as of three– and four-membered ring systems. Concerning the Cp∗ reactivity, we have studied: I) nucleophilic substitution reactions, II) homolytic Cp∗[sbnd]El bond cleavages, and III) Cp∗ migration to Transition Metals.

Structure of Sodium Aluminosilicate Glass Surfaces

Sodium aluminosilicate (NAS) glass surfaces with compositions containing approximately 63% SiO2 and Al/Na ratios, R, of 0.25 R 2.0 were simulated using the molecular dynamics technique with a multibody interaction potential. There were changes to the surface structure and composition in comparison to bulk NAS glasses. The changes included an increased concentration of sodium and oxygen and the formation of nonbridging oxygen at the outermost surfaces, although the increases were smaller with increased Al concentration. In addition, the formation of small‐membered rings and three‐coordinated aluminum occurred in the subsurface regions. These changes were accompanied by a change in the ratio of Al/Na in the region extending to 4 Å below the surface.

Gem-Dialkyl effect in the intramolecular Diels-Alder reaction of 2-furfuryl methyl fumarates: the reactive rotamer effect, the enthalpic basis for acceleration, and evidence for a polar transition state

Investigation of the rates of cyclization of a series of substituted 2-furfuryl methyl fumarates has allowed us to determine which of the two explanations for the gem-dialkyl effect is more important. Studies with compounds substituted with small-membered rings showed that the rate acceleration is due primarily to the reactive rotamer effect and not to angle compresion («Thorpe-Ingold effect»)

Polycyclic systems containing small-membered rings

Abstract

The calculation of strain energy by molecular orbital theories

Strain energies are calculated for a series of polycyclic alkanes containing small-membered rings by the MINDO molecular orbital theory. Comparison of experimental and theoretical strain energies indicates that while the MINDO method accounts for the total strain semiquantitatively, it fails to predict correctly the nonadditivity of strain in bicyclic and spirocyclic systems. Significant strain energies are also predicted for cyclopropane and cyclobutane by the Extended Huckel Method, although these effects are swamped by errors introduced in the handling of nonbonded interactions by the method.