Formation Of Small Rings Research Articles

Comparative structural study of Al2O3–SiO2 glasses and amorphous thin films

AbstractWe compared the impact of alumina doping on the structure of Al2O3–SiO2 amorphous thin films and bulk glasses using Raman spectroscopy and x‐ray diffraction. In both thin films and bulk glasses, the addition of Al2O3 is accompanied by an increase in the mean Si–O–T angle and an evolution of the ring statistics with a decrease in the proportion of small rings. We evidenced structural differences between sputtered films and fused bulk glasses. Sputtered Al2O3–SiO2 thin films are about 6%–7% denser than their equivalent Al2O3–SiO2 bulk glasses. This difference is mainly due to a change in ring statistics with the formation of small rings within the sputtered thin films. These structural differences in atomic structural organization highlight the impact of the synthesis conditions and open the door to further investigation of the structure–functional property relationships in sputtered Al2O3–SiO2 thin films.

Critical Role of Dynamic Flexibility in Ge‐Containing Zeolites: Impact on Diffusion

Incorporation of germanium in zeolites is well known to confer static flexibility to their framework, by stabilizing the formation of small rings. In this work, we show that the flexibility associated to Ge atoms in zeolites goes beyond this static effect, manifesting also a clear dynamic nature, in the sense that it leads to enhanced molecular diffusion. Our study combines experimental and theoretical methods providing evidence for this effect, which has not been described previously, as well as a rationalization for it, based on atomistic grounds. We have used both pure‐silica and silico‐germanate ITQ‐29 (LTA topology) zeolites as a case study. Based on our simulations, we identify the flexibility associated to the pore breathing‐like behavior induced by the Ge atoms, as the key factor leading to the enhanced diffusion observed experimentally in Ge‐containing zeolites.

ChemInform Abstract: Ylide Formal [4 + 1] Annulation

AbstractReview: [32 refs.

Ylide formal [4 + 1] annulation.

Over the past few decades, ylide chemistry has been significantly extended to an area beyond olefination and small ring formation, among which ylide [4 + 1] annulation has been extensively explored, and five-membered ring structures, such as dihydrofurans, isoxazolines, pyrrolines, indoles, dihydropyrazoles and cyclopentenones, can be readily constructed through this type of transformation. An overview of the recent advances in this field is presented herein. Ylide [4 + 1] annulations are reviewed by highlighting their product diversity, selectivity and applicability, and the mechanistic rationale is presented when possible.

Effects of a protic ionic liquid on the reaction pathway during non-aqueous sol-gel synthesis of silica: a Raman spectroscopic investigation.

The reaction pathway during the formation of silica via a two-component “non-aqueou” sol-gel synthesis is studied by in situ time-resolved Raman spectroscopy. This synthetic route is followed with and without the addition of the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) in order to investigate its effect on the reaction pathway. We demonstrate that Raman spectroscopy is suitable to discriminate between different silica intermediates, which are produced and consumed at different rates with respect to the point of gelation. We find that half-way to gelation monomers and shorter chains are the most abundant silica species, while the formation of silica rings strongly correlates to the sol-to-gel transition. Thus, curling up of linear chains is here proposed as a plausible mechanism for the formation of small rings. These in turn act as nucleation sites for the condensation of larger rings and thus the formation of the open and polymeric silica network. We find that the protic ionic liquid does not change the reaction pathway per se, but accelerates the cyclization process, intermediated by the faster inclusion of monomeric species.

Strategic collusion in auctions with externalities

We study a first price auction preceded by a negotiation stage, during which bidders may form a bidding ring. We prove that in the absence of external effects the all-inclusive ring forms in equilibrium, allowing ring members to gain the auctioned object for a minimal price. However, identity dependent externalities may lead to the formation of small rings, as often observed in practice. Potential ring members may condition their participation on high transfer payments, as a compensation for their expected (negative) externalities if the ring forms. The others may therefore profitably exclude such demanding bidders, although risking tougher competition in the auction. We also analyze ring inefficiency in the presence of externalities, showing that a ring may prefer sending an inefficient member to the auction, if the efficient member exerts threatening externalities on bidders outside the ring, which in turn leads to a higher winning price.

Germanium coordination and the germanate anomaly

The structural mechanism responsible for the germanate anomaly along the X 2 O-GeO 2 join, where X = Li, Na, K, Rb, and Cs, was investigated using Raman spectroscopy. Density maxima were measured at ∼17.5–20 mol%, 15 mol%, and 10 mol% for Li 2 O, Na 2 O and K 2 O, respectively. A maximum at 15 mol% and a minimum at 32.5 mol% were measured for Rb 2 O while the Cs 2 O glasses exhibited a maximum at 17.5 mol%. A coordination change of IV Ge to VI Ge was not observed. It is proposed that the germanate anomaly is a consequence of formation of small 3-membered GeO 4 rings as alkali oxide is added to the glass. The small rings cause a density increase. The anomaly maximum is reached when continued small ring formation cannot occur without straining the glass network. At this point the network generates large numbers of Q 3 non-bridging oxygens (NBOs) and at higher alkali oxide contents, Q 2 NBOs. The formation of the NBOs causes a decline in the density. The Rb 2 O- and Cs 2 O-containing glasses do not exhibit similar density trends to the lighter alkali-containing glasses. This is because of competing density effects from smallring formation and the increased mass of the alkalis. The overall effect on the density trends of these glasses is to skew the anomaly maxima to higher alkali oxide compositions.

Intramolecular Cyclizations via Arylnitrenium Ions. Formation of a Six-Membered Ring Rather than a Macrocycle

The stereochemistry of 1-(3-benzyloxyphenyl)-2-(4-nitrophenyl)ethane has been studied. MMX calculations predicted, and 2D NOESY confirmed, that the bent conformation (global energy minimum) was such that six-membered ring formation, and not macrocyclization, would occur using the corresponding nitrenium ion, and this was found to be the case. Acid-catalyzed decomposition of 1-(3-benzyloxyphenyl)-2-(4-azidophenyl)ethane followed by treatment with (CF3CO)2O gave 48% of 2-benzyloxy-6-trifluoroacetamido-9,10-dihydrophenanthrene and 18% of 1-(3-benzyloxyphenyl)-2-(4-trifluoroacetamidophenyl)ethane. Blocking the original point of attack with a bromine atom led to the prediction (MMX, 2D NOESY) that, once again, small ring formation would take place, with macrocyclization possible but less likely. Again, this was found to be so. It is suggested that simple MMX calculations may provide a very rapid, empirical indicator of which precursors would have a readily accessible conformation that could result in intramolcular cyclization leading to macrocycles being preferred over intermolecular reactions.

A novel small ring formation using methylsulfinyl group as a leaving group

A novel small ring formation using methylsulfinyl group as a leaving group

Complex formation of small rings with chlorine at low temperatures

Complex formation of small rings with chlorine at low temperatures

Study of the kinetics of fast unimolecular decomposition processes and of organic rearrangement reactions by field ionization mass spectrometry

It is shown that extremely fast unimolecular decomposition processes of organic ions in the gas phase can be studied by means of the field ionization method. The shortest decomposition time which can be attributed to a process that remains virtually unmodified by the electrical field is about 10 −11 sec. A retarding potential technique was developed permitting measurement of the distribution of the average rate constants between 10 −8 and 10 −6 sec. A continuous distribution of rate constants was found for a number of large molecular ions in the range of ion times-of-flight between several 10 −11 and 10 −6 sec, in accordance with the quasi-equilibrium theory of the mass spectra of large molecules. Organic rearrangement reactions have been studied by combined electron impact and field ionization mass spectrometry. Three different types of rearrangement reactions were studied: 1. the formation of smaller rings from larger ones; 2. the McLafferty and other hydrogen rearrangement reactions; 3. skeletal rearrangement reactions. It is postulated that these reactions have lower rates than the single-step direct single-bond ruptures of the same molecules (the rearrangement reactions have large negative entropies of activation). This is confirmed by the field ionization mass spectra where the intensities of these rearrangement-process peaks are extremely small as compared with other fragment peaks. This is due to the extremely short time (several 10 −14−10 −12 sec) available for “normal” fragment formation after field ionization.

Group association equilibria in isolated polymer coils

AbstractWhen two or more groups attached to a single chain molecule participate in the formation of an association complex, the equilibrium for complex formation in dilute polymer solution depends on the local concentrations in the swollen polymer coil rather than on the stoichiometric concentration in the bulk of the solution. The theory of intramolecular group association is developed using two models. In the first, the interacting groups are evenly spaced along the polymer chain and the probabilities of their association are governed by Kuhn's statistics of chain configuration. In the second model, every segment of the chain has the same probability of carrying an interacting group and the Kuhn statistics are applied only to large rings, the probability of small ring formation being governed by steric factors. The predictions of the theory are in agreement with the extent of carboxyl association in dilute solutions of seven styrene‐methacrylic acid copolymers, measured by infrared spectroscopy. The chelation of cations in dilute polyelectrolyte solution also involves the participation of groups attached to a single macromolecule, but the interpretation of experimental data requires that the electrostatic free energy of complex formation be taken into account. Spectroscopic evidence shows that in copper(II) binding to poly(methacrylic acid) or poly(N, ε‐methacrylyllysine) higher complexes are formed than in solutions of simple carboxylic acids or α‐amino acids, respectively. Dialysis equilibrium studies indicate that, in the concentration range 4.7 × 10−3 to 15.6 × 10−3 N poly(acrylic acid), the polymer coils act independently of each other in chelating small amounts of copper(II), while the copper (II) affinity of poly(methacrylic acid) molecules drops off with rising polyacid concentration.

FUNDAMENTAL CONDITION OF GLASS FORMATION*

AbstractThe fundamental condition for glass formation is the existence of strongly bonded large networks or long chains of atoms in the liquid. Such chains or networks prevent or hinder the orderly orientation of atoms in the chains or networks themselves and also that of atoms, not in the networks in the liquid, near the melting point or liquidus temperature. A liquid structure will persist when the liquid is cooled below the melting point or liquidus temperature and will continue until free, long‐range movement of atoms becomes impossible. In order to obtain such long chains or large networks, it is necessary that (1) the bond strength of atoms in the chains or networks be very strong; (2) the small ring formation of these strongly bonded atoms be at a minimum; and (3) the relative numbers of various atoms in the chains or networks be such that at least a continuous chain is configuratively and structurally possible; at the same time, the coordination numbers of the glassforming atoms should be as small as possible to keep the bond strong. The general treatment is applied to all kinds of glasses, particularly oxide glasses. The bond strengths of all M—O single‐bond linkages in these glasses are calculated and tabulated here for the first time. The bond strength of all the glassformers was found to be greater than 80 kilocalories per Avogadro bond, that of the intermediates between 60 and 80, and that of the modifiers below 60; the transition is continuous, and the division into glassformers, intermediates, and modifiers is arbitrary.