Catalytic Regioselective Isomerization of 2,2‐Disubstituted Oxetanes to Homoallylic Alcohols

The selective isomerization of strained heterocyclic compounds is an important tool in organic synthesis. An unprecedented regioselective isomerization of 2,2‐disubstituted oxetanes into homoallylic alcohols is described. The use of tris(pentafluorophenyl)borane (B(C₆F₅)₃)_, a commercially available Lewis acid was key to obtaining good yields and selectivities since other Lewis acids afforded mixtures of isomers and substantial polymerization. The reaction took place under exceptionally mild reaction conditions and very low catalyst loading (0.5 mol %). DFT calculations disclose the mechanistic features of the isomerization and account for the high selectivity displayed by the B(C₆F₅)₃ catalyst. The synthetic applicability of the new reaction is demonstrated by the preparation of γ‐chiral alcohols using iridium‐catalyzed asymmetric hydrogenation.     

Is there a reentrant glass in binary mixtures?

By employing computer simulations for a model binary mixture, we show that a reentrant glass transition upon adding a second component occurs only if the ratio alpha of the short-time mobilities between the glass-forming component and the additive is sufficiently small. For alpha approximately 1, there is no reentrant glass, even if the size asymmetry between the two components is large, in accordance with the two-component mode-coupling theory. For alpha<1, on the other hand, the reentrant glass is observed and reproduced only by an effective one-component mode-coupling theory.     

Trends in the Synthesis of Polymer Nano- and Microscale Materials for Bio-Related Applications

Polymeric nano- and microscale materials bear significant potential in manifold applications related to biomedicine. This is owed not only to the large chemical diversity of the constituent polymers, but also to the various morphologies these materials can achieve, ranging from simple particles to intricate self-assembled structures. Modern synthetic polymer chemistry permits the tuning of many physicochemical parameters affecting the behavior of polymeric nano- and microscale materials in the biological context. In this Perspective, an overview of the synthetic principles underlying the modern preparation of these materials is provided, aiming to demonstrate how advances in and ingenious implementations of polymer chemistry fuel a range of applications, both present and prospective.     

Cluster-cluster correlation during irreversible diffusion-limited aggregation

Summary In this paper, we discuss the evolution of the scattered intensityI(q) during irreversible diffusion-limited cluster-cluster aggregation. We analyse twodimensional simulations and interpret the results within the framework of a recently proposed theoretical approach. The theory describes the correlation among different clusters which develops during the irreversible aggregation process. The model is based on two coupled differential equations, controlling the growth of the average cluster mass and the time dependence of the probability of finding pairs of clusters as a function of their distance. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Quantitative description of the self-assembly of patchy particles into chains and rings

We numerically study a simple fluid composed of particles having a hard-core repulsion complemented by two patchy attractive sites on the particle poles. An appropriate choice of the patch angular width allows for the formation of ring structures which, at low temperatures and low densities, compete with the growth of linear aggregates. The simplicity of the model makes it possible to compare simulation results and theoretical predictions based on the Wertheim perturbation theory, specialized to the case in which ring formation is allowed. Such a comparison offers a unique framework for establishing the quality of the analytic predictions. We find that the Wertheim theory describes remarkably well the simulation results.     

Physics of the liquid-liquid critical point

Within the inherent structure thermodynamic formalism introduced by Stillinger and Weber [Phys. Rev. A 25, 978 (1982)]], we address the basic question of the physics of the liquid-liquid transition and of density maxima observed in some complex liquids such as water by identifying, for the first time, the statistical properties of the potential energy landscape responsible for these anomalies. We also provide evidence of the connection between density anomalies and the liquid-liquid critical point. Within the simple (and physically transparent) model discussed, density anomalies do imply the existence of a liquid-liquid transition.     

Crossover (or Kovacs) effect in an aging molecular liquid

We study by means of molecular dynamics simulations the aging behavior of a molecular model of ortho-terphenyl. We find evidence of a nonmonotonic evolution of the volume during an isothermal-isobaric equilibration process, a phenomenon known in polymeric systems as the crossover (or Kovacs) effect. We characterize this phenomenology in terms of landscape properties, providing evidence that, far from equilibrium, the system explores regions of the potential energy landscape distinct from the one explored in thermal equilibrium.     

High-performance nanowire-grid polarizers

We developed a new type of wire-grid polarizer that has achieved excellent optical performance and reliability. The nanowire-grid polarizer is based on a fully optimized innovative design structure that consists of not only the core nanowire grid but also the surrounding multilayer thin-film structures. The surrounding structures are designed for antireflectivity to provide the best possible efficiency as well as for device reliability to provide the best possible handling robustness and environmental durability. The core nanowire grid utilizes nanosized high-aspect-ratio dielectric walls as a support for forming a high-aspect-ratio metal nanowire grid that significantly reduces energy loss as a result of metal absorption for the transmitted beam while providing a high extinction ratio of the blocked beam. The developed high-quality nanowire-grid polarizer has potential for use in many integrated optical applications.     

Interplay between time-temperature transformation and the liquid-liquid phase transition in water

We study the new water model proposed by Mahoney and Jorgensen [J. Chem. Phys. 112, 8910 (2000)], which is closer to real water than previously proposed classical pairwise additive potentials. We simulate the model in a wide range of deeply supercooled states and find (i) the existence of a nonmonotonic "nose-shaped" temperature of maximum density line and a nonreentrant spinodal, (ii) the presence of a low-temperature phase transition, (iii) the free evolution of bulk water to ice, and (iv) the time-temperature-transformation curves at different densities.     

Liquid stability in a model for ortho-terphenyl

We report an extensive study of the phase diagram of a simple model for ortho-terphenyl, focusing on the limits of stability of the liquid state. Reported data extend previous studies of the same model to both lower and higher densities and to higher temperatures. We estimate the location of the homogeneous liquid-gas nucleation line and of the spinodal locus. Within the potential energy landscape formalism, we calculate the distributions of depth, number, and shape of the potential energy minima and show that the statistical properties of the landscape are consistent with a Gaussian distribution of minima over a wide range of volumes. We report the volume dependence of the parameters entering in the Gaussian distribution (amplitude, average energy, variance). We finally evaluate the locus where the configurational entropy vanishes, the so-called Kauzmann line, and discuss the relative location of the spinodal and Kauzmann loci.