Confinement effects and acid strength in zeolites

Chemical reactivity and sorption in zeolites are coupled to confinement and—to a lesser extent—to the acid strength of Brønsted acid sites (BAS). In presence of water the zeolite Brønsted acid sites eventually convert into hydronium ions. The gradual transition from zeolite Brønsted acid sites to hydronium ions in zeolites of varying pore size is examined by ab initio molecular dynamics combined with enhanced sampling based on Well-Tempered Metadynamics and a recently developed set of collective variables. While at low water content (1–2 water/BAS) the acidic protons prefer to be shared between zeolites and water, higher water contents (n > 2) invariably lead to solvation of the protons within a localized water cluster adjacent to the BAS. At low water loadings the standard free energy of the formed complexes is dominated by enthalpy and is associated with the acid strength of the BAS and the space around the site. Conversely, the entropy increases linearly with the concentration of waters in the pores, favors proton solvation and is independent of the pore size/shape

Electronic structure of the ground and low-lying excited states of TiP

The electronic structure of TiP in its ground 2Σ+ and low-lying excited states (2Δ, 2Πr, and 4Δ) has been studied, using MCSCF and multireference CI techniques. We report bond energies, bond lengths, vibrational frequencies, dipole moments, and charge distributions. Additionally, we compare these results with previously reported results for TiN. © 1996 American Chemical Society

On the Role of Enthalpic and Entropic Contributions to the Conformational Free Energy Landscape of MIL‐101(Cr) Secondary Building Units

The thermo‐structural behavior of metal‐organic framework (MOF) precursors is responsible for regulating the introduction of defects in MOF structures during synthesis. In this paper, factors affecting the flexibility of MIL‐101(Cr) half‐secondary building units (half‐SBUs) are evaluated in solution using enhanced sampling methods. In particular, entropic and enthalpic contributions to the conformational free energy landscape of isolated MIL‐101(Cr) half‐SBUs are calculated in water, in the presence and absence of ionic species (Na^{+} and F^{−}), and in N, N‐dimethylformamide (DMF). This analysis leads to the observation that the interplay between enthalpy and entropy determines the most probable conformational state for half‐SBUs. This observation extends to the most relevant SBU intermediate, in which conformational entropy plays a key role in stabilizing configurations that differ from those found in the MIL‐101(Cr) crystal structure. The findings highlight the importance of explicitly considering entropic effects, associated with finite‐temperature sampling when estimating the relative stability of different conformers of SBUs

Properties of polyvinylchloride in solution: An hydrodynamic and vibrational spectroscopy study

The polymer polyvinylchloride has been studied in binary solvent mixtures and as a function of temperature in solution. A discontinuity of the polymer chain dimensions has been observed, as measured by hydrodynamic methods. This phenomenon is further examined by infrared and Raman spectroscopy

Molecular level understanding of the free energy landscape in early stages of MOF nucleation

The assembly mechanism of hierarchical materials controlled by the choice of solvent and presence of spectator ions. In this paper, we use enhanced sampling molecular dynamics methods to investigate these effects on the configurational landscape of metal-linker interactions in the early stages of synthesis, using MIL-101(Cr) as a prototypical example. Microsecond-long well-tempered metadynamics (WTmetaD) uncover a complex free energy structural landscape, with distinct crystal (C) and non-crystal (NC) like configurations and their equilibrium population. In presence of ions (Na+, F-), we observe a complex effect on the crystallinity of secondary building units (SBUs), by encouraging/ suppressing salt bridges between C configurations and consequently controlling the percentage of defects. Solvent effects are assessed by introducing N, N-dimethylformamide (DMF) instead of water, where SBU adducts are appreciably more stable and compact. These results shed light on how solvent nature and ionic strength impact the free energy of nucleation phenomena that ultimately control materials synthesis and defect formation

Estereotipos e identidad en los chistes.

The contention of this paper is jokes operate on stereotypes and that they are organized into a simulacrum of positive representation(s) that a specific group constructs about itself. In support of this argument, a set of jokes about blondes and gauchos (inhabitants from the State of Rio Grande do Sul, Brazil) are analyzed in this article.En este trabajo se defiende la hipótesis de que los chistes operan a partir de estereotipos, y que ellos se constituyen probablemente en un simulacro de la imagen positiva que un grupo construye de sí mismo. La hipótesis es defendida mediante un análisis sucinto de un conjunto de chistes sobre gauchos y rubias