scholarly journals DFT Study of Si/Al Ratio and Confinement Effects on the Energetics and Vibrational Properties of some Aza-Aromatic Molecules Adsorbed on H-ZSM-5 Zeolite

Computation ◽  
2020 ◽  
Vol 8 (3) ◽  
pp. 81
Author(s):  
Martine Castellà-Ventura ◽  
Alain Moissette ◽  
Emile Kassab
Keyword(s):  
Acid Sites ◽  
Vibrational Frequencies ◽  
Van Der Waals Interactions ◽  
Closed Region ◽  
Straight Channel ◽  
Confinement Effects ◽  
Narrow Region ◽  
Adsorption Complexes ◽  
Open Region ◽  
The Stability

The Si/Al ratio and confinement effects of zeolite framework on energetics and vibrational frequencies of pyridine and 4,4′-bipyridine adsorbed on Brønsted acid sites in the straight channel of H-ZSM-5 are investigated by DFT calculations at the B3LYP and M06-2X+D3 levels. The straight channel of H-ZSM-5 is simulated by a cluster of 32 tetrahedral centers covering the intersection between straight and zigzag channels. Pyridine and 4,4′-bipyridine adsorption at two different sites in the intersection (open region) and/or in the narrow region situated between two intersections (closed region) is studied. For two Si/Al ratios (31, 15), the ion pair complexes formed by proton transfer upon pyridine and 4,4′-bipyridine adsorption in the open region and for the first time in the closed region are characterized. Our results indicate: (i) the stability for all adsorption complexes is essentially governed by the dispersive van der Waals interactions and the open region is energetically more favorable than the closed region owing to the predominance of the dispersive interactions over the steric constraints exerted by the confinement effects; (ii) as the Al centers are sufficiently spaced apart, Si/Al ratio does not influence pyridine adsorption energy, but significantly affects the adsorption energies and the relative stability of 4,4′-bipyridine complexes; (iii) neither Si/Al ratio nor confinement significantly influence pyridine and 4,4′-bipyridine vibrational frequencies within their complexes.

Adhesion of Partially and Fully Collapsed Nanotubes

2018 ◽  
Vol 86 (1) ◽  
Author(s):  
Ming Li ◽  
Hao Li ◽  
Fengwei Li ◽  
Zhan Kang
Keyword(s):  
Finite Deformation ◽  
Adhesion Energy ◽  
Van Der Waals Interactions ◽  
Critical Conditions ◽  
Nano Composites ◽  
Deformation Model ◽  
Analysis And Design ◽  
Structural Rigidity ◽  
Multiwall Nanotubes ◽  
The Stability

The competition between the structural rigidity and the van der Waals interactions may lead to collapsing of aligned nanotubes, and the resulting changes of both configurations and properties promise the applications of nanotubes in nano-composites and nano-electronics. In this paper, a finite-deformation model is applied to study the adhesion of parallel multiwall nanotubes with both partial and full collapsing, in which the noncontact adhesion energy is analytically determined. The analytical solutions of both configurations and energies of collapsed nanotubes are consistent with the molecular dynamics (MD) results, demonstrating the effectiveness of the finite-deformation model. To study the critical conditions of generating the partially and fully collapsed multiwall nanotubes, our analytical model gives the predictions for both the geometry- and energy-related critical diameters, which are helpful for the stability analysis and design of nanotube-based nano-devices.

scholarly journals Energetics of carbohydrate binding by a 14 kDa S-type mammalian lectin

1995 ◽  
Vol 308 (1) ◽  
pp. 237-241 ◽  
Author(s):  
R Ramkumar ◽  
A Surolia ◽  
S K Podder
Keyword(s):  
Binding Constants ◽  
Van Der Waals Interactions ◽  
Titration Calorimetry ◽  
Carbohydrate Binding ◽  
Glucose Binding ◽  
The Stability ◽  
L 14 ◽  
Derivatives Of ◽  
Binding Lectin ◽  
Site Binding

The thermodynamics of the binding of derivatives of galactose and lactose to a 14 kDa beta-galactoside-binding lectin (L-14) from sheep spleen has been studied in 10 nM phosphate/150 mM NaCl/10 mM beta-mercaptoethanol buffer, pH 7.4, and in the temperature range 285-300 K using titration calorimetry. The single-site binding constants of various sugars for the lectin were in the following order: N-acetyl-lactosamine thiodigalactoside > 4-methylumbelliferyl lactoside > lactose > 4-methylumbelliferyl alpha-D-galactoside > methyl-alpha-galactose > methyl-beta-galactose. Reactions were essentially enthalpically driven with the binding enthalpies ranging from -53.8 kJ/mol for thiodigalactoside at 301 K to -2.2 kJ/mol for galactose at 300 K, indicating that hydrogen-bonding and van der Waals interactions provide the major stabilization for these reactions. However, the binding of 4-methylumbelliferyl-alpha-D-galactose displays relatively favourable entropic contributions, indicating the existence of a non-polar site adjacent to the galactose-binding subsite. From the increments in the enthalpies for the binding of lactose, N-acetyl-lactosamine and thiodigalactoside relative to methyl-beta-galactose, the contribution of glucose binding in the subsite adjacent to that for galactose shows that glucose makes a major contribution to the stability of L-14 disaccharide complexes. Observation of enthalpy-entropy compensation for the recognition of saccharides such as lactose by L-14 and the absence of it for monosaccharides such as galactose, together with the lack of appreciable changes in the heat capacity (delta Cp), indicate that reorganization of water plays an important role in these reactions.

Pressure- and temperature-induced unfolding studies: thermodynamics of core hydrophobicity and packing of ribonuclease A

2006 ◽  
Vol 387 (3) ◽  
pp. 285-296 ◽  
Author(s):  
Josep Font ◽  
Antoni Benito ◽  
Joan Torrent ◽  
Reinhard Lange ◽  
Marc Ribó ◽  
...  
Keyword(s):  
Hydrophobic Interactions ◽  
Rnase A ◽  
Van Der Waals Interactions ◽  
Hydrophobic Core ◽  
Experimental Conditions ◽  
Unfolded State ◽  
Different Temperatures ◽  
Methylene Groups ◽  
The Stability ◽  
Core Packing

Abstract In this work we demonstrate that heat and pressure induce only slightly different energetic changes in the unfolded state of RNase A. Using pressure and temperature as denaturants on a significant number of variants, and by determining the free energy of unfolding at different temperatures, we estimated the stability of variants unable to complete the unfolding transition owing to the experimental conditions required for pressure experiments. The overall set of results allowed us to map the contributions to stability of the hydrophobic core residues of RNase A, with the positions most critical for stability being V54, V57, I106 and V108. We also show that the stability differences can be attributed to both hydrophobic interactions and packing density with an equivalent energetic magnitude. The main hydrophobic core of RNase A is tightly packed, as shown by the small-to-large and isosteric substitutions. In addition, we found that large changes in the number of methylene groups have non-additive positive stability interaction energies that are consistent with exquisite tight core packing and rearrangements of van der Waals' interactions in the protein interior, even after drastic deleterious substitutions.

scholarly journals Optimized Pore Structures of Hierarchical HY Zeolites for Highly Selective Production of Methyl Methoxyacetate

Catalysts ◽  
2019 ◽  
Vol 9 (10) ◽  
pp. 865 ◽  
Author(s):  
Fei Chen ◽  
Dongxi Zhang ◽  
Lei Shi ◽  
Yan Wang ◽  
Guangwen Xu
Keyword(s):  
Strong Acid ◽  
Treatment Method ◽  
Alkaline Treatment ◽  
Acid Sites ◽  
Relative Crystallinity ◽  
Mesoporous Zeolites ◽  
Pore Structures ◽  
The Stability ◽  
Strong Acid Sites ◽  
Hy Zeolites

Several organic templates were introduced during acid or alkaline treatment to optimize pore structures of hierarchical HY zeolites. The influences of category and concentration of templates on the pore structures and acidity of hierarchical HY zeolites were systemically studied. The N2 adsorption-desorption showed that the micropore amount of the optimized HY zeolites obviously increased, while both the large mesopore size and amount remained almost unchanged. The XRD and NH3-TPD revealed that the optimized HY zeolites exhibited higher relative crystallinity and medium-strong acid sites amount than those of hierarchical HY zeolites produced without the addition of templates. The optimized HY zeolites were used for the synthesis of methyl methoxyacetate (MMAc) from dimethoxymethane (DMM) carbonylation. In comparison with parent HY, the conversion and the selectivity clearly increased from 36.43% to 96.32% and from 11.06% to 92.35%, respectively. The stability of the optimized zeolite was also conducted under the same conditions. The conversion and the selectivity remained nearly unchanged even through 24 h reaction, showing that the performance was extremely stable. The TG-DTA and GC-MS also indicated that the generation of coke was effectively inhibited. This catalyst treatment method, which is facile and highly efficient, provided a route for producing mesoporous zeolites.

scholarly journals Structure and Stability of Gas Adsorption Complexes in Periodic Porous Solids as Studied by VTIR Spectroscopy: An Overview

2020 ◽  
Vol 10 (23) ◽  
pp. 8589
Author(s):  
Montserrat R. Delgado
Keyword(s):  
Gas Adsorption ◽  
Variable Temperature ◽  
Spectroscopic Technique ◽  
Porous Solids ◽  
Large Temperature ◽  
Adsorption Complexes ◽  
The Stability ◽  
Standard Enthalpy Change

Variable-temperature infrared (VTIR) spectroscopy is an instrumental technique that enables structural characterization of gas-solid adsorption complexes by analysis of meaningful vibrational modes, and simultaneous determination of the standard enthalpy change (ΔH0) involved in the gas adsorption process, which allows one to quantify the stability of the corresponding complex. This is achieved by a van’t Hoff analysis of a set of IR spectra recorded over a sufficiently large temperature range. Herein, the use of this versatile spectroscopic technique is demonstrated by reviewing its application to the study of carbon monoxide, carbon dioxide and dinitrogen adsorption on several (alkaline) zeolites, which can be regarded as the archetype of periodic porous solids.

On Counting Rooted Triangular Maps

1965 ◽  
Vol 17 ◽  
pp. 373-382 ◽  
Author(s):  
R. C. Mullin
Keyword(s):  
Finite Number ◽  
Euclidean Plane ◽  
Single Point ◽  
Simple Closed Curve ◽  
Closed Curve ◽  
The Other ◽  
Closed Region ◽  
Open Region ◽  
Simply Connected ◽  
Triangular Maps

Let R be a simply connected closed region in the Euclidean plane E2 whose boundary is a simple closed curve C. A triangular map, or simply "map," is a representation of R as the union of a finite number of disjoint point sets called cells, where the cells are of three kinds, vertices, edges, and faces (said to be of dimension 0, 1, and 2, respectively), where each vertex is a single point, each edge is an open arc whose ends are distinct vertices, and each face is a simply connected open region whose boundary consists of the closure of the union of three edges. Two cells of different dimension are incident if one is contained in the boundary of the other.

scholarly journals Effect of van der Waals interactions on the stability of SiC polytypes

2016 ◽  
Vol 119 (17) ◽  
pp. 175101 ◽  
Author(s):  
Sakiko Kawanishi ◽  
Teruyasu Mizoguchi
Keyword(s):  
Van Der Waals ◽  
Van Der Waals Interactions ◽  
The Stability
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