A comparative molecular dynamics study of sulfuric and methanesulfonic acids

The molecular dynamics computer simulation method has been used to study sulfuric and methanesulfonic acids. Calculations have been carried out between 200 K and 400 K using reliable force fields. Thermodynamic properties, such as the density, the heat of vaporization and the melting temperature, have been computed. Moreover, structural and dynamical quantities, such as the radial distribution functions, the shear viscosity and the diffusion coefficients, have also been calculated. The results display a noticeable good agreement with the available experimental data. A hydrogen bond analysis has also been performed, which shows, on one hand, that sulfuric acid has a hydrogen bond network which resembles the one of water; and, on the other hand, that methanesulfonic acid has a hydrogen bond structure which, in some details, recalls the one of methanol, but with a more important presence of single bonds and, to a lesser extent, of branching. Finally, the dynamics of the formation and rupture of hydrogen bonds has also been analyzed. To this end, the interrupted or slow hydrogen bonding lifetimes have been calculated using two different procedures. Our findings suggest that the sulfuric acid hydrogen bond network is more labile than the methanesulfonic acid one.Postprint (author's final draft

Dynamically Slow Processes in Supercooled Water Confined Between Hydrophobic Plates

We study the dynamics of water confined between hydrophobic flat surfaces at low temperature. At different pressures, we observe different behaviors that we understand in terms of the hydrogen bonds dynamics. At high pressure, the formation of the open structure of the hydrogen bond network is inhibited and the surfaces can be rapidly dehydrated by decreasing the temperature. At lower pressure the rapid ordering of the hydrogen bonds generates heterogeneities that are responsible for strong non-exponential behavior of the correlation function, but with no strong increase of the correlation time. At very low pressures, the gradual formation of the hydrogen bond network is responsible for the large increase of the correlation time and, eventually, the dynamical arrest of the system and of the dehydration process.Comment: 14 pages, 3 figure

Probing defects and correlations in the hydrogen-bond network of ab initio water

The hydrogen-bond network of water is characterized by the presence of coordination defects relative to the ideal tetrahedral network of ice, whose fluctuations determine the static and time-dependent properties of the liquid. Because of topological constraints, such defects do not come alone, but are highly correlated coming in a plethora of different pairs. Here we discuss in detail such correlations in the case of ab initio water models and show that they have interesting similarities to regular and defective solid phases of water. Although defect correlations involve deviations from idealized tetrahedrality, they can still be regarded as weaker hydrogen bonds that retain a high degree of directionality. We also investigate how the structure and population of coordination defects is affected by approximations to the inter-atomic potential, finding that in most cases, the qualitative features of the hydrogen bond network are remarkably robust

Crystal structures of N ′-Aminopyridine-2-carboximidamide and N ′-{[1-(pyridin-2-yl)ethylidene]amino}pyridine-2-carboximidamide

© Eya'ane Meva et al. 2017. The crystal structures of N,-Aminopyridine-2-carboximidamide (C 6 H 8 N 4 ), 1, and N,-{[1-(pyridin-2-yl)ethylidene]amino}pyridine-2-carboximidamide (C 13 H 13 N 5 ), 2, are described. The non-H atoms in compound 1 are nearly planar (r.m.s. deviation from planarity = 0.0108 Å ), while 2 is twisted about the central N-N bond by 17.8 (2)-. Both molecules are linked by intermolecular N-H·N hydrogen-bonding interactions; 1 forms a two-dimensional hydrogen-bonding network and for 2 the network is a one-dimensional chain. The bond lengths of these molecules are similar to those in other literature reports of azine and diimine systems

Systematic Hydrogen‐Bond Manipulations To Establish Polysaccharide Structure–Property Correlations

A dense hydrogen‐bond network is responsible for the mechanical and structural properties of polysaccharides. Random derivatization alters the properties of the bulk material by disrupting the hydrogen bonds, but obstructs detailed structure–function correlations. We have prepared well‐defined unnatural oligosaccharides including methylated, deoxygenated, deoxyfluorinated, as well as carboxymethylated cellulose and chitin analogues with full control over the degree and pattern of substitution. Molecular dynamics simulations and crystallographic analysis show how distinct hydrogen‐bond modifications drastically affect the solubility, aggregation behavior, and crystallinity of carbohydrate materials. This systematic approach to establishing detailed structure–property correlations will guide the synthesis of novel, tailor‐made carbohydrate materials

Systematic Hydrogen‐Bond Manipulations To Establish Polysaccharide Structure–Property Correlations

A dense hydrogen‐bond network is responsible for the mechanical and structural properties of polysaccharides. Random derivatization alters the properties of the bulk material by disrupting the hydrogen bonds, but obstructs detailed structure–function correlations. We have prepared well‐defined unnatural oligosaccharides including methylated, deoxygenated, deoxyfluorinated, as well as carboxymethylated cellulose and chitin analogues with full control over the degree and pattern of substitution. Molecular dynamics simulations and crystallographic analysis show how distinct hydrogen‐bond modifications drastically affect the solubility, aggregation behavior, and crystallinity of carbohydrate materials. This systematic approach to establishing detailed structure–property correlations will guide the synthesis of novel, tailor‐made carbohydrate materials