Hydrogen-bond network in an equimolar acetic acid-water mixture as studied by neutron scattering and density functional theory.

The present study explores the hydrogen-bond network in an equimolar mixture of acetic acid and water (AA-W). The investigation was conducted using a combination of neutron scattering and Density Functional Theory (DFT). New neutron scattering data at large scattering wave vectors were analyzed to determine the total structure factor SM(q) and the molecular form factor F1(q) of the system. DFT calculations using the 6-311++G(d, p) basis set were performed to optimize the monomers and various AA-W H-bonded clusters, including one acetic acid (AA) molecule connected to one, two, and three water molecules. Consequently, three dimers, three trimers, and one tetramer have been considered in order to describe the local order in the mixture. In addition, this study focused on the H-bond interactions in the most probable clusters in the solution, using the natural bond orbital and the atoms in molecules analyses. Our analysis particularly shows that stronger H-bond interactions occur in the ring structures.

Intermolecular associations in an equimolar formamide-water solution based on neutron scattering and DFT calculations.

In the present work, we have investigated the intermolecular associations of formamide with water in an equimolar formamide-water solution (FA-Water) by means of neutron scattering in combination with density functional theory calculations. The neutron scattering data were analyzed to deduce the structure factor SM(q) and the intermolecular pair correlation function gL(r). By considering different hydrogen bonded FA-Water associations, it has been shown that some of them describe well the local order in the solution. Natural bond orbital and atoms in molecules analyses have been performed to give more insight into the properties of hydrogen bonds involved in the more probable models.

H-Bonding network in fully deuterated N-methylformamide-water mixtures as studied by neutron scattering. Complementarity to X-ray study.

In complementarity with X-ray scattering and as extension of our previous publication, neutron scattering measurements are combined to DFT calculation to investigate the structural features of N-methylformamide-water mixtures (NMF-water) for three water molar fractions x w = 0.5 , 0.66 and x w = 0.75 . The recorded data at atmospheric pressure and room temperature are analyzed to yield the structure factor, the molecular form factor and the pair correlation function. Neutron investigations corroborate the X-ray ones and clearly show that liquid order in solutions is well accounted for by a tetramer. In this cluster, an NMF molecule is connected to three water molecules by one N-D···O and two O-D···O hydrogen bonds.

Structural investigations of N-methylformamide-water mixtures at various concentrations.

Structural investigations of N-methylformamide-water mixtures (NMF-water) are performed at room temperature and atmospheric pressure for two water molar fractions x w = 0.66 and x w = 0.75 . This paper extends our recent study on the equimolar system. H-bond networks are preferentially formed between NMF and water molecules. Among a large variety of DFT optimized models, X-ray scattering data shows that the local order of each mixture is better described by a tetramer where one NMF molecule is connected to three water molecules. No self-association is observed in the considered systems. The effect of hydration is compared to the temperature and pressure effects in some hydrogen-bonded liquids.

Local order in fully deuterated liquid N-methylacetamide (C3D7NO) as studied by neutron diffraction and density-functional theory calculations.

A structural investigation of fully deuterated liquid N-methylacetmide (NMAd7) is performed at 308 K and atmospheric pressure by using neutron diffraction together with density-functional theory (DFT). The analysis of experimental data yields the total structure factor SM(Q), the molecular form factor F1(Q), and the distinct pair correlation function gL(r). The DFT calculations are performed to study the relative stability of the two possible isomers (trans and cis) and to examine some possible clusters recently published that may describe the intermolecular arrangement in the liquid state. Neutron measurements can be interpreted in terms of trans linear trimer (T1) and cis cyclic trimer (T2) where the total number of hydrogen bonds is respectively equal to two and three. The theoretical structure factors obtained on the basis of intermolecular arrangements agree fairly well with the experimental one beyond Q = 2 A-1. All through the study, a comparison is made with complementary X-ray results.

Local order in aqueous NaCl solutions and pure water: X-ray scattering and molecular dynamics simulations study.

The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O...O and O...H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O...O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.

Neutron scattering study of the H-bond network in amorphous N-methylformamide.

Original investigations of structural properties in noncrystalline solid N-methylformamide (HCONHCH(3)) are presented in this paper. After description of the sample preparation, the elastic neutron scattering formalism is applied to deduce the coherent scattering cross section for amorphous N-methylformamide. The contribution of each possible monomer (cis or trans) is computed and the corresponding distinct structure factor is deduced. Our measurements clearly show that the hydrogen-bond network occurs in amorphous N-methylformamide between C=O and N-H groups. The experimental spectra are consistent with an open-chain structure similar to the one existing in the crystalline form. The more probable clusters that have been identified in the liquid are also examined.

X-ray scattering and density-functional theory calculations to study the presence of hydrogen-bonded clusters in liquid N-methylacetamide.

A structural investigation of liquid N-methylacetamide (NMA) is performed by x-ray scattering and density functional theory (DFT). Experimental data are analyzed to yield the total structure function SM(Q) and the pair correlation function g(r). The DFT calculations, using the standard triple zeta valence basis set augmented by a diffuse function for carbon, nitrogen and oxygen atoms, are performed on the one hand to study the structure and stability of the two possible conformers cis and trans. On the other hand, they are meant to examine some possible clusters which may describe the intermolecular arrangement in liquid NMA. Among two series of dimers and trimers associations, the spectra are particularly interpreted in terms of: Trans NMA dimers and trimers which resemble the short-range crystal structure, mixed cis and trans trimers and cis cyclic trimers. The H-bonding parameters and the intermolecular energy for each model are described.

Neutron scattering experiments on fully deuterated liquid N-methylformamide DCONDCD3 at various temperatures and under pressure. Comparison to X-ray results.

Neutron scattering experiments are performed on fully deuterated liquid N-methylformamide (C2D5NO) at various temperatures and under pressure. The recorded data at atmospheric pressure and room temperature are analyzed to yield the molecular form factor and the distinct pair correlation function. Our measurements clearly show that the hydrogen-bond network, of which the parameters are deduced, persists locally in the liquid. The experimental structure factor could be explained in terms of short-range crystal structure. The r(N...O) distance decreases with increasing temperature from 293 to 398 K, whereas no significant variation of the intermolecular structure is detected when varying pressure from 1 bar to 4 kbar. Along the study, some comparison is made with complementary X-ray results.

Intermolecular association in liquid N-methylacetamide as studied by x-ray scattering.

A structural investigation of liquid N methylacetamide was performed at 308 K using x-ray scattering. To extract the molecular form factor F1(q), the geometry of the conformer which has been found in the crystal is considered. The intermolecular structure function DM(q) is interpreted in terms of H-bonding interactions. The crystal N...O distance is taken into accounted and the number of H bond(s) is assumed to be, respectively, equal to one and two. The liquid structure can be described by a linear dimer or chainlike trimer similar to the ones existing in the crystal. The structure factors SM(q) extracted from these clusters fairly agree with the experimental one beyond q=2.5 A(-1).

Water-silica gel interactions, X-ray diffraction study at room and low temperature.

X-ray diffraction experiments on water confined in silica gel powder hydrated at about 20% are presented and analyzed at room temperature and down to 77 K. The structural modification of confined water observed at second neighbors is due to the competition between the confinement effect and the water-silica interaction.