Salah Nasr

X-ray and neutron scattering studies of the temperature and pressure dependence of the structure of liquid formamide

A structural investigation of liquid formamide was performed by neutron scattering at pressures of up to 4 kbar. We have also deduced the molecular pair correlation functions from x-ray studies at ambient pressure and in the 263–326 K temperature range. The spectra obtained are consistent with the presence of hydrogen-bonded open chains of molecules in the liquid state. The effect of both pressure and temperature on the hydrogen-bond network is examined.

Short-range order in liquid N-methylformamide HCONHCH3, as studied by x-ray scattering and density functional theory calculations

The local order in liquid N-methylformamide has been studied by using x-ray diffraction and a density functional theory (DFT). Experimental data were analyzed to yield the total structure function SM(q) and the intermolecular pair correlation function gL(r). DFT calculations, using, namely, the standard TZV basis set, were performed to study the relative stability of the two possible isomers (cis and trans) and to examine some possible intermolecular arrangements in the liquid state. X-ray measurements can be interpreted in term of cyclic trimers of cis form molecules where each monomer can establish two N–H⋯O hydrogen bonds that are described.

X-ray and neutron scattering studies of liquid formic acid DCOOD at various temperatures and under pressure

A structural investigation of fully deuterated liquid formic acid was performed by neutron scattering at pressure up to 3 kbar. The molecular pair correlation function was also deduced from x-ray study of DCOOD at ambient pressure and at 294 K. The results could be explained in terms of an open-chain structure with only two H bonds per molecule. The mean O⋯O distance is about 2.72 A. The effect of both temperature and pressure on the hydrogen bond network is examined.

H-bonding in liquid acetamide as studied by x-ray scattering

A structural investigation of liquid acetamide was performed at 346 K using x-ray scattering. The data are analyzed to yield the molecular structure factor and the distinct pair correlation function. Two H-bonds per molecule are found on average. The mean N⋯O distance is about 3.03 A. The liquid structure can be described by some structural units similar to the ones existing either in the rhombohedral or in the orthorhombic crystalline form.

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.

X-ray scattering study of amorphous formamide

Noncrystalline solid formamide can be prepared by vapor deposition onto substrates kept at 77 K. A structural investigation of the so-obtained samples using x-ray scattering indicates that there are, on average, four hydrogen-bonds per molecule. The amorphous structure can be described by a short-range order which resembles the one in the crystalline solid.

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).

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

Abstract.Structural investigations of N-methylformamide-water mixtures (NMF-water) are performed at room temperature and atmospheric pressure for two water molar fractions xw = 0.66 and xw = 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.Graphical abstract

H-bonding in amorphous acetamide CH3CONH2 as studied by x-ray scattering

Noncrystalline solid acetamide (CH3CONH2) can be prepared by vapor deposition onto precooled substrates at 77 K. The structure factor SM(Q) derived from x-ray diffraction patterns is compared to that recently obtained for liquid acetamide near the melting point. The data are analyzed to yield the molecular structure factor F1(Q) and the distinct pair correlation function gL(r). Our measurements clearly show that the hydrogen bonded network occurs in amorphous acetamide between C=O and N–H groups. Two H-bonds per molecule are found in average. The mean N⋯O distance is about 3.01 A. The results could be explained in terms of an open-chain structure which resembles preferentially the one existing in the metastable crystalline form. The crystallization of the glassy film which occurs on heating is also studied.