Electrostatic properties in zeolite-type materials from high-resolution x-ray diffraction: The case of natrolite

Tracing electron density changes in langbeinite under pressure

Pressure is well known to dramatically alter physical properties and chemical behaviour of materials, much of which is due to the changes in chemical bonding that accompany compression. Though it is relatively easy to comprehend this correlation in the discontinuous compression regime, where phase transformations take place, understanding of the more subtle continuous compression effects is a far greater challenge, requiring insight into the finest details of electron density redistribution. In this study, a detailed examination of quantitative electron density redistribution in the mineral langbeinite was conducted at high pressure. Langbeinite is a potassium magnesium sulfate mineral with the chemical formula [K2Mg2(SO4)3], and crystallizes in the isometric tetartoidal (cubic) system. The mineral is an ore of potassium, occurs in marine evaporite deposits in association with carnallite, halite and sylvite, and gives its name to the langbeinites, a family of substances with the same cubic structure, a tetrahedral anion, and large and small cations. Single-crystal X-ray diffraction data for langbeinite have been collected at ambient pressure and at 1 GPa using a combination of in-house and synchrotron techniques. Experiments were complemented by theoretical calculations within the pressure range up to 40 GPa. On the basis of changes in structural and thermal parameters, all ions in the langbeinite structure can be grouped into 'soft' (potassium cations and oxygens) and 'hard' (sulfur and magnesium). This analysis emphasizes the importance of atomic basins as a convenient tool to analyse the redistribution of electron density under external stimuli such as pressure or temperature. Gradual reduction of completeness of experimental data accompanying compression did not significantly reduce the quality of structural, electronic and thermal parameters obtained in experimental quantitative charge density analysis.

Crystal engineering of a co-crystal of antipyrine and 2-chlorobenzoic acid: relative energetic contributions based on multipolar refinement

The growth and stability of a new 1 : 1 antipyrene–dichlorobenzoic acid cocrystal system has been analyzed in terms of electron density analysis and electrostatic interaction energy contributions.

CO diffusion as a re-orientation mechanism in the NaY zeolite

Our work is devoted to DFT calculations of the relative rotational and diffusional barriers for CO motions in zeolite NaY. The diffusion jump of CO adsorbed in NaY from NaII to Na'II has been confirmed as the favored way for CO re-coordination via either the C or the O atom to the Na cations instead of the CO rotation, hence explaining the mechanism which is responsible for the CO exchange between different positions and the changes in the intensities of the vibrational IR spectra. The fine structure of the vibrational C-O bands is explained by the different CO locations of adsorbed mono- and dicarbonyl species. The calculated activation energy of intra-cage CO diffusion from NaII-CO to Na'II-OC matches the respective experimental barrier observed in the NaX zeolite.

Experimental observation of charge-shift bond in fluorite CaF2

On the basis of a multipole refinement of single-crystal X-ray diffraction data collected using an Ag source at 90 K to a resolution of 1.63 Å^-1, a quantitative experimental charge density distribution has been obtained for fluorite (CaF₂). The atoms-in-molecules integrated experimental charges for Ca²⁺ and F^- ions are +1.40 e and -0.70 e, respectively. The derived electron-density distribution, maximum electron-density paths, interaction lines and bond critical points along Ca²⁺...F^- and F^-...F^- contacts revealed the character of these interactions. The Ca²⁺...F^- interaction is clearly a closed shell and ionic in character. However, the F^-...F^- interaction has properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This conclusion is supported by the topology of the electron localization function and analysis of the quantum theory of atoms in molecules and crystals topological parameters. The Ca²⁺...F^- bonded radii - measured as distances from the centre of the ion to the critical point - are 1.21 Å for the Ca²⁺ cation and 1.15 Å for the F^- anion. These values are in a good agreement with the corresponding Shannon ionic radii. The F^-...F^- bond path and bond critical point is also found in the CaF₂ crystal structure. According to the quantum theory of atoms in molecules and crystals, this interaction is attractive in character. This is additionally supported by the topology of non-covalent interactions based on the reduced density gradient.

Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials

DDEC6 atomic population analysis gives excellent performance for small and large molecules, porous solids, dense solids, solid surfaces, organometallic complexes, nanoclusters, and magnetic materials.

Experimental determination of electrostatic properties of Na-X zeolite from high resolution X-ray diffraction

High-resolution single crystal X-ray diffraction is used for the first time to obtain the charge density distribution in dehydrated Na-X zeolite. The electron density is extracted according to the Hansen & Coppens multipolar-model, from which Pval-κ-type atomic charges are derived. In order to compare the experimental electron density with theoretical calculations on zeolites and other minerals, a topological analysis is performed to derive AIM charges and electron density properties at bond critical points. The results are compared with that described in the literature. Finally, the electrostatic potential is evaluated in a periodic, mean field approach (disordered cation distribution in the Fd3[combining macron] space group) and for a given distribution of the cations (space group P1). The electrostatic energy is, then, derived in the neighbourhood of cation sites where the molecules are usually physisorbed.

Rapid topography mapping of scalar fields: large molecular clusters

An efficient and rapid algorithm for topography mapping of scalar fields, molecular electron density (MED) and molecular electrostatic potential (MESP) is presented. The highlight of the work is the use of fast function evaluation by Deformed-atoms-in-molecules (DAM) method. The DAM method provides very rapid as well as sufficiently accurate function and gradient evaluation. For mapping the topography of large systems, the molecular tailoring approach (MTA) is invoked. This new code is tested out for mapping the MED and MESP critical points (CP's) of small systems. It is further applied to large molecular clusters viz. (H(2)O)(25), (C(6)H(6))(8) and also to a unit cell of valine crystal at MP2/6-31+G(d) level of theory. The completeness of the topography is checked by extensive search as well as applying the Poincaré-Hopf relation. The results obtained show that the DAM method in combination with MTA provides a rapid and efficient route for mapping the topography of large molecular systems.

Dynamical properties of confined water nanoclusters: Simulation study of hydrated zeolite NaA: structural and vibrational properties

Water nanoclusters confined to zeolitic cavities have been extensively investigated by various experimental techniques. We report a series of molecular dynamics simulations at different temperatures and for water nanoclusters of different sizes in order to attempt an atomistic interpretation of the properties of these systems. The cavities of zeolite NaA are spherical in shape and about 1 nm in diameter and can host nanoclusters of water containing nearly up to 24 water molecules. A modified interaction potential, yielding a better reproduction of experimental hydration energy and water diffusivity across a number of different zeolites, is proposed. Molecular dynamics simulations reproduce the known experimental structural features obtained by X-ray diffraction. Variations of simulated vibrational IR and IINS spectra with temperature and size of nanoclusters are in good agreement with experiment. The simulated water nanoclusters in zeolite NaA are found to be too small to crystallize and, at low temperature, behave as amorphous ice, in agreement with recent experimental results for similar water nanoclusters in reverse micelles.

Electric field convergence versus atomic basis sets in all-siliceous zeolites

The electrostatic potential (EP) and electric field (EF) are calculated in the TON and CHA zeolites using periodic hybrid B3LYP, PBE, and PW91 functionals considering eight basis sets. Relative root mean square differences between the EP or EF values estimated between the different basis sets are evaluated in several domains available for adsorbate molecules in both zeolites. The EP is interpreted in terms of ionicity of the framework.

Molecular Dynamics Simulation Study of Superhydrated Perdeuterated Natrolite Using a New Interaction Potential Model

To test a new interaction potential, molecular dynamics simulations of zeolite natrolite were performed for the structures under ambient conditions hydrated by perdeuterated water and at high pressure (1.87 GPa) in the superhydrated phase, which were recently studied by neutron diffraction. The experimental structures were reproduced with reasonable accuracy, and the hydrogen bond features are discussed. As in ordinary natrolite, a flip motion of water molecules around the HOH bisector is found, which, together with translational oscillations, gives rise to transient hydrogen bonds between water molecules, which do not appear from experimental equilibrium coordinates. The dynamics of water molecules can explain some problems encountered in refining the experimental structure. Vibrational spectra of natrolite containing perdeuterated water, which are not yet measured, were simulated, and their qualitative trend is discussed.

Nonempirical quantification of molecular interactions in supramolecular assemblies

A new nonempirical scheme for the computation of realistic atomic charges from the sole knowledge of molecular or crystalline structures and of two ab initio parameters per chemical element (a configuration energy and the radius of the most diffuse valence orbital) is presented. Charge distributions obtained from this formalism are consistent with common chemical knowledge and found to be in agreement with available X-ray diffraction studies. They are also compatible with electrical-field gradients derived from solid-state NMR measurements. It is also shown how the total electrostatic energy associated with each charge distribution may be used to quantify van der Waals as well as hydrogen bond interactions from an energetic point of view. Finally, the position of the new formalism relative to other ab initio schemes is discussed both qualitatively and quantitatively.