Quantum Dynamical Investigation of Dihydrogen-Hydride Exchange in a Transition-Metal Polyhydride Complex.

The ongoing shift toward clean, sustainable energy is a primary driving force behind hydrogen fuel research. Safe and effective storage of hydrogen is a major challenge (particularly for mobile applications) and requires a detailed understanding of the atomic level interactions of hydrogen with its host materials. The light mass of hydrogen, however, implies that quantum effects are important, so a quantum dynamical treatment is required to properly account for these effects in computational simulations. As one such example, we describe herein the hydrogen exchange dynamics between a hydride and a dihydrogen ligand in the [FeH(H2)(PH3)4]+ model complex. A global three-dimensional (3D) potential energy surface (PES) was constructed by fitting to and interpolating from a discrete set of grid points computed using density functional theory; exact quantum dynamical calculations were then carried out on the 3D PES using discrete variable representation basis sets. Energy levels and their quantum tunneling splittings were computed up to 3000 cm-1 above the ground state. Within that energy range, all three fundamentals have been identified using wave function plots, as well as the first three overtones of the exchange (reaction coordinate) motion and several of its combination bands. From the tunneling splittings, the Boltzmann-averaged tunneling rates were computed. The Arrhenius plot of the total exchange rate shows a clear transition around 150 K, below which the activation energy is essentially zero and above which it is less than half of the electronic structure barrier. This indicates that exchange rates are governed by quantum tunneling throughout the relevant temperature range with the low-temperature regime dominated by a single quantum (ground) state. This work is the first-ever fully quantum dynamical study to investigate the hydrogen exchange dynamics between hydride and dihydrogen ligands coordinated to a transition-metal complex.

Investigating H2 Adsorption in Isostructural Metal-Organic Frameworks M-CUK-1 (M = Co and Mg) through Experimental and Theoretical Studies.

A combined experimental and theoretical study of H2 adsorption was carried out in Co-CUK-1 and Mg-CUK-1, two isostructural metal-organic frameworks (MOFs) that consist of M2+ ions (M = Co and Mg) coordinated to pyridine-2,4-dicarboxylate (pdc2-) and OH- ligands. These MOFs possess saturated metal centers in distorted octahedral environments and narrow pore sizes and display high chemical and thermal stability. Previous experimental studies revealed that Co-CUK-1 exhibits a H2 uptake of 183 cm3 g-1 at 77 K/1.0 atm [ Angew. Chem., Int. Ed. 2007, 46, 272-275, DOI: 10.1002/anie.200601627], while that for Mg-CUK-1 under the same conditions is 240 cm3 g-1 on the basis of the experimental measurements carried out herein. The theoretical H2 adsorption isotherms are in close agreement with the corresponding experimental measurements for simulations using electrostatic and polarizable potentials of the adsorbate. Through simulated annealing calculations, it was found that the primary binding site for H2 in both isostructural analogues is localized proximal to the center of the aromatic rings belonging to the pdc2- linkers. Inelastic neutron scattering (INS) spectroscopic studies of H2 adsorbed in both MOFs revealed a rotational tunnelling transition occurring at around 8 meV in the corresponding spectra; this peak represents H2 adsorbed at the primary binding site. Two-dimensional quantum rotation calculations for H2 localized at the primary and secondary binding sites in both MOFs yielded rotational energy levels that are in agreement with the transitions observed in the INS spectra. Even though both M-CUK-1 analogues possess different metal ions, they exhibit similar electrostatic environments, modeled structures at H2 saturation, and rotational potentials for H2 adsorbed at the most favorable adsorption site. Overall, this study demonstrates how important molecular-level details of the H2 adsorption mechanism inside MOF micropores can be derived from a combination of experimental measurements and theoretical calculations using two stable and isostructural MOFs with saturated metal centers and small pore windows as model systems.

Ammonia Storage in Hydrogen Bond-Rich Microporous Polymers.

The fascinating structural flexibility of porous polymers is highly attractive because it can result in optimized materials with specific host-guest interactions. Nevertheless, the fundamental mechanisms responsible for controlling the weak interactions of these hydrogen bond-rich networks-essential for developing smart task-specific materials used in recognition, capture, and sequestration processes-remain unexplored. Herein, by systematically comparing performance changes between poly(amic acid) (PAA)- and polycyclic imide (PI)-based porous polymers before and after NH3 adsorption, the role of hydrogen bonds in conformational lability and responsiveness toward guest molecules is highlighted. By combining thermal gravimetric analysis with neutron spectroscopy supported by DFT calculations, we demonstrate that PAA's chemical and physical stability is enhanced by the presence of stronger host-guest interactions. This observation also emphasizes the idea that efficient adsorption relies on having a high number of sites, upon which gas molecules can adsorb with greater affinity via strong hydrogen bonding interactions.

Microstructure and Mechanical Properties of Al-(12-20)Si Bi-Material Fabricated by Selective Laser Melting.

In this study, a combination of Al-12Si and Al-20Si (Al-(12-20)Si) alloys was fabricated by selective laser melting (SLM) as a result of increased component requirements such as geometrical complexity and high dimensional accuracy. The microstructure and mechanical properties of the SLM Al-(12-20)Si in as-produced as well as in heat-treated conditions were investigated. The Al-(12-20)Si interface was in the as-built condition and it gradually became blurry until it disappeared after heat treatment at 673 K for 6 h. This Al-(12-20)Si bi-material displayed excellent mechanical properties. The hardness of the Al-20Si alloy side was significantly higher than that of the Al-12Si alloy side and the disparity between both sides gradually decreased and tended to be consistent after heat treatment at 673 K for 6 h. The tensile strength and elongation of the Al-(12-20Si) bi-material lies in between the Al-12Si and Al-20Si alloys and fracture occurs in the Al-20Si side. The present results provide new insights into the fabrication of bi-materials using SLM.

Hydrogen bond dynamics and conformational flexibility in antipsychotics.

Effective treatment of disorders of the central nervous system can often be achieved using bioactive molecules of similar moieties to those known to be tolerable. A better understanding of the solid-state characteristics of such molecules could thereby create new opportunities for research on pharmaceutical preparations and drug prescriptions, while information about their rich intramolecular dynamics may well add an important aspect in the field of in silico drug discovery. We have therefore investigated three different antipsychotic drugs: haloperidol (C21H23ClFNO2, HAL), aripiprazole (C23H27Cl2N3O2, APZ) and quetiapine hemifumarate (C21H25N3O2S·0.5C4H4O4, QTP) based on similarities either in their structures, hydrophobic and hydrophilic moieties, or in their modes of action, typical or atypical. Our aim was to test the structural and molecular stability of these three different antipsychotics. To this end, we compared the molecular vibrations observed by inelastic neutron spectroscopy of these systems with those from theoretical periodic calculations of the crystalline antipsychotics using the Vienna ab initio simulation package (VASP). While most of the observed features in the lattice region were reasonably well represented by the calculations, the overall spectra were relatively complex, and hence traditional assignment procedures for the approximately 600 normal modes in the unit cell were not possible. These results indicate that in the search for new drug candidates, not only analysis of the flexibility of the receptor, but also the dynamics of the active molecules play a role in improving the prediction of binding affinities.

Response to comment on 'Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers'.

A response is given to comments by Bürgi & Macchi [IUCrJ (2018), 5, 654-657] about Belo et al. [IUCrJ (2018), 5, 6-12.].

A nano-silicate material with exceptional capacity for CO2 capture and storage at room temperature.

In order to mitigate climate change driven by the observed high levels of carbon dioxide (CO2) in the atmosphere, many micro and nano-porous materials are being investigated for CO2 selectivity, capture and storage (CCS) purposes, including zeolites, metal organic frameworks (MOFs), functionalized polymers, activated carbons and nano-silicate clay minerals. Key properties include availability, non-toxicity, low cost, stability, energy of adsorption/desorption, sorbent regeneration, sorption kinetics and CO2 storage capacity. Here, we address the crucial point of the volumetric capture and storage capacity for CO2 in a low cost material which is natural, non-toxic, and stable. We show that the nano-silicate Nickel Fluorohectorite is able to capture 0.79 metric tons of CO2 per m3 of host material - one of the highest capacities ever achieved - and we compare volumetric and gravimetric capacity of the best CO2 sorbent materials reported to date. Our results suggest that the high capture capacity of this fluorohectorite clay is strongly coupled to the type and valence of the interlayer cation (here Ni2+) and the high charge density, which is almost twice that of montmorillonite, resulting in the highest reported CO2 uptake among clay minerals.

A quantum dynamical study of the rotation of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 coordination complex.

Progress in the hydrogen fuel field requires a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen nuclei, any theoretical studies of these systems must be treated quantum dynamically. One class of material that has been examined in this context are dihydrogen complexes. Since their discovery by Kubas in 1984, many such complexes have been studied both experimentally and theoretically. This particular study examines the rotational dynamics of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 complex, allowing for full motion in both the rotational degrees of freedom and treating the quantum dynamics (QD) explicitly. A "gas-phase" global potential energy surface is first constructed using density functional theory with the Becke, 3-parameter, Lee-Yang-Parr functional; this is followed by an exact QD calculation of the corresponding rotation/libration states. The results provide insight into the dynamical correlation of the two rotation angles as well as a comprehensive analysis of both ground- and excited-state librational tunneling splittings. The latter was computed to be 6.914 cm-1-in excellent agreement with the experimental value of 6.4 cm-1. This work represents the first full-dimensional ab initio exact QD calculation ever performed for dihydrogen ligand rotation in a coordination complex.

Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers.

Enantiomeric amino acids have specific physiological functions in complex biological systems. Systematic studies focusing on the solid-state properties of d-amino acids are, however, still limited. To shed light on this field, structural and spectroscopic studies of d-alanine using neutron powder diffraction, polarized Raman scattering and ab initio calculations of harmonic vibrational frequencies were carried out. Clear changes in the number of vibrational modes are observed as a function of temperature, which can be directly connected to variations of the N-D bond lengths. These results reveal dissimilarities in the structural properties of d-alanine compared with l-alanine.

Raman and Infrared spectroscopies and X-ray diffraction data on bupivacaine and ropivacaine complexed with 2-hydroxypropyl-ß-cyclodextrin.

The data presented in this article are related to the research article entitled "Probing the dynamics of complexed local anesthetics via neutron scattering spectroscopy and DFT calculations (http://dx.doi.org/10.1016/j.ijpharm.2017.03.051)" (Martins et al., 2017) [1]. This work shows the molecular and structural behavior of the local anesthetics (LAs) bupivacaine (BVC, C18H28N2O) and ropivacaine (RVC, C17H26N2O) before and after complexation with the water-soluble oligosaccharide 2-hydroxypropyl-ß-cyclodextrin (HP-ß-CD).

The rotational dynamics of H2 adsorbed in covalent organic frameworks.

A combined inelastic neutron scattering (INS) and theoretical study was carried out on H2 adsorbed in two covalent organic framework (COF) materials: COF-1 and COF-102. These COFs are synthesized from self-condensation reactions of 1,4-benzenediboronic acid (BDBA) and tetra(4-(dihydroxy)borylphenyl)methane (TBPM) molecules, respectively. Molecular simulations of H2 adsorption in COF-1 revealed that the H2 molecules occupy the region between two eclipsed layers of the COF. The most favorable H2 binding site in COF-1 is located between two B3O3 clusters of the eclipsed layers. Two distinct H2 binding sites were identified in COF-102 from the simulations: the B3O3 clusters and the phenyl rings of the tetraphenylmethyl units. Two-dimensional quantum rotation calculations for H2 adsorbed at the considered sites in both COFs resulted in rotational transitions that are in good agreement with those that appear in the corresponding INS spectra. Such calculations were important for interpreting the INS spectra in these materials. Calculation of the rotational potential energy surface for H2 bound at the most favorable adsorption site in COF-1 and COF-102 revealed unusually high rotational barriers that are attributed to the nature of the B3O3 rings. The values for these barriers to rotation are greater than or comparable to those observed in some metal-organic frameworks (MOFs) that possess open-metal sites. This study demonstrates the power of using INS experiments in conjunction with theoretical calculations to gain valuable insights into the nature of the binding sites and, for the first time, the rotational dynamics of H2 adsorbed in COFs.

Probing the dynamics of complexed local anesthetics via neutron scattering spectroscopy and DFT calculations.

Since potential changes in the dynamics and mobility of drugs upon complexation for delivery may affect their ultimate efficacy, we have investigated the dynamics of two local anesthetic molecules, bupivacaine (BVC, C18H28N2O) and ropivacaine (RVC, C17H26N2O), in both their crystalline forms and complexed with water-soluble oligosaccharide 2-hydroxypropyl-ß-cyclodextrin (HP-ß-CD). The study was carried out by neutron scattering spectroscopy, along with thermal analysis, and density functional theory computation. Mean square displacements suggest that RVC may be less flexible in crystalline form than BVC, but both molecules exhibit very similar dynamics when confined in HP-ß-CD. The use of vibrational analysis by density functional theory (DFT) made possible the identification of molecular modes that are most affected in both molecules by insertion into HP-ß-CD, namely those of the piperidine rings and methyl groups. Nonetheless, the somewhat greater structure in the vibrational spectrum at room temperature of complexed RVC than that of BVC, suggests that the effects of complexation are more severe for the latter. This unique approach to the molecular level study of encapsulated drugs should lead to deeper understanding of their mobility and the respective release dynamics.

Self-Terminating Confinement Approach for Large-Area Uniform Monolayer Graphene Directly over Si/SiOx by Chemical Vapor Deposition.

To synthesize graphene by chemical vapor deposition (CVD) both in large area and with uniform layer number directly over Si/SiOx has proven challenging. The use of catalytically active metal substrates, in particular Cu, has shown far greater success and therefore is popular. That said, for electronics applications it requires a transfer procedure, which tends to damage and contaminate the graphene. Thus, the direct fabrication of uniform graphene on Si/SiOx remains attractive. Here we show a facile confinement CVD approach in which we simply "sandwich" two Si wafers with their oxide faces in contact to form uniform monolayer graphene. A thorough examination of the material reveals it comprises faceted grains despite initially nucleating as round islands. Upon clustering, they facet to minimize their energy. This behavior leads to faceting in polygons, as the system aims to ideally form hexagons, the lowest energy form, much like the hexagonal cells in a beehive, which requires the minimum wax. This process also leads to a near minimal total grain boundary length per unit area. This fact, along with the high graphene quality, is reflected in its electrical performance, which is highly comparable with graphene formed over other substrates, including Cu. In addition, the graphene growth is self-terminating. Our CVD approach is easily scalable and will make graphene formation directly on Si wafers competitive against that from metal substrates, which suffer from transfer. Moreover, this CVD route should be applicable for the direct synthesis of other 2D materials and their van der Waals heterostructures.

Computational study of inelastic neutron scattering vibrational spectra of water clusters and their relevance to hydration water in proteins.

BACKGROUND: Inelastic neutron scattering (INS) vibrational spectra for hydration water in proteins can be obtained from spectral differences, but their interpretation has mainly been limited to comparisons with various forms of ice at high hydration levels without making use of available structural information from neutron protein crystallography. METHODS: The INS vibrational spectra of free and partially constrained water clusters (up to n=17) were calculated with DFT methods using published energy-minimized structures. RESULTS: Reference is made to neutron diffraction studies of hydrated proteins, which contain a wealth of structural information both on individual water molecules and small clusters in the inner "shell" in order to select representative clusters to serve as models for bound, rather than free clusters as they would occur in a protein. CONCLUSIONS: INS spectra of the water librational region calculated for a combination of model bound clusters provide a qualitative account of the essentially featureless experimental spectra on water in proteins at very low hydration levels, but do indicate that the well-known rise in intensity near 500cm-1 is connected to increasing numbers of four-coordinate water molecules in larger clusters. GENERAL SIGNIFICANCE: The combination of structural information of hydration water from neutron protein crystallography with much more sophisticated computational methods than used herein should lead to a much more detailed picture of the hydration of proteins. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Dynamics of H2 adsorbed in porous materials as revealed by computational analysis of inelastic neutron scattering spectra.

The inelastic scattering of neutrons from adsorbed H2 is an effective and highly sensitive method for obtaining molecular level information on the type and nature of H2 binding sites in porous materials. While these inelastic neutron scattering (INS) spectra of the hindered rotational and translational excitations on the adsorbed H2 contain a significant amount of information, much of this can only be reliably extracted by means of a detailed analysis of the spectra through the utilization of models and theoretical calculations. For instance, the rotational tunneling transitions observed in the INS spectra can be related to a value for the barrier to rotation for the adsorbed H2 with the use of a simple phenomenological model. Since such an analysis is dependent on the model, it is far more desirable to use theoretical methods to compute a potential energy surface (PES), from which the rotational barriers for H2 adsorbed at a particular site can be determined. Rotational energy levels and transitions for the hindered rotor can be obtained by quantum dynamics calculations and compared directly with experiment with an accuracy subject only to the quality of the theoretical PES. In this paper, we review some of the quantum and classical mechanical calculations that have been performed on H2 adsorbed in various porous materials, such as clathrate hydrates, zeolites, and metal-organic frameworks (MOFs). The principal aims of these calculations have been the interpretation of the INS spectra for adsorbed H2 along with the extraction of atomic level details of its interaction with the host. We describe calculations of the PES used for two-dimensional quantum rotation as well as rigorous five-dimensional quantum coupled translation-rotation dynamics, and demonstrate that the combination of INS measurements and computational modeling can provide important and detailed insights into the molecular mechanism of H2 adsorption in porous materials.

Restricted mobility of specific functional groups reduces anti-cancer drug activity in healthy cells.

The most common cancer treatments currently available are radio- and chemo-therapy. These therapies have, however, drawbacks, such as, the reduction in quality of life and the low efficiency of radiotherapy in cases of multiple metastases. To lessen these effects, we have encapsulated an anti-cancer drug into a biocompatible matrix. In-vitro assays indicate that this bio-nanocomposite is able to interact and cause morphological changes in cancer cells. Meanwhile, no alterations were observed in monocytes and fibroblasts, indicating that this system might carry the drug in living organisms with reduced clearance rate and toxicity. X-rays and neutrons were used to investigate the carrier structure, as well as to assess the drug mobility within the bio-nanocomposite. From these unique data we show that partial mobility restriction of active groups of the drug molecule suggests why this carrier design is potentially safer to healthy cells.

Exceptional H2 sorption characteristics in a Mg(2+)-based metal-organic framework with small pores: insights from experimental and theoretical studies.

Experimental sorption measurements, inelastic neutron scattering (INS), and theoretical studies of H2 sorption were performed in α-[Mg3(O2CH)6], a metal-organic framework (MOF) that consists of a network of Mg(2+) ions coordinated to formate ligands. The experimental H2 uptake at 77 K and 1.0 atm was observed to be 0.96 wt%, which is quite impressive for a Mg(2+)-based MOF that has a BET surface area of only 150 m(2) g(-1). Due to the presence of small pore sizes in the MOF, the isosteric heat of adsorption (Qst) value was observed to be reasonably high for a material with no open-metal sites (ca. 7.0 kJ mol(-1)). The INS spectra for H2 in α-[Mg3(O2CH)6] is very unusual for a porous material, as there exist several different peaks that occur below 10 meV. Simulations of H2 sorption in α-[Mg3(O2CH)6] revealed that the H2 molecules sorbed at three principal locations within the small pores of the framework. It was discovered through the simulations and two-dimensional quantum rotation calculations that different groups of peaks correspond to particular sorption sites in the material. However, for H2 sorbed at a specific site, it was observed that differences in the positions and angular orientations led to distinctions in the rotational tunnelling transitions; this led to a total of eight identified sites. An extremely high rotational barrier was calculated for H2 sorbed at the most favorable site in α-[Mg3(O2CH)6] (81.59 meV); this value is in close agreement to that determined using an empirical phenomenological model (75.71 meV). This rotational barrier for H2 exceeds those for various MOFs that contain open-metal sites and is currently the highest yet for a neutral MOF. This study highlights the synergy between experiment and theory to extract useful and important atomic level details on the remarkable sorption mechanism for H2 in a MOF with small pore sizes.

Investigating H2 Sorption in a Fluorinated Metal-Organic Framework with Small Pores Through Molecular Simulation and Inelastic Neutron Scattering.

Simulations of H2 sorption were performed in a metal-organic framework (MOF) consisting of Zn(2+) ions coordinated to 1,2,4-triazole and tetrafluoroterephthalate ligands (denoted [Zn(trz)(tftph)] in this work). The simulated H2 sorption isotherms reported in this work are consistent with the experimental data for the state points considered. The experimental H2 isosteric heat of adsorption (Qst) values for this MOF are approximately 8.0 kJ mol(-1) for the considered loading range, which is in the proximity of those determined from simulation. The experimental inelastic neutron scattering (INS) spectra for H2 in [Zn(trz)(tftph)] reveal at least two peaks that occur at low energies, which corresponds to high barriers to rotation for the respective sites. The most favorable sorption site in the MOF was identified from the simulations as sorption in the vicinity of a metal-coordinated H2O molecule, an exposed fluorine atom, and a carboxylate oxygen atom in a confined region in the framework. Secondary sorption was observed between the fluorine atoms of adjacent tetrafluoroterephthalate ligands. The H2 molecule at the primary sorption site in [Zn(trz)(tftph)] exhibits a rotational barrier that exceeds that for most neutral MOFs with open-metal sites according to an empirical phenomenological model, and this was further validated by calculating the rotational potential energy surface for H2 at this site.