Petko St. Petkov

Defects in MOFs: a thorough characterization.

As indicated by nearly perfect XRD data, but challenged by a two-signal IR spectrum of CO guest molecules, it is confirmed by computer simulations and XPS experiments that the most defect-free SURMOFs contain about 4% defective Cu sites.

Structural properties of metal‐organic frameworks within the density‐functional based tight‐binding method

Density-functional based tight-binding (DFTB) is a powerful method to describe large molecules and materials. Metal-organic frameworks (MOFs), materials with interesting catalytic properties and with very large surface areas, have been developed and have become commercially available. Unit cells of MOFs typically include hundreds of atoms, which make the application of standard density-functional methods computationally very expensive, sometimes even unfeasible. The aim of this paper is to prepare and to validate the self-consistent charge-DFTB (SCC-DFTB) method for MOFs containing Cu, Zn, and Al metal centers. The method has been validated against full hybrid density-functional calculations for model clusters, against gradient corrected density-functional calculations for supercells, and against experiment. Moreover, the modular concept of MOF chemistry has been discussed on the basis of their electronic properties. We concentrate on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn4O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)), and aluminum oxide AlO4(OH)2 octahedron (MIL-53). We show that SCC-DFTB predicts structural parameters with a very good accuracy (with less than 5% deviation, even for adsorbed CO and H2O on HKUST-1), while adsorption energies differ by 12 kJ mol−1 or less for CO and water compared to DFT benchmark calculations.

Ab Initio Molecular Dynamics of Na+ and Mg2+ Countercations at the Backbone of RNA in Water Solution

The interactions between sodium or magnesium ions and phosphate groups of the RNA backbone represented as dinucleotide fragments in water solution have been studied using ab initio Born-Oppenheimer molecular dynamics. All systems have been simulated at 300 and 320 K. Sodium ions have mobility higher than that of the magnesium ions and readily change their position with respect to the phosphate groups, from directly bonded to completely solvated state, with a rough estimate of the lifetime of bonded Na(+) of about 20-30 ps. The coordination number of the sodium ions frequently changes in irregular intervals ranging from several femtoseconds to about 10 ps with the most frequently encountered coordination number five, followed by six. The magnesium ion is stable both as directly bonded to an oxygen atom from the phosphate group and completely solvated by water. In both states the Mg(2+) ion has exactly six oxygen atoms in the first coordination shell; moreover, during the whole simulation of more than 100 ps no exchange of ligand in the first coordination shells has been observed. Solvation of the terminal phosphate oxygen atoms by water molecules forming hydrogen bonds in different locations of the ions is also discussed. The stability of the system containing sodium ions essentially does not depend on the position of the ions with respect to the phosphate groups.

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20 K.

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu(2+) ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu(2+) ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1/2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion.

Controlling embedment and surface chemistry of nanoclusters in metal-organic frameworks†

A combined theoretical and experimental approach demonstrates that nanocluster embedment into the pores of metal-organic frameworks (MOF) may be influenced by the chemical functionalisation of the MOF. Furthermore, this results in the surface functionalisation of the embedded nanoclusters, highlighting the potential of MOF scaffolds for the design and synthesis of novel functional materials.

Computational evaluation of the capability of transition metal exchanged zeolites for complete purification of hydrogen for fuel cell applications: the cheapest performs the best

The study reports an estimation of the capability of zeolites exchanged with different monovalent transition metal cations to be used for the deep purification of hydrogen for PEMFC applications from CO, ammonia and hydrogen sulfide impurities. The estimation is based on thermodynamic data derived by computational modeling using the periodic DFT approach, and allows determination of the minimal impurity gas concentration in the H2 feed that could be achieved with the specific adsorbent. The results suggest that zeolites exchanged with Co+, Ni+, Cu+, Rh+ or Ir+ cations can purify hydrogen under standard conditions to CO concentrations of 10−12–10−16, depending on the metal. A theoretically recommended material for hydrogen purification is Cu exchanged zeolite, since it is able to reduce CO concentrations down to 10−11 and has weaker binding of CO compared to the other modeled cations, which facilitates the regeneration of the adsorbent. In addition, this zeolite can capture NH3 and H2S impurities and reduce their concentration in the H2 feed to 10−10, while the other modeled cation exchanged zeolites show a higher permeability of these impurities and are less appropriate. Thus, the modeled adsorbents, in particular Cu exchanged zeolite, are good candidates for H2purification due to their low cost and predicted high efficiency, and could be considered as an appropriate alternative to the other currently applied approaches.

CO coordination at XNi4 clusters with impurities X = H, C, O. A density functional study.

We report a computational investigation of CO adsorption on small nickel clusters that contain single impurity atoms H, C, or O. At bare Ni 4 and clusters with H or O impurity, the most stable coordination of the probe molecule is on top of a Ni atom which interacts with the impurity. The CNi 4 cluster is an exception where 3-fold coordination of CO was determined to be more stable than that on top, however, by 4 kJ/mol only. Our results suggest that the heteroatoms X (X = H, C, O) affect only weakly the reactivity of the cluster with respect to CO; the binding energy of CO in the most stable complexes (CO)XNi 4 increases at most by 10% compared to the value for bare Ni 4, 194 kJ/mol. The impurity induces a small decrease of the CO infrared frequency shift for on-top coordinated CO, compared to Ni 4, because of partial oxidation of the metal moiety. A notable difference is predicted for clusters that contain a C impurity because of the different preferred coordination mode which results in a strong CO frequency red shift of approximately 300 cm (-1). The calculated characteristic CO frequency shifts may be helpful in identifying experimentally clusters with impurity atoms.

Hydrogen Atom Transfer from Water or Alcohols Activated by Presolvated Electrons.

High-energy irradiation of protic solvents can transiently introduce excess electrons that are implicated in a diverse range of reductive processes. Here we report the evolution of electron solvation in water and in alcohols following photodetachment from aqueous hydroxide or the corresponding alkoxides studied by two- and three-pulse femtosecond spectroscopy and ab initio molecular dynamic simulations. The experiments reveal an ultrafast recombination channel of the excess electrons. Through the calculations this channel emerges as an H-atom transfer process to the hydroxyl or alkoxy radical species from neighboring solvent molecules, which are activated as the presolvated electron occupies their antibonding orbitals. The initially low activation barrier in the early stages of electron solvation was found to increase (from 12 to 44 kJ/mol in water) as full solvation proceeded.

High-mobility band-like charge transport in a semiconducting two-dimensional metal–organic framework

Metal–organic frameworks (MOFs) are hybrid materials based on crystalline coordination polymers that consist of metal ions connected by organic ligands. In addition to the traditional applications in gas storage and separation or catalysis, the long-range crystalline order in MOFs, as well as the tunable coupling between the organic and inorganic constituents, has led to the recent development of electrically conductive MOFs as a new generation of electronic materials. However, to date, the nature of charge transport in the MOFs has remained elusive. Here we demonstrate, using high-frequency terahertz photoconductivity and Hall effect measurements, Drude-type band-like transport in a semiconducting, π–d conjugated porous Fe3(THT)2(NH4)3 (THT, 2,3,6,7,10,11-triphenylenehexathiol) two-dimensional MOF, with a room-temperature mobility up to ~ 220 cm2 V–1 s–1. The temperature-dependent conductivity reveals that this mobility represents a lower limit for the material, as mobility is limited by impurity scattering. These results illustrate the potential for high-mobility semiconducting MOFs as active materials in thin-film optoelectronic devices.Semiconducting metal–organic frameworks (MOFs) can be of interest for optoelectronics, but charge transport property is rarely elucidated. Here, a π–d conjugated 2D MOF shows band-like charge transport, with room-temperature mobility of 220 cm2 V–1 s–1.

Computational Modelling of Nanoporous Materials

Publisher Summary This chapter reviews the contemporary computational approaches based on quantum chemical or hybrid methods, which are used for modeling of nanoporous materials such as zeolites and other molecular sieves. The computational methods for modeling of micro- and mesoporous materials and various processes on them are divided in two groups according to the level at which the interactions in the system are described – molecular mechanical and quantum chemical methods. The combination between them, the so-called hybrid QM/MM methods, is also applied in various cases, but it is usually considered together with the higher level method—quantum chemical. Since the quantum chemical approaches are orders of magnitude more demanding computationally, they are applied for smaller systems or systems with smaller unit cells compared to molecular mechanical methods. The selection of the model depends on the structure of the system and the properties to be investigated, as well as on the computational method. For crystalline microporous materials, three types of models are used—isolated cluster models, hybrid embedded cluster models, and periodic models. The quantum chemical methods are used for description of problems and processes connected with chemical interactions and reactions, redistribution of electron density, simulation of spectral features connected with the electronic structure of the material, its active centers, or guest species. The simplest way for modeling of processes in zeolites and mesoporous materials are isolated cluster models, which have several drawbacks, but in many cases the obtained results are reasonable.