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The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO

Flexible metal-organic frameworks (MOFs) can undergo structural transitions with significant pore volume changes upon guest adsorption or other external triggers while maintaining their porosity. In computational studies of this breathing behavior, molecular dynamics (MD) simulations within periodic boundary conditions (PBCs) are commonly performed. However, to account for the finite size and surface effects affecting the phase transition mechanism, the simulation of non-periodic nanocrystallite (NC) models without the constraint of PBCs is an important alternative. In this study, we present an approach allowing the analysis and control of the volume of finite-size structures during MD simulations by a tetrahedral tessellation of the (deformed) NC's volume. The new method allows for defining the current NC's volume during the simulation and manipulating it regarding a particular reference volume to compute free energies for the phase transformation via umbrella sampling. The application on differently sized DMOF‑1 and DUT‑128 NCs reveals flexible pore closing mechanisms without significant biasing of the transition pathway. The concept provides the theoretical foundation for further research on flexible materials regarding targeted initialization of the breathing phase behavior to elucidate the underlying mechanism, which can be used to improve the applications of flexible materials by targeted controlling of the phase transition.

The chemical potential of adsorbed or confined fluids provides insight into their unique thermodynamic properties and determines adsorption isotherms. However, it is often difficult to compute this quantity from atomistic simulations using existing statistical mechanical methods. We introduce a computational framework that utilizes static structure factors, thermodynamic integration, and free energy perturbation for calculating the absolute chemical potential of fluids. For demonstration, we apply the method to compute the adsorption isotherms of carbon dioxide in a metal-organic framework and water in carbon nanotubes.

The edge chlorination of the benchmark nanographenes triphenylene and hexa-peri-hexabenzocoronene is conducted mechanochemically. This approach overcomes solubility limitations and eliminates the need for elaborate chlorination conditions. Additionally, the planarization of oligophenylenes and their edge-chlorination can be combined in a one-pot approach requiring as little as 60 minutes.

Oxygenated poly aromatic hydrocarbons (OPAH) are widely produced in biomass combustion. Recent studies suggest significantly higher toxicity for OPAH in comparison to PAH and soot. However, the pre...

Phase equilibria of fluid substances and their mixtures are important in numerous scientific as well as industrial applications and are, therefore, a major focus of thermophysical property research...

Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge- and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements and temperature-dependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature. Overall, the best performance amongst the approximate methodologies is observed for a system-specifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit cell volume resulting in a pronounced overestimation of the phonon energies in the low frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments., Supporting Information is available free of charge at https://pubs.acs.org/doi/abs/10.1021/acs.jctc.0c00119

Trihalide salts were found to efficiently promote photochemical dediazotizing halogenations of diazonium salts. In contrast to classical Sandmeyer reactions, no metal catalysts are required to achieve high yields and outstanding selectivities for halogenation over competing hydridodediazotization. Convenient protocols are disclosed for synthetically meaningful brominations, iodinations, and chlorinations of diversely functionalized derivatives.

If complex systems are to be studied in molecular simulation, one usually attempts to combine existing interaction models in order to describe the new system. This is, however, not always feasible. We thus propose here a new pairwise-additive interaction model for liquid methanol and solvated Cl− to be used to study the immersion of Metal-Organic Frameworks (MOFs) in methanol. Practically, it entails that all interactions must be written to be compatible with the family of MOF-FF models, which have been specifically developed and then widely employed in molecular simulations of such MOFs, in particular flexible ones. The new model for liquid methanol has been mostly tailored to provide densities and dielectric constants as close to experiment as possible in a large temperature domain. This is important since the flexible MOFs modify their shapes according to their loading with guest molecules of various types, and also according to the thermodynamic conditions. The model yields excellent agreement for the density-temperature, dielectric constant-temperature, and self-diffusion-temperature relationships, properties. Other properties such as e.g. the compressibilities or thermal expansion coefficients are of the correct order of magnitude. Since some MOF frameworks are electrically charged, counterions will be present in these cases. The interactions of Cl− with the liquid are thus also considered here. The solvation of this ion is also found to be satisfactory when compared to other MD studies.

A robust and efficient numerical solver for three-dimensional linear elliptic partial differential equations without cross derivative terms, but variable coefficients is presented. The proposed algorithm is based on a general meshsize fourth-order compact finite-difference scheme and an accelerated geometric multigrid solver. The former not only increases the accuracy of the discretization, but also does not affect the bandwidth of the common second-order finite-difference scheme, which is relevant for distributed memory parallel implementations of the multigrid preconditioner. The acceleration technique, on the other hand, allows for a dramatic reduction of the number of cycles and smoothing steps required to achieve convergence. Furthermore, it is found that depending on the coefficients of the elliptic partial differential equation, the multigrid preconditioned solver can be the only way to obtain a reliable numerical solution or attain convergence.

Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system.

Controlling the transport of thermal energy is key to most applications of metal-organic frameworks. Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node-linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parametrized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. Still, the key findings prevail for different node structures and node-linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction.

Flexible metal–organic frameworks (MOFs) show large structural flexibility as a function of temperature or (gas)pressure variation, a fascinating property of high technological and scientific relevance. The targeted design of flexible MOFs demands control over the macroscopic thermodynamics as determined by microscopic chemical interactions and remains an open challenge. Herein we apply high‐pressure powder X‐ray diffraction and molecular dynamics simulations to gain insight into the microscopic chemical factors that determine the high‐pressure macroscopic thermodynamics of two flexible pillared‐layer MOFs. For the first time we identify configurational entropy that originates from side‐chain modifications of the linker as the key factor determining the thermodynamics in a flexible MOF. The study shows that configurational entropy is an important yet largely overlooked parameter, providing an intriguing perspective of how to chemically access the underlying free energy landscape in MOFs., By combining high‐pressure powder X‐ray diffraction and molecular dynamics simulation we gain insight into the microscopic chemical factors that determine the high‐pressure macroscopic thermodynamics of two flexible pillared‐layer MOFs, identifying configurational entropy as originating from side‐chain modifications as the determining factor of the macroscopic thermodynamics.

Most conventional materials display expansion in response to heating, so there is considerable interest in identifying materials that display the opposite behavior, negative thermal expansion (NTE). The current study investigated the temperature-induced contraction of seven mesoporous metal–organic frameworks (MOFs) of varying topology and composition, which exhibit outstanding porosity, using molecular simulation. We found exceptional NTE for the most porous MOFs, as well as a correlation between the coefficient of NTE and porosity. The large molecular subunits of the MOFs were further studied to find they intrinsically display NTE, corresponding to terahertz vibrational modes. As a result, NTE has a considerable effect on the mechanical properties of these MOFs and is an important consideration for understanding the mechanical stability of new extremely porous materials.

The effect of linkers with extended π-system on the topological preference of (4,4)-connected copper paddle-wheel-based metal–organic frameworks (MOFs) was investigated using the reverse topological approach (RTA) in which a genetic algorithm (GA) and the DFT-derived force field MOF-FF were used for ranking and predicting the most stable phase. Three tetracarboxylate linkers bearing different functionality, namely, phenylene (L1), naphthalene (L2), and anthracene (L3) groups, were studied. All potential topologies including nbo-b, ssa, ssb, pts, and lvt-b were considered. The computational results reveal that nbo-b is the most stable topology for all three investigated linkers. However, L2 is also formed in ssb according to experimental findings. Our simulation results show that the CH−π interactions with a Y-shaped configuration between naphthalene moieties of L2 stabilize the ssb framework. Unlike L2, CH−π interactions are not favorable for L1 and L3 because of unsuitable size of the π-system. The resul...

We present a computational multiscale study of a metal–organic framework (MOF)/polymer composite combining micro- and mesoscopic resolution, by coupling atomistic and coarse grained (CG) force field-based molecular dynamics simulations. As a proof of concept, we describe the copper paddlewheel-based HKUST-1 MOF/poly(vinyl alcohol) composite. Our newly developed CG model reproduces the salient features of the interface in excellent agreement with the atomistic model and allows the investigation of substantially larger systems. The polymer penetrates into the open pores of the MOF as a result of the interactions between its OH groups and the O and Cu atoms in the pores, suggesting an excellent MOF/polymer compatibility. Polymer structure is affected by the MOF surface up to a distance of ∼2.4 times its radius of gyration. This study paves the way toward understanding important interfacial phenomena such as aggregation and phase separation in these mixed matrix systems.

The prototypical pillared layer MOFs, formed by a square lattice of paddle-wheel units and connected by dinitrogen pillars, can undergo a breathing phasetransition by a “wine-rack” type motion of the square lattice. We studied this notyet fully understood behavior using an accurate first principles parameterized forcefield (MOF-FF) for larger nanocrystallites on the example of Zn 2 (bdc) 2 (dabco) [bdc:benzenedicarboxylate, dabco: (1,4-diazabicyclo[2.2.2]octane)] and found clear indi-cations for an interface between a closed and an open pore phase traveling throughthe system during the phase transformation [Adv. Theory Simul. 2019, 2, 11]. Inconventional simulations in small supercells this mechanism is prevented by periodicboundary conditions (PBC), enforcing a synchronous transformation of the entirecrystal. Here, we extend this investigation to pillared layer MOFs with flexibleside-chains, attached to the linker. Such functionalized (fu-)MOFs are experimen-tally known to have different properties with the side-chains acting as fixed guestmolecules. First, in order to extend the parameterization for such flexible groups,1a new parametrization strategy for MOF-FF had to be developed, using a multi-structure force based fit method. The resulting parametrization for a library offu-MOFs is then validated with respect to a set of reference systems and shows verygood accuracy. In the second step, a series of fu-MOFs with increasing side-chainlength is studied with respect to the influence of the side-chains on the breathingbehavior. For small supercells in PBC a systematic trend of the closed pore volumewith the chain length is observed. However, for a nanocrystallite model a distinctinterface between a closed and an open pore phase is visible only for the short chainlength, whereas for longer chains the interface broadens and a nearly concerted trans-formation is observed. Only by molecular dynamics simulations using accurate forcefields such complex phenomena can be studied on a molecular level.

Das Laborpraktikum zur Qualitativen Analyse (QA) war uber Jahrzehnte Bestandteil der universitaren Chemieausbildung in den ersten Semestern. In der QA werden durch nasschemische Verfahren anorganische Salzgemische in Anionen und Kationen aufgetrennt und nachgewiesen. Hierbei lernt der Studierende das chemische Verhalten der verschiedenen Elemente kennen.

This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization ${P}_{r}$ was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature ${T}_{c}$ between 425 and $450\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in excellent agreement with the experimental value of $437\phantom{\rule{4pt}{0ex}}\mathrm{K}$. In addition, the anomalous lattice behavior (shrinking of the $c$ axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a $c$-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by ${\mathrm{NaNO}}_{2}$.

One of the intriguing features of certain metal-organic frameworks (MOFs) is the large volume change upon external stimuli like pressure or guest molecule adsorption, referred to as “breathing”. This displacive phase transformation from an open to a closed pore has been investigated intensively by theoretical simulations within periodic boundary conditions (PBC). However, the actual free energy barriers for the transformation under real conditions and the impact of surface effects on it can only be studied beyond PBC for nanocrystallites. In this work, we used the first-principles parameterized forcefield MOF-FF to investigate the thermal- and pressure induced transformations for nanocrystallites of the pillared-layer DMOF-1 (Zn 2 (bdc) 2 (dabco); bdc: 1,4-benzenedicarboxylate; dabco: 1,4-diazabicyclo[2.2.2]octane) as a model system. By heating of prepared closed pore nanocrystallites of different size, a spontaneous opening is observed within a few tenth of picoseconds with an interface between the closed and open pore phase moving with a velocity of several 100 m/s through the system. The critical nucleation temperature for the opening transition raises with size. On the other hand, by forcing the closing transition with a distance restraint between paddle-wheel units placed on opposite edges of the crystallite, the free energy barrier can be determined by umbrella sampling. As expected, this barrier is substantially lower than the one determined for a concerted process under PBC. Interestingly, the barrier reduces with the size of the crystallite, indicating a hindering surface effect. The results demonstrate the need consider domain boundaries and surfaces, for example by simulations that go beyond PBC and to large system sizes in order to properly predict and describe first order phase transitions in MOFs.

In this paper we parameterized in a consistent way a new force field for a range of different zeolitic imidazolate framework systems (ZIF-8, ZIF-8(H), ZIF-8(Br) and ZIF- 8(Cl)), extending the MOF-FF parameterization methodology in two aspects. First, we implemented the possibility to use periodic reference data in order to prevent the difficulty of generating representative finite clusters. Second, a more efficient global optimizer based on the covariance matrix adaptation evolutionary strategy (CMA-ES) was employed during the parameterization process. We confirmed that CMA-ES, as a state-of-the-art black box optimizer for problems on continuous variables, is more suitable for force field optimization than the previous genetic algorithm. The obtained force field was then fully validated with respect to static and dynamic properties. Much effort was spent to ensure that the FF is able to describe the crucial linker swing effect in a large number of ZIF-8 derivatives. For this reason we compared our force field to ab initio molecular dynamic simulations and found an accuracy comparable to those obtained by different exchange–correlation functionals.

Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal–organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O2 and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O2-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O2-binding site are replaced with other metal ions (Mg2+, Ni2+, Zn2+, Co2+, or Mn2+), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O2 binding indirectly.

Atomistic simulations were performed in order to investigated the effect of pore dimension on the interaction of guest molecules with the inner surface of metal–organic frameworks (MOFs). In these systems, which only differ in the metric of their open structure, the chemical nature is conserved, and less impact on the host–guest interaction is expected compared to chemically funcionalized MOFs. However, by performing molecular dynamics simulations of benzene loaded MOF-5 derivatives (IRMOFs), which differ just in the length of the organic linker, it can be shown that impacts are present. The influence of the soft-modification can be explained only by a detailed analysis of the free energy topology and the diffusion mechanism. Note that the calculated self-diffusivity of benzene Dself shows no change with respect to the elongation of the linkers. The apparent contradiction between the macroscopic observable Dself and the microscopic free energy landscape could be resolved by introducing a hopping model for...

The temperature-responsive behavior of functionalized metal–organic frameworks (fu-MOF) with the general formula [Zn2(fu-L)2dabco]n has been investigated using molecular dynamics simulations (fu-L = alkoxy-functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The studied frameworks show a narrow pore (np) form at low temperatures, while at higher temperatures, large pore (lp) structures can be observed. The transition temperature is controlled by the chemical nature of the linker’s side chains as well as their length. In general, enhancing the side chain length decreases the transition temperature. On the other hand, more polar linkers shift the transition temperature to higher values. The so-called opening process of the narrow pores is caused by the thermally induced motion of the alkoxy side chains of the functionalized linkers. For qualitative comparison, the difference in internal energy as well as entropy between two forms (np and lp) was calculated for all studied linker ...

The thermal conductivity is an important property of metal-organic frameworks (MOFs). For gas storage applications, a too low thermal conductivity might turn out to be a game-stopper. Conversely, for thermoelectric devices the thermal conductivity has to be low enough. The chemical and structural flexibility of MOFs provides handles for conveniently tuning thermal properties. Still, for realizing the full potential of these materials, a detailed understanding of the corresponding structure-to-property relationships is required. For achieving such an understanding, we employ quantum-mechanical and force-field based simulations to study heat transport processes in MOFs both in real as well as in reciprocal space. The former approach relies on analyzing the effective local temperature in different parts of the MOF during a non-equilibrium molecular dynamics simulation. This allows identifying specific elements and bonds within the MOF that act as heat-transport bottlenecks. For a variety of MOF structures we show that the main local inhibitor to heat flow is the bond between the metal ions in the nodes and the carboxylic oxygens in the linkers. Further computer experiments then allow assessing the impact of the strength of that bond and of parameters like the mass of the nodes, the length and density of the linkers, bonding motives, the network topology, etc.. Typically, we find that stronger bonds, lighter nodes, and longer and more dense linkers increase thermal transport. As an alternative approach, analyzing heat transport in reciprocal space enables us to connect the values of the thermal conductivity to the properties of phonons as the quanta of mechanical and thermal lattice energy. Of particular interest in this context is the impact of structural variations on quantities like phonon densities of states, group velocities, and phonon lifetimes, which all directly impact heat transport processes. An analysis of these quantities provides insight into the nature of the vibrations that most strongly impact thermal conductivities. Consistent with the results from the real-space analysis, we find that the group velocities of the relevant phonons are increased by lighter nodes and by an increased mode-linker coupling. Notably, the bonds between nodes and linkers are also a major source of anharmonicities, thus reducing phonon lifetimes and decreasing thermal conductivities. Finally, we show that phonon properties are also crucial for other parameters, like elastic constants, thermal expansion, and - by virtue of electron-phonon coupling - also charge transport.