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The methyl transfer pathway is the most favorable pathway in the polyMB cycle and the polyMB cycle is more competitive than the alkene cycle. The addition of the phosphorus atom in the FAU could improve the productivity of propene in the MTP process.

We have theoretically investigated the hydrodesulphurisation (HDS) mechanism of hydrogenated thiophene over Cu–Mo-modified FAU zeolite using a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) study. The thiophene is hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT) due to moderate free energy barriers. Hydrogenolysis desulphurisation (HYD) and concerted direct desulphurisation (DDS) are discussed. Ring-opening, hydrogen transfer and C−S bond cleavage steps of thiophene derivatives are involved in the HYD process. The rate-determining steps are the hydrogen transfer step for 2,5-DHT and C−S bond cracking step for 2,3-DHT and THT. The concerted DDS pathway is probably more favourable than the HYD pathway in the desulphurisation of 2,5-DHT. The difference charge density (DCD) analysis reveals that for the ring-opening process, the electrons are migrated from the organic chain to the Cu–Mo catalytic centre. The reduced density gradient (RDG) plots indicate that both steric hindrance and a weak van der Waals (VDW) interaction exist between organic fragment and catalytic centre for all transition states (TSs). The localised orbital locator (LOL) maps suggest that there are strong covalent interactions and weak VDW interactions between the atoms in the forming chemical bonds.

We have theoretically studied the hydrodesulphurisation (HDS) mechanism of thiophene and its hydrogenated derivatives over sulphided Co–Mo catalysts supported by four zeolites (ZSM-5, FAU, Beta, MCM-22) by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. Due to low energy barriers, the thiophene can be hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT). Direct desulphurisation (DDS) and hydrogenolysis desulphurisation (HYD) pathways were investigated in detail. The calculated results show that in DDS mechanism of thiophene, the rate-determining step is the C–S bond cleavage. In HYD mechanisms of 2,5-DHT, 2,3-DHT and THT, the rate-determining step is hydrogen transfer on ZSM-5, FAU and Beta zeolites except MCM-22 zeolite. The HYD of 2,3-DHT and THT should occur by the stepwise pathway. But for the 2,5-DHT, the concerted pathway is probably more favourable than the stepwise pathway. The reduced density gradient (RDG), differential charge density (DCD), and local orbital locator (LOL) maps show that for all transition states (TSs), there are complicated van der Waals (VDW) and electrostatic interactions between the organic fragment and the catalytic centre.

This work proposes a state-of-the-art hybrid kernel to calculate molecular similarity. Combined with Gaussian process models, the performance of the hybrid kernel in predicting molecular properties is comparable to that of the directed message-passing neural network (D-MPNN). The hybrid kernel consists of a marginalized graph kernel (MGK) and a radial basis function (RBF) kernel that operate on molecular graphs and global molecular features, respectively. Bayesian optimization was used to obtain the optimal hyperparameters for both models. The comparisons are performed on 11 publicly available data sets. Our results show that their performances are similar, their prediction errors are correlated, and the ensemble predictions of the two models perform better than either of them. Through principal component analysis, we found that the molecular embeddings of the hybrid kernel and the D-MPNN are also similar. The advantage of D-MPNN lies in the computational efficiency and scalability of large-scale data, while the advantage of the graph kernel models lies in the accurate uncertainty quantification.

The TEAM force-field database is extended to cover room-temperature ionic liquids (IL). The training set for parametrization includes 11 cations in five classes, imidazolium, pyridinium, pyrrolidin...

Determining reaction mechanisms and kinetic models, which can be used for chemical reaction engineering and design, from atomistic simulation is highly challenging. In this study, we develop a novel methodology to solve this problem. Our approach has three components: (1) a procedure for precisely identifying chemical species and elementary reactions and statistically calculating the reaction rate constants; (2) a reduction method to simplify the complex reaction network into a skeletal network which can be used directly for kinetic modeling; and (3) a deterministic method for validating the derived full and skeletal kinetic models. The methodology is demonstrated by analyzing simulation data of hydrogen combustion. The full reaction network comprises 69 species and 256 reactions, which is reduced into a skeletal network of 9 species and 30 reactions. The kinetic models of both the full and skeletal networks represent the simulation data well. In addition, the essential elementary reactions and their rate constants agree favorably with those obtained experimentally. © 2019 Wiley Periodicals, Inc.

Single-crystalline sponge-like MFI mesoporous zeolites (SSMZs) have been synthesized by using bolaform surfactants with an axial chiral binaphthyl core in the hydrophobic tail and triquaternary ammonium head groups, as bifunctional organic structure-directing agents (OSDAs). By changing the length of alkyl chain between a triquaternary ammonium head group and a binaphthyl group from 4 to 10 carbons, SSMZs with high specific surface area (382-434 m2 g-1 ), abundant micropore-mesopore connectivity, and uniform mesopore diameter (4-10 nm) were obtained. OSDAs with an alkyl chain length of 11 and 12 carbons led to the formation of nanorod-constructed mesoporous MFI zeolites. A geometrical matching between the cylindrical arrangement of the binaphthyl groups and the zeolitic framework is speculated to be the key factor for the formation of mesoporous zeolites. The SSMZ zeolites, with abundant mesopores beneficial for the diffusion of reactants, exhibited significantly higher catalytic efficiencies than those of the conventional ZSM-5 with a microcrystal morphology (≈1.5 μm).

Knowledge of the thermodynamic properties of molecules is essential for chemical process design and the development of new materials. Experimental measurements are often expensive and not environmentally friendly. In the past, studies using molecular simulations have focused on a specific class of molecules, owing to the lack of a consistent force field and simulation protocol. To solve this problem, we have developed a high-throughput force field simulation (HT-FFS) procedure by combining a recently developed general force field with a validated simulation protocol to calculate thermodynamic properties for large number of molecules. This procedure is applied to calculate liquid densities, heats of vaporization, heat capacities, vapor-liquid equilibrium curves, critical temperatures, critical densities and surface tensions for a wide range of alkanes. The predictions agree well with available experimental data in terms of accuracy and precision, demonstrating that HT-FFS is a valid approach to supplementing experimental measurements. Furthermore, the large amount of data generated by HT-FFS lays a foundation for machine learning. We have developed an artificial neural network that demonstrates the feasibility of expanding predictions beyond simulation using a machine learning model.

Calcium and magnesium ions play important roles in many physicochemical processes. To facilitate the investigation of phenomena related to these ions that occur over large length and time scales, a coarse-grained force field (CGFF) is developed for MgCl2 and CaCl2 aqueous solutions. The ions are modeled by CG beads with characteristics of hydration shells. To accurately describe the nonideal behavior of the solutions, osmotic coefficients in a wide range of concentrations were used as guidance for parametrization. The osmotic coefficients were obtained from the chemical potential increments of water calculated using the Bennett acceptance ratio (BAR) method. The result CGFF accurately reproduces experimental osmotic coefficients, densities, surface tensions, and cation–anion separations of calcium chloride and magnesium chloride solutions at molalities up to 3.0 mol/kg. As a preliminary application, the force field is applied to simulate aggregations of sodium dodecyl sulfate (SDS) in calcium chloride sol...

By combining the bottom-up and top-down approaches, we have developed a new all-atom (AA) force field from quantum mechanics and experimental data and a new coarse grained (CG) force field from AA simulation and experimental data, for polydimethylsiloxane (PDMS). The AA force field is developed based on the TEAM force field database. The CG force field uses a mapping rule that splits the connecting oxygen into neighbouring CG beads to maintain the charge neutrality of the beads, analytical functional forms including anharmonic terms in the valence terms, and the temperature-dependent free-energy functional form to describe the inter-bead interactions. Broad range of thermodynamic properties of PDMS including density, surface tension, solubility parameter, radius of gyration and glass transition temperature are calculated to validate the force fields, and good agreements with the experimental data are obtained.

Molecular dynamics simulation is performed to compute densities and solubility parameters of polymers. TEAM force field is validated on monomers and then employed for predictions for polymers built...

The adsorption isotherm of n-perfluorohexane on BAM-P109 activated carbon was predicted using an ab initio force field and a slit-like pore model representing the activated carbon. The force field parameters were derived from data calculated using quantum mechanics density functional theory with dispersion energy correction, and the total adsorption amount was calculated as the sum of the adsorption amounts of microscopic and mesoscopic pores. It was found that the Grand Canonical Monte Carlo method can efficiently calculate adsorption on microscopic pores but not on mesoscopic pores. Thus, we divided the mesoscopic pores into surface and free-volume regions and calculated the density and thickness of the adsorbed layers and the density of the vapor phase using canonical ensemble simulations. Using this protocol, we can calculate the adsorption isotherms in good agreement with the experimental data.

Designing a templating strategy for directing mesopore growth along different crystallographic directions is essential for fabricating two- or three-dimensional single-crystalline mesoporous zeolites. However, so far, mesopores formed in MFI zeolites by soft templates have mostly been generated by disrupting growth along the b axis; generating mesopores by disrupting growth along the a axis is rare. Herein, a single-crystalline mesoporous MFI zeolite (SCMMZ) with sheet-like mesopores layered along the a and b axes was synthesized using a triply branched surfactant with diquaternary ammonium groups connected to 1,3,5-triphenylbenzene by a six- and eight-carbon alkyl chain (TPB-6 and 8). The sheet-like mesopores were embedded in the MFI framework and were retained even after calcination. Molecular mechanics calculations provided evidence of low binding energy configurations of the surfactant that directed the growth of straight and zigzag channels along the b and a axes, respectively. The formation of nanosheets was attributed to the geometric matching of the arrangement of the aromatic groups to the zeolite framework.

The understanding of cholesteric liquid crystals at a molecular level is challenging. Limited insights are available to bridge between molecular structures and macroscopic chiral organization. In the present study, we introduce a novel coarse-grained (CG) molecular model, which is represented by flexible chain particles with helical interactions (FCh), to study the liquid crystalline phase behavior of cholesteric molecules such as double strand DNA and α-helix polypeptides using molecular dynamics (MD) simulations. The isotropic–cholesteric phase transitions of FCh molecules were simulated for varying chain flexibilities. A wall confinement was used to break the periodicity along the cholesteric helix director in order to predict the equilibrium cholesteric pitch. The left-handed cholesteric phase was shown for FCh molecules with right-handed chiral interactions, and a spatially inhomogeneous distribution of the nematic order parameter profile was observed in cholesteric phases. It was found that the chain flexibility plays an important role in determining the macroscopic cholesteric pitch and the structure of the cholesteric liquid crystal phase. The simulations provide insight into the relationship between microscopic molecular characteristics and the macroscopic phase behavior.

Single-crystalline mesostructured zeolite nanosheets (SCZN) are synthesized by using designed surfactants with an aromatic group and only single quaternary ammonium head. Both the number of benzene rings and the length of the carbon chain play important roles in the ordered self-assembly of these alternating MFI (mordenite framework inverted) nanosheets. The surfactants, only with two benzene rings and a carbon chain larger than 4, lead to the formation of SCZN because of their highly ordered orientation through π–π stacking; the interlamellar spacing of SCZN could be controlled in the range of 1.7–2.1 nm through variation of the carbon chain length from 6 to 10. A combination of X-ray diffraction patterns and electron microscopy provides visible evidence for the mesostructural transformation from two amorphous aluminosilicate layers to one MFI sheet. The highly ordered orientation of the aromatic groups through π–π stacking geometrically matches the MFI framework to form the crystallographically correlat...

We develop here the methodology for dramatically accelerating the ReaxFF reactive force field based reactive molecular dynamics (RMD) simulations through use of the bond boost concept (BB), which we validate here for describing hydrogen combustion. The bond order, undercoordination, and overcoordination concepts of ReaxFF ensure that the BB correctly adapts to the instantaneous configurations in the reactive system to automatically identify the reactions appropriate to receive the bond boost. We refer to this as adaptive Accelerated ReaxFF Reactive Dynamics or aARRDyn. To validate the aARRDyn methodology, we determined the detailed sequence of reactions for hydrogen combustion with and without the BB. We validate that the kinetics and reaction mechanisms (that is the detailed sequences of reactive intermediates and their subsequent transformation to others) for H_2 oxidation obtained from aARRDyn agrees well with the brute force reactive molecular dynamics (BF-RMD) at 2498 K. Using aARRDyn, we then extend our simulations to the whole range of combustion temperatures from ignition (798 K) to flame temperature (2998K), and demonstrate that, over this full temperature range, the reaction rates predicted by aARRDyn agree well with the BF-RMD values, extrapolated to lower temperatures. For the aARRDyn simulation at 798 K we find that the time period for half the H_2 to form H_2O product is 538 s, whereas the computational cost was just 1289 ps, a speed increase of 0.42 trillion (10^(12)) over BF-RMD. In carrying out these RMD simulations we found that the ReaxFF-COH2008 version of the ReaxFF force field was not accurate for such intermediates as H_3O. Consequently we reoptimized the fit to a quantum mechanics (QM) level, leading to the ReaxFF–OH2014 force field that was used in the simulations.

Using force field parameters developed and validated for zeolitic imidazolate frameworks (ZIFs) and carbon dioxide (CO2) independently from adsorption data, we predicted CO2/ZIFs adsorption isotherms using the Grand Canonic Monte Carlo (GCMC) method. The results are in sharp contradiction: the calculated adsorption data agree well with the experimental data for SOD-type ZIF-8, but are more than 100% higher than the experimental data for GME-type ZIFs. Using non-equilibrium molecular dynamics simulations and potential of mean force (PMF) calculations, we reveal that the discrepancies are due to the kinetic blockage which is significant for ZIF-68 but negligible for ZIF-8. This study demonstrates that a force field developed independently from the adsorption data can be used to predict the adsorptions accurately; and the kinetic factor must be considered if the bottlenecks exist in the adsorption paths due to geometric and energetic features of adsorbate and adsorbent. It could be very misleading if the force field parameters are adjusted by fitting the GCMC simulation data to experimental data without considering the kinetic factors.

It is of great interest to understand how surfactant molecules influence the formation of micro-emulsion particles in order to support application of microemulsions in various fields. In this paper, the effect of surfactants on the formation of microemulsion particles was studied using molecular dynamics simulations coupled with Martini force field. n -Dodecane (C12H26) and sodium dodecyl sulfate (SDS) were used as the models-representing oil and surfactant molecules respectively. Twelve simulation boxes containing C12H26 and SDS with different concentrations were used in simulations running for 100 ns. Simulation results showed that the phase separations occurred rapidly for systems without surfactants, and were accompanied with decreases of potential energies. For the systems with surfactants, stable oil particles covered by surfactants were observed in the same time. The initial kinetic characteristic of the formation of microemulsion particles can be expressed as the second order reaction approximately, and the activation energy is estimated to be 14.6 kJ/mol.

The crystallization fields of initial mixtures with compositions of Al2O3:3.0 P2O5:x pip:277 H2O, where x = 2.4-12 and pip = piperazine, were investigated. Pure and highly crystalline three-dimensional (3D) AIPO-CJB2, two-dimensional (2D) AP2pip, 3D AlPO-JU88, and 2D AlPO-CJ9 were obtained when x = 3, 3.4, 6.5, and 12, respectively. Along with the increase in the ratio of piperazine to Al2O3 in the initial mixture, the density of piperazine in the final structure as well as the non-bonding interaction between the inorganic framework and the protonated piperazine increased. The crystallization processes of AlPO-CJB2, AP2pip, and AlPO-JU88 were investigated using multiple techniques. In the crystallization of AlPO-CJB2, a pure and highly crystalline AP2pip intermediate phase was first formed and then dissolved. A species that may promote the dissolution of AP2pip and the formation of AlPO-CJB2 was identified. Under the synthetic condition of AP2pip, however, this species was not formed; thus, the transformation of AP2pip to AlPO-CJB2 was not observed even for significantly prolonged crystallization times. Additionally, the crystallization rate of AP2pip was significantly accelerated by an increased molar ratio of the reactants with respect to the aluminum source. The results indicate that the type and concentration of the species in the liquid phase were the governing factors in determining the final products and that the formation of particular structures is energy-directed. (c) 2013 Elsevier Inc. All rights reserved.

The free energy based Lennard-Jones 12-6 (FE-12-6) coarse-grained (CG) force field developed for alkanes1 has been extended to model small molecules of light hydrocarbons (methane, ethane, propane, butane, and isobutane), nitrogen, oxygen, and carbon dioxide. The adjustable parameters of the FE-12-6 potential are determined by fitting against experimental vapor-liquid equilibrium (VLE) curves and heat of vaporization (HOV) data for pure substance liquids. Simulations using the optimized FE-12-6 parameters correctly reproduced experimental measures of the VLE, HOV, density, vapor pressure, compressibility, critical point, and surface tension for pure substances over a wide range of thermodynamic states. The force field parameters optimized for pure substances were tested on methane/butane, nitrogen/decane, and carbon dioxide/decane binary mixtures to predict their vapor-liquid equilibrium phase diagrams. It is found that for nonpolar molecules represented by different sized beads, a common scaling factor (0.08) that reduces the strength of the interaction potential between unlike beads, generated using Lorentz-Berthelot (LB) combination rules, is required to predict vapor-liquid phase equilibria accurately.

The products of piperazine-directed synthesis of two-dimensional (20) aluminophosphates, AP2pip and AIPO-CJ9, that were crystallized under different conditions for different periods of time were freeze-dried and characterized using XRD and NMR techniques. The core units of the layered aluminophosphates including AP2pip, AIPO-CJ9, and L-DAP, which was hydrothermally synthesized in the presence of racemic 1,2-diaminopropane, were obtained by applying a "reverse evolution" process to their structures. The factors affecting the structure-directing ability of piperazine under acidic and basic conditions were analyzed. The cooperative role of water in directing the layered aluminophosphate of AP2pip under acidic conditions is discussed. After a thorough analysis of the core units of AIPO-CJ9 and L-DAP, a new phenomenon of a "topological-structure-directing effect" or a "topological-templating effect" was observed. The observation of the new phenomenon of a topological-structure-directing effect or a topological-templating effect may contribute to a deeper understanding of the nature or origin of the "structure-directing or templating effect". (C) 2012 Elsevier Inc. All rights reserved.

The products of microporous aluminophosphate AlPO4-11 crystallized for different periods of time were freeze-dried and characterized by XRD and NMR techniques. Six crystallographically distinct Al- and P-centered large fragments and 34 small fragments of dimer (Al-P), trimer (Al-P-Al and P-Al-P), tetramer (Al-P-3 and P-Al-3). pentamer (Al-P-4 and P-Al-4), and 4- and 6-membered rings were extracted from the framework of AlPO4-11. Each of the six large fragments contained two completed coordination layers surrounding the centered atom. The shielding tensors of the Al and P atoms of the fragments were calculated using the quantum mechanics density functional theory. The experimentally observed chemical shifts of Al or P from the well-crystallized AlPO4-11 were assigned to the shielding tensors of the center atoms of the six large fragments and were further used as references in the determination of the chemical shifts of the Al or P atoms in the small fragments. A comparison of the calculated chemical shifts of the Al and P atoms in the small fragments to the experimental data of the products isolated at different crystallization periods suggested that the fragments of dimers, trimers of the form Al-P-Al, tetramers of the form P-Al-3, and pentamers may exist in the crystallization process of AlPO4-11. On the basis of these observations regarding the putative small fragments, a possible crystallization process of AlPO4-11 was proposed. (C) 2011 Elsevier Inc. All rights reserved.

Molecular simulation, as a powerful tool for investigating science of matter at a molecular level, is widely applied in interdisciplinary fields between chemistry, physics, biology, engineering and...

A gemini type of supramolecular amphiphile with a double ion-pair structure was fabricated through proton-transfer hydrogen bonding. In dioxane, the complexes could self-assemble into reverse micelles and vesicles.

DFT calculations were carried out to study heme complexes with diatomic ligand (CO, NO, or O(2)) and trans-imidazole ligand. The optimized electronic ground states of CO, NO, and O(2) adducts are singlet, doublet, and open-shell singlet, respectively. For O(2) adduct, the open-shell singlet is slightly lower in energy than the close-shell singlet. However, important differences are found in optimized structures and vibrational frequencies. Particularly, the trans-imidazole-induced frequency up-shift of the Fe-O(O) stretching mode can be predicted only with the open-shell singlet as ground state. An analysis of normal modes confirms that the up-shifts in the bent (NO and O(2) ) adducts are mainly due to mixing of Fe-X(O) stretching mode with Fe-X-O bending coordinate. Our study of binding mechanism indicates that a secondary source of the upshifts is the diminished weakening of the Fe-X(O) bonds. The Fe-X(O) bond strengths are modulated by σ competition mechanism, which weakens the Fe-X(O) bond and σ-π cooperation mechanism, which only exists in the bent adducts and enforce the Fe-X(O) bond.

Density functional theory was employed to study the hydrothermal stability of P-modified ZSM-5 zeolites using cluster models. The calculations of hydrolysis energies indicated that the introduction of phosphorus increases the hydrothermal stability of ZSM-5 zeolites. The initial paths of dealumination were studied with explicit water molecules. It was found that the framework Al-O coordination bond can be replaced by coodination bonds between water molecules and the aluminium. One to three water molecules can form coordination bonds with framework Al and release energies. The P-modification restrain the dealumination. The calculated 27Al NMR chemical shifts for the obtained structures are consistent with the experimental measurements.

Computational prediction of adsorption of small molecules in porous materials has great impact on the basic and applied research in chemical engineering and material sciences. In this work, we report an approach based on grand canonical ensemble Monte Carlo (GCMC) simulations and ab initio force fields. We calculated the adsorption curves of ammonia in ZSM-5 zeolite and hydrogen in MOF-5 (a metal-organic-framework material). The predictions agree well with experimental data. Because the predictions are based on the first principle force fields, this approach can be used for the adsorption prediction of new molecules or materials without experimental data as guidance.

To establish force-field-based (molecular) modeling capability that will accurately predict condensed-phase thermophysical properties for materials containing aliphatic azide chains, potential parameters for atom types unique to such chains have been developed and added to the COMPASS force field. The development effort identified the need to define four new atom types: one for each of the three azide nitrogen atoms and one for the carbon atom bonded to the azide. Calculations performed with the expanded force field yield (gas-phase) molecular structures and vibrational frequencies for hydrazoic acid, azidomethane, and the anti and gauche forms of azidoethane in good agreement with values determined experimentally and/or through computational quantum mechanics. Liquid densities calculated via molecular dynamics (MD) simulations were also in good agreement with published values for 13 of 15 training set compounds, the exceptions being hydrazoic acid and azidomethane. Of the 13 compounds whose densities are well simulated, nine have experimentally determined heats of vaporization reported in the open literature, and in all of these cases, MD simulated values for this property are in reasonable agreement with the published values. Simulations with the force field also yielded reasonable density estimates for a series of 2-azidoethanamines that have been synthesized and tested for use as hydrazine-alternative fuels.

One of the challenges in material science has been to prepare macro- or mesoporous zeolite. Although examples of their synthesis exist, there is a need for a facile yet versatile approach to such hierarchical structures. Here we report a concept for designing a single quaternary ammonium head amphiphilic template with strong ordered self-assembling ability through π-π stacking in hydrophobic side, which stabilizes the mesostructure to form single-crystalline mesostructured zeolite nanosheets. The concept is demonstrated for the formation of a new type of MFI (zeolite framework code by International Zeolite Association) nanosheets joined with a 90° rotational boundary, which results in a mesoporous zeolite with highly specific surface area even after calcination. Low binding energies for this self-assembling system are supported by a theoretical analysis. A geometrical matching between the arrangement of aromatic groups and the zeolitic framework is speculated for the formation of single-crystalline MFI nanosheets.

Density functional calculations were carried out on a series of oligomers of polyphosphazenes to characterize molecular structures and conformational properties of the polymers in isolation. Polyph...

A CFF931 all-atom force field for aromatic polyesters based on ab initio calculations is reported. The force field parameters are derived by fitting to quantum mechanical data which include total energies, first and second derivatives of the total energies, and electrostatic potentials. The valence parameters and the ab initio electrostatic potential (ESP) derived charges are then scaled to correct the systematic errors originating from the truncation of the basis functions and the neglect of electron correlation in the HF/6-31G* calculations. Based on the force field, molecular mechanics calculations are performed for homologues of poly(p-hydroxybenzoic acid) (PHBA) and poly(ethylene terephthalate) (PET). The force field results are compared with available experimental data and the ab initio results. © 1994 by John Wiley & Sons, Inc.

An all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported. Force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies, and electrostatic potentials, all generated from ab initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory. Valence parameters and ab initiocharges are then scaled to correct for differences between experiment and the Hartree-Fock approximation. The van der Waals parameters and the scaling factors for atomic partial charges are determined from crystal structures. Based on the force field, molecular mechanics calculations are performed for several model compounds, and the results are compared with experimental values and with the results of the ab initio calculations.

■ ASSOCIATED CONTENT *S Supporting Information H2O generation curves of all 19 samples, H2O generation curves of ΔV = 50, 60, 70 and 80, potential energy and force curves, H2 loss curve at 2498 K, H2O2 generation curve at 2498 K, species generated in aARRDyn simulations with ΔV = 20 and 40 kcal at 2498 K; the new ReaxFF−OH2014 force field parameters plus some of the validations against QM; results for the Arrhenius analysis, reaction mechanisms and kinetics from the older ReaxFF-CHO2008 force field, the force field file, and the plot script. This material is available free of charge via the Internet at http://pubs.acs.org.

The adsorption and decomposition of ammonia–borane (AB) on the surface of silicon carbide nanotubes (SiCNTs) was investigated using density functional theory. Five adsorption types and four reaction channels were identified. The most favorable reaction channel that generates a H2 molecule is slightly endothermic; furthermore, the energy barrier for the decomposition of the AB molecule is only 9.6 kcal mol−1. The side reactions that generate NH3 or BH3 are highly endothermic; therefore, the generation of side products can be depressed by decreasing the temperature. However, desorption of hydrogen atoms from the surface appears to be a more difficult step. The energy-barrier height for generation of a H2 molecule and its subsequent desorption from the surface is approximately 34.2 kcal mol−1. The migration of hydrogen atoms on the surface of SiCNTs involves lower energy than the desorption process, indicating that the desorption of H2 molecules from the surface may be more complicated.

Cation-alloyed ternary PbxSn1−xS semiconductor nanocrystals were synthesized via a solvothermal method. Both the optical and structural characterization measurements were utilized to analyze the nanocrystals. Several interesting variations on the crystal structures and band gap energy were indicated. The differences in reactivity of precursors were employed to explain the phenomenon.

Unprecedented non-interconvertable conformational isomers derived from cyclic amide ligands in a paddlewheel dimetallic framework are reported.