Prediction of adsorption of small molecules in porous materials based on ab initio force field method

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.     

Molecular force fields obtained using the nonempirical stabilizer of the regularizing functional

A new approach to the analysis of the force fields of polyatomic molecules is discussed. The results of quantum chemical calculations in combination with experimental data are used in a regularizing procedure, where the nonempirical matrix of the force constants determines the stabilizer of Tikhonov's functional. The use of stable numerical methods allows the specific modeling of the force fields of polyatomic molecules with due account of rotational isomerism. M. V. Lomonosov Moscow State University, Chemical Faculty. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 204–216, March–April, 1995. Translated by I. Izvekova     

Ab initio studies on vibrational spectra of XSO2NCO (X = F,Cl): harmonic force fields and frequency assignments

The harmonic vibrational force fields and the IR spectrum of XSO₂NCO (X= F, C1) molecules have been studied usingab initio HF/SCF method with the 6-31G’ basis set. Theab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors are optimized by the least-squares fitting to the experimental frequencies of FSO₂NCO and then are transferred to CISO₂NCO to give ana priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental values for FSO₂NCO and C1SO₂NCO are 3 and 5 cm^-1, respectively. The assignments of the fundamentals for these two molecules are also made atcording to the potential energy distributions and theab initio IR intensities Project supported by the National Natural Science Foundation of China (Grant No. 29673029)     

Combined experimental–theoretical NMR study on 2,5‐bis(5‐aryl‐3‐hexylthiophen‐2‐yl)‐thiazolo[5,4‐d]thiazole derivatives for printable electronics

Four 2,5‐bis(5‐aryl‐3‐hexylthiophen‐2‐yl)thiazolo[5,4‐d]thiazole derivatives have been synthesized and thoroughly characterized. The extended aromatic core of the molecules was designed to enhance the charge transport characteristics, and solubilizing hexyl side chains were introduced on the thiophene subunits to enable possible integration of these semiconducting small molecules in printable electronics. Complete elucidation of the chemical structures by detailed one‐dimensional/two‐dimensional NMR spectroscopy is described, providing interesting input for chemical shift prediction software as well, because limited experimental data on these types of compounds are currently available. Furthermore, theoretical calculations have assisted experimental observations—giving support for the chemical shift assignment and providing a springboard for future screening and predictions—demonstrating the benefits of a coordinated theoretical–experimental approach. Copyright © 2012 John Wiley & Sons, Ltd.     

Force fields of halide-substituted methane derivatives

The paper describes methods for determining the force fields of large sets of molecules which share the common property that in their constituent groups of molecules the equivalent force constants always change monotonically with the properties of substituent atoms belonging to the same subgroup of the periodic table. The force fields of the CH_4−nX_n (n=0–4, X=F, Cl, Br, I) molecules of the methane series are obtained. The applications of these force fields are discussed. In particular, they can serve as a basis for a data bank of the force fields of fragments suitable for constructing the force fields of more complex molecules. Russian Scientific Center “Applied Chemistry”, St. Petersburg. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 250–256, March–April, 1996. Translated by I. Izvekova     

Comment on the <Emphasis Type="Italic">ab initio</Emphasis> vibrational analysis of the rotational isomers of Acrolein

Due to the publication of a number of contradictory assignments of the vibrational wave numbers of rotational isomers of Acrolein in the ground electronic state, the analysis of their vibrational spectra is repeated based on the previously calculated scaled ab initio force fields. With the use of the reported results that predicted the force fields of trans-acrolein in the ¹(n,π*) and ³(n, π*) states at the CASSCF/cc-pVTZ level, the experimental vibrational bands are analyzed in these excited electronic states based on well-established regularities. It is noted that in the assignment of the calculated vibrational wave numbers of the molecule, the isotopic shifts in the ground and excited electronic states ¹(n, π*) and ³(n, π*) are taken into account. The previously considered calculated potential curves of the internal rotation of acrolein in combination with the data on the difference in the enthalpies (ΔH ₀) of conformers allow a choice to be made in favor of one of the variants of the torsional vibration wave numbers that have been reported in the literature.     

A simple adsorption model for ionic surfactants

In the past, few theoretical attempts have been made to describe quantitatively the adsorption of ionic surfactants at liquid interfaces. Well-known adsorption isotherms due to Frumkin or Hill–de Boer cannot respond to the specific electrostatic and geometric properties of the surfactant molecules. Our approach is based on a combination of the Gouy–Chapman theory with a modified Frumkin isotherm. The modification implies that the system is free to choose an optimal head group area and an optimal arrangement of the surfactant molecules in the interface as a function of bulk concentration. Interaction energies between neighbouring adsorbed surfactant molecules and between surfactant and water molecules are taken into consideration. The minimum of the Gibbs free energy of the system is equivalent to a minimal interfacial tension. Thus, the thermodynamically stable isotherm can be obtained as the lower envelope of the family of σ versus ln c isotherms resulting from different choices of the model parameters, including the area per molecule. According to the Gibbs equation, the Γ versus ln c adsorption isotherm is obtained as the derivative of this envelope. By variation of the model parameters, the envelope of the calculated adsorption isotherms can be fitted to experimental data of the interfacial tension versus bulk concentration. A computer program is used to calculate the σ versus c and the Γ versus ln c curves as well as to fit the parameters. Received: 28 October 1999/Accepted: 8 February 2000     

Study on hexane adsorption in zeolite ITQ-29 by molecular simulation

Adsorption isotherms and isosteric heat of adsorption of n-hexane in zeolite ITQ-29 were simulated using the Configurational Bias Monte Carlo (CBMC) technique in the grand-canonical (μ VT) ensemble and compared with experimental results published by Gribov et al. and obtained by IR spectroscopy where the fractional loadings of n-hexane in ITQ-29 are presented in units from integral intensities of the absorption bands [u.a.]. In this work we present the simulation loadings of n-hexane in ITQ-29 converted to fractional coverages and compared to the experimental results. The simulations were performed using a united atom force field. In addition, we calculated equilibrium adsorption isotherms of ethane and propane in ITQ-29 in excellent agreement with published experiments. This force field successfully reproduces adsorption properties of linear alkanes in cation-free LTA zeolite and is suitable for fast and accurate adsorption data predictions.     

Adsorption of Phenolic Compounds from Water on Activated Carbon: Prediction of Multicomponent Equilibrium Isotherms using Single-Component Data

Batch-type experiments were carried out to obtain equilibrium isotherms for the adsorption of phenol and m-cresol in aqueous solutions on activated carbon. Single solute systems, at 20 and 40^∘C, were tested for Langmuir, Freundlich and Sips adsorption isotherms in the range of concentrations up to 200 mg/L. Equilibrium data were more closely followed by the Freundlich and Sips equations for all cases. Adsorption isotherms for bisolute systems at 20^∘C, with two different initial concentrations of phenol and m-cresol, were predicted solely on the basis of single solute equilibrium parameters by using the equations of Butler and Ockrent and the IAS theory. The best agreement with the experimental loading values was afforded with the IAS theory based on Sips isotherm for pure compounds. However, this theory is found to be not able to predict with success the binary isotherms in this work where significant displacement of one solute by the other is observed. Chemical interactions in the adsorbed phase, estimated by a modified Butler–Ockrent model, can be responsible for this lack of success of the conventional IAS theory. The predictions based on the IAS theory are compared with the results of some empirical models.     

分子模拟预测含配位键不饱和Cu的有机骨架(MOFs)材料的储氢性能

本文根据RI-UMP2方法、TZVPP基组和BSSE校正计算得到的能量数据,推导了描述氢分子与含有不饱和配位键的Cu的金属羧酸配合物相互作用的分子力学力场. 用巨正则系综蒙特卡洛模拟(GCMC)计算了氢在含四方形配位Cu的MOFs材料上的吸附等温线. 通过对比CuBTC的实验吸附数据发现,虽然理论预测含有四方形配位Cu的MOFs具有比含有四面体配位Zn的MOFs与氢分子更强的相互作用,实验合成得到的材料尚未反映这一区别. 以现有的CuBTC实验数据为参照消除理论计算和实验测定的系统偏差,预测了3种含有四方形配位Cu的MOFs材料以CuBTC作为参照的储氢能力.     

Osmotic ensemble methods for predicting adsorption-induced structural transitions in nanoporous materials using molecular simulations

Osmotic framework adsorbed solution theory is a useful molecular simulation method to predict the evolution of structural transitions upon adsorption of guest molecules in flexible nanoporous solids. One challenge with previous uses of this approach has been the estimation of free energy differences between the solid phases of interest in the absence of adsorbed molecules. Here we demonstrate that these free energy differences can be calculated without reference to experimental data via the vibrational density of states of each phase, a quantity that can be obtained from molecular dynamics simulations. We show the applicability of this method through case studies of the swelling behaviors of two representative systems in which swelling upon adsorption of water is of importance: single-walled aluminosilicate nanotube bundles and cesium montmorillonite. The resulting predictions show that the aluminosilicate nanotube bundles swell significantly with increasing interstitial adsorption and that the layer spacing of cesium montmorillonite expands up to about 12.5 A?, giving good agreement with experiments. The method is applicable to a wide range of flexible nanoporous materials, such as zeolites, metal-organic frameworks, and layered oxide materials, when candidate structures can be defined and a force field to describe the material is available.     

Grand challenges in quantum‐classical modeling of molecule–surface interactions

A detailed understanding of the adsorption of small molecules or macromolecules to a materials surface is of importance, for example, in the context of material and biomaterial research. Classical atomistic simulations in principle provide microscopic insight in the complex entropic and enthalpic interplay at the interface. However, an application of classical atomistic simulation techniques to such interface systems is a nontrivial problem, mostly because commonly used force fields cannot be straightforwardly applied, as they are usually developed to reproduce bulk properties of either solids or liquids but not the interfacial region between two phases. Therefore, a dual‐scale modeling approach has often been the method of choice in the past, in which the classical force field is parameterized such that quantum chemical information on near‐surface conformations and adsorption energies is reproduced by the classical force field. We will discuss in this review the current state‐of‐the‐art of quantum‐classical modeling of molecule–surface interactions and outline the major challenges in this field. In this context, we will, among other things, lay emphasis on discussing ways to obtain representable force fields and propose systematic and system‐independent strategies to optimize the quantum‐classical fitting procedure. © 2013 Wiley Periodicals, Inc.     

Influence of electrostatic interactions on the properties of cyanobiphenyl liquid crystals predicted from atomistic molecular dynamics simulations

The influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4?-pentylbiphenyl (5CB) systems have been investigated in the 292–368 K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (T_NI) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the T_NI by about 30 K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of T_NI as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.     

Adsorption of n-alkanes in faujasite zeolites: molecular simulation study and experimental measurements

We report an application of a previously developed force field for adsorption of hydrocarbons in silicalite (Pascual, P., et al. in Phys. Chem. Chem. Phys. 5:3684–3693, 2003), to the case of the linear alkane-sodium faujasite systems. In order to extend this force field from siliceous to cationic zeolites, we propose to take into account the polarization part of the zeolite-molecule interaction energy. A first order polarization term is explicitly considered for this purpose, using standard molecular polarizabilities. Polarization appears to amount to 30–40% of the zeolite-alkane interaction energy, as a consequence of the strong electric field created by the sodium cation distribution and negatively charged framework. This approach is compared with experimental adsorption isotherms of ethane, propane, n-octane and n-decane in NaY from the literature and with original measurements of n-butane isotherms in NaY obtained by thermal gravimetry. Henry constants and heats of adsorption at zero coverage of n-alkanes (n=6–10) are also compared with experimental measurements. Although no specific parameter has been calibrated for extending the force field, the general agreement between simulation results and experiments is satisfactory. Cation redistribution upon alkane adsorption is not observed in these simulations.     

Quantum chemical model for the adsorption of organic molecules on metal surfaces in electrolyte solutions

A quantum chemical model is proposed for the adsorption of organic molecules on metal surfaces in electrolyte solutions. This approach is based on the method of electrostatic images modefied for the case of a double electrical layer within the framework of the PPP method. The calculated data are in good accord with the available experimental results. Donbas State Building and Architecture Academy, 2 Derzhavina ul., Makeevka 339023, Donetsk Oblast', Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 5, pp. 284–288, September–October, 1999.     

Enthalpies of mixing predicted using molecular dynamics simulations and OPLS force field

Enthalpy of mixing (EOM) is one of the most basic thermodynamic properties of mixtures. To assess feasibility of predicting EOM using force field simulation methods, fifteen (15) representative binary mixtures were investigated using MD simulations based on OPLS and TIP4P force fields. The simulation conditions and errors were carefully examined. The precision level of 0.04 kJ/mol was obtained for calculated EOM data. However, the predictions, measured by deviations from experimental data, were only qualitatively correct. Among various factors influencing the accuracy of predictions, force field quality representing interactions among different molecules plays the most significant role. Using methanol/benzene and ethanol/benzene as examples, we demonstrated that non-additive interaction terms between polarizable atoms can be used to significantly improve the quality of predictions. In addition, it appears that charge-dependent LJ parameters are required in order to represent the polarization effects accurately.     

Influence of Confinement on The Phase Transition And Spectral Characteristics of Nematic Liquid Crystals

The results of nematic liquid crystal - isotropic liquid phase transition study by the method of differential scanning calorimetry for inclusion compounds - mesoporous aluminosilicate molecular sieves of MCM-41 type (including Cu-exchanged samples) with encapsulated in inner-crystalline space nematic liquid crystal (5CB), as well as IR spectroscopic data for such compounds were represented. It was shown, that the 5CB molecules are able to interact with the active centers in the MCM-41 channels forming strong enough bonds of -C≡N⋅⋅⋅H-O- type. The relative amount of 5CB molecules interacting with the walls of channels and those retaining ‘liquid crystalline’ state in binary systems of molecular sieves MCM-41 and CuMCM-41 was estimated. This conclusion was confirmed by the data obtained from differential scanning calorimetry measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Efficient parametrization of complex molecule–surface force fields

We present an efficient scheme for parametrizing complex molecule–surface force fields from ab initio data. The cost of producing a sufficient fitting library is mitigated using a 2D periodic embedded slab model made possible by the quantum mechanics/molecular mechanics scheme in CP2K. These results were then used in conjunction with genetic algorithm (GA) methods to optimize the large parameter sets needed to describe such systems. The derived potentials are able to well reproduce adsorption geometries and adsorption energies calculated using density functional theory. Finally, we discuss the challenges in creating a sufficient fitting library, determining whether or not the GA optimization has completed, and the transferability of such force fields to similar molecules. © 2015 Wiley Periodicals, Inc.