The treatment of the solvation charges using Gaussian functions in the polarizable continuum model results in a smooth potential energy surface. These charges are placed on top of the surface of the solute cavity. In this article, we study the effect of the solute cavity (van der Waals‐type or solvent‐excluded surface‐type) using the Gaussian charge scheme within the framework of the conductor‐like polarizable continuum model on (a) the accuracy and computational cost of the self‐consistent field (SCF) energy and its gradient and on (b) the calculation of free energies of solvation. For that purpose, we have considered a large set of systems ranging from few atoms to more than 200 atoms in different solvents. Our results at the DFT level using the B3LYP functional and the def2‐TZVP basis set show that the choice of the solute cavity does neither affect the accuracy nor the cost of calculations for small systems (< 100 atoms). For larger systems, the use of a vdW‐type cavity is recommended, as it prevents small oscillations in the gradient (present when using a SES‐type cavity), which affect the convergence of the SCF energy gradient. Regarding the free energies of solvation, we consider a solvent‐dependent probe sphere to construct the solvent‐accessible surface area required to calculate the nonelectrostatic contribution to the free energy of solvation. For this part, our results for a large set of organic molecules in different solvents agree with available experimental data with an accuracy lower than 1 kcal/mol for both polar and nonpolar solvents.

Continuum solvation models have become a widespread approach for the study of environmental effects in Density Functional Theory (DFT) methods. Adding solvation contributions mainly relies on the solution of the Generalized Poisson Equation (GPE) governing the behavior of the electrostatic potential of a system. Although multigrid methods are especially appropriate for the solution of partial differential equations, up to now, their use is not much extended in DFT-based codes because of their high memory requirements. In this Article, we report the implementation of an accelerated multigrid solver-based approach for the treatment of solvation effects in the Vienna ab initio Simulation Package (VASP). The stated implicit solvation model, named VASP-MGCM (VASP-Multigrid Continuum Model), uses an efficient and transferable algorithm for the product of sparse matrices that highly outperforms serial multigrid solvers. The calculated solvation free energies for a set of molecules, including neutral and ionic species, as well as adsorbed molecules on metallic surfaces, agree with experimental data and with simulation results obtained with other continuum models.

The 30 cationic {Mo(V)2O4(acetate)}(+) units linking 12 negatively charged pentagonal "ligands," {(Mo(VI))Mo(VI)5O21(H2O)6}(6-) of the porous metal-oxide capsule, [{Mo(VI)6O21(H2O)6}12{Mo(V)2O4(acetate)}30](42-) provide active sites for catalytic transformations of organic "guests". This is demonstrated using a well-behaved model reaction, the fully reversible cleavage and formation of methyl tert-butyl ether (MTBE) under mild conditions in water. Five independent lines of evidence demonstrate that reactions of the MTBE guests occur in the ca. 6 × 10(3) Å(3) interior of the spherical capsule. The Mo atoms of the {Mo(V)2O4(acetate)}(+) linkers--spanning an ca. 3-nm truncated icosahedron--are sterically accessible to substrate, and controlled removal of their internally bound acetate ligands generates catalytically active {Mo(V)2O4(H2O)2}(2+) units with labile water ligands, and Lewis- and Brønsted-acid properties. The activity of these units is demonstrating by kinetic data that reveal a first-order dependence of MTBE cleavage rates on the number of acetate-free {Mo(V)2O4(H2O)2}(2+) linkers. DFT calculations point to a pathway involving both Mo(V) centers, and the intermediacy of isobutene in both forward and reverse reactions. A plausible catalytic cycle--satisfying microscopic reversibility--is supported by activation parameters for MTBE cleavage, deuterium and oxygen-18 labeling studies, and by reactions of deliberately added isobutene and of a water-soluble isobutene analog. More generally, pore-restricted encapsulation, ligand-regulated access to multiple structurally integral metal-centers, and options for modifying the microenvironment within this new type of nanoreactor, suggest numerous additional transformations of organic substrates by this and related molybdenum-oxide based capsules.

The adsorption of CO 2 , CH 4 , and N 2 at 303 K by MIL-91(Al), one of the few porous phosphonate-based-MOFs, has been investigated by combining advanced experimental and computational tools. Whereas CH 4 and N 2 adsorption isotherms exhibit type I behavior, the reversible CO 2 isotherm displays an unusual inflection point at low pressure. In situ X-ray powder diffraction and infrared spectroscopy showed structural changes of this small-pore MOF upon CO 2 adsorption. Grand canonical Monte Carlo simulations delivered a detailed picture of the adsorption mechanisms at the microscopic level. The so-predicted arrangements of the confined CO 2 molecules were supported by analysis of the in situ diffraction and infrared experiments. It was shown that while adsorbed CH 4 and N 2 are located mainly in the center of the pores, CO 2 molecules interact with the hydrogen-bonded POH−N acid−base pairs. This results in a relatively high adsorption enthalpy for CO 2 of ca. −40 kJ mol −1 , which suggests that this material might be of interest for CO 2 capture at low pressure (postcombustion).

Solvation is crucial in many chemical and electrochemical processes related to alcohol conversion on metal surfaces. Particularly, understanding the dehydrogenation mechanism of methanol on solvated Pd, Pt, and Ru surfaces could allow the design of efficient methanol fuel cells. The large computational cost related to adopting an explicit solvation approach into density functional theory can be reduced drastically by using implicit solvation methods. In this study, we use our recently developed continuum solvation model (MGCM) to elucidate the minimum number of explicit water molecules to add to the solvated methanol/metal surface systems to reproduce experimental data with an optimized balance between time and reliability. Our results stress the importance of adding two explicit water molecules, especially for the case of Ru surfaces. For this later system, we provide a first insight into the decomposition mechanism of methanol using first-principles calculations. Our predictions can be then a useful reference for future studies that aim at designing more efficient heterogeneous catalysts with solvents.

Polyether polyols are used widely in the plastic and coating industries in the form of polyurethanes. The polymerization of epoxides can be catalyzed by double metal cyanides (DMCs), Zn3[Co(CN)6]2. These catalysts were first reported in the 1960s by General Tire Inc. and provide products with excellent technical features, which are better than those that result from traditional alkaline polymerization as side reactions are alleviated. However, DMC‐catalyzed polymerization is not free of drawbacks as high‐molecular‐weight side products (1–3 wt %) can form in the propylene process. These tails are detrimental to the subsequent use of these polymers, in particular to foam stability. Despite the wide industrial interest in DMCs, there are only a few experimental studies and a complete lack of theoretical research of their structure, activity, and performance. The present work is thus the first attempt to describe the nature of the active site, the main polymerization mechanism, and two potential origins for the high‐weight tails from a theoretical standpoint by analyzing three crucial steps in the polymerization process.

Mass diffusion coefficients of CO(2)/brine mixtures under thermodynamic conditions of deep saline aquifers have been investigated by molecular simulation. The objective of this work is to provide estimates of the diffusion coefficient of CO(2) in salty water to compensate the lack of experimental data on this property. We analyzed the influence of temperature, CO(2) concentration,and salinity on the diffusion coefficient, the rotational diffusion, as well as the electrical conductivity. We observe an increase of the mass diffusion coefficient with the temperature, but no clear dependence is identified with the salinity or with the CO(2) mole fraction, if the system is overall dilute. In this case, we notice an important dispersion on the values of the diffusion coefficient which impairs any conclusive statement about the effect of the gas concentration on the mobility of CO(2) molecules. Rotational relaxation times for water and CO(2) increase by decreasing temperature or increasing the salt concentration. We propose a correlation for the self-diffusion coefficient of CO(2) in terms of the rotational relaxation time which can ultimately be used to estimate the mutual diffusion coefficient of CO(2) in brine. The electrical conductivity of the CO(2)-brine mixtures was also calculated under different thermodynamic conditions. Electrical conductivity tends to increase with the temperature and salt concentration. However, we do not observe any influence of this property with the CO(2) concentration at the studied regimes. Our results give a first evaluation of the variation of the CO(2)-brine mass diffusion coefficient, rotational relaxation times, and electrical conductivity under the thermodynamic conditions typically encountered in deep saline aquifers.

The structure and dynamics of water confined inside a polyoxomolybdate molecular cluster [{(Mo)Mo(5)O(21)(H(2)O)(6)}(12){Mo(2)O(4)(SO(4))}(30)](72-) metal oxide nanocapsule have been studied by means of molecular dynamics simulations under ambient conditions. Our results are compared to experimental data and theoretical analyses done in reverse micelles, for several properties. We observe that the characteristic three-dimensional hydrogen bond network present in bulk water is distorted inside the cavity where water organizes instead in concentric layered structures. Hydrogen bonding, tetrahedral order, and orientational distribution analyses indicate that these layers are formed by water molecules hydrogen bonded with three other molecules of the same structure. The remaining hydrogen bond donor/acceptor site bridges different layers as well as the whole structure with the hydrophilic inner side of the cavity. The most stable configuration of the layers is thus that of a buckyball with 12 pentagons and a variable number of hexagons. The geometrical constraints make it so that the bridges between the layers display a significant degree of frustration. The main modes of motion at short times are correlated fluctuations of the entire system with a characteristic frequency. Switches of water molecules between layers are rare events, due to the stability of the layers. At long times, the system shows a power law decay (pink noise) in properties like the fluctuations in the number of molecules in the structures and the total dipole moment. Such behavior has been attributed to the complex relaxation of the hydrogen bond network, and the exponents found are close to those encountered in bulk water for the relaxation of the potential energy. Our results reveal the importance of the competition between the confinement and the long-range structure induced in this system by the hydrogen bond network.