Ammonia Solvation vs Aqueous Solvation of Samarium Diiodide. A Theoretical and Experimental Approach to Understanding Bond Activation Upon Coordination to Sm(II).

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O-H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI was examined using Born-Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI-ammonia system is compared to the SmI-water system.

Proton donor effects on the reactivity of SmI. Experimental and theoretical studies on methanol solvation vs. aqueous solvation.

Proton donors are important components of many reactions mediated by samarium diiodide (SmI). The addition of water to SmI creates a reagent system that enables the reduction of challenging substrates through proton-coupled electron-transfer (PCET). Simple alcohols such as methanol are often used successfully in reductions with SmI but often have reduced reactivity.

Solving the CH_{4}^{-} Riddle: The Fundamental Role of Spin to Explain Metastable Anionic Methane.

Several types of experiments showed the existence of negative methane ions CH_{4}^{-} over a period of 50 years but the nature of this elusive species remains unknown. A benchmark study has shown that the experimentally observed species cannot be described by the attachment of an electron in the doublet ground state of CH_{4}^{-}. Here we find CH_{4}^{-} as being a metastable species in its lowest quartet spin state, a CH_{2}^{-}:H_{2} exciplex with three open shells lying ca.

Tracing source and mobility of arsenic and trace elements in a hydrosystem impacted by past mining activities (Morelos state, Mexico).

The Sierra Huautla (Morelos State, Mexico) is a biological reserve with historical mines of Ag and Pb. In this area, waters used by inhabitants are contaminated by arsenic (As). An integrated environmental survey was realized both in waters and sediments to better constrain the source and the mobility of As and other trace elements.

Experimental and Theoretical Studies on the Aqueous Solvation and Reactivity of SmCl and Comparison with SmBr and SmI.

Water addition to Sm(II) has been shown to increase reactivity for both SmI and SmBr. Previous work in our groups has demonstrated that this increase in reactivity can be attributed to coordination induced bond weakening enabling substrate reduction through proton-coupled electron transfer. The present work examines the interaction of water with samarium dichloride (SmCl) and illustrates the importance of the Sm-X interaction and bond distance upon water addition critical for the reactivity of the reagent system.

Experimental and Theoretical Studies on the Implications of Halide-Dependent Aqueous Solvation of Sm(II).

The addition of water to samarium(II) has been demonstrated to have a significant impact on the reduction of organic substrates, with the majority of research dedicated to the most widely used reagent, samarium diiodide (SmI). The work presented herein focuses on the reducing capabilities of samarium dibromide (SmBr) and demonstrates how the modest change in halide ligand results in observable mechanistic differences between the SmBr-water and the SmI-water systems that have considerable implications in terms of reactivity between the two reagents. Quantum chemical results from Born-Oppenheimer molecular dynamics simulations show significant differences between SmI-water and SmBr-water, with the latter displaying less dissociation of the halide, which results in a lower coordination number for water.

Hydration of CHHgOH and CHHgCl compared to HgCl, HgClOH, and Hg(OH): A DFT microsolvation cluster approach.

We address the aqueous microsolvation of the CHHgCl and CHHgOH molecules using a stepwise hydration scheme including up to 33 water molecules and compare our results with the previously studied HgCl, HgClOH, and Hg(OH) complexes. Optimized geometries and Gibbs free energies were obtained at the B3PW91/aug-RECP(Hg)-6-31G(d,p) level. At least 33 water molecules were required to build the first solvation shell around both methylmercury compounds.

A Four-Site Molecular Model for Simulations of Liquid Methanol and Water-Methanol Mixtures: MeOH-4P.

In this work, we present a new four-site potential for methanol, MeOH-4P, fitted to reproduce the dielectric constant ε, the surface tension γ , and the liquid density ρ of the pure liquid at T = 298.15 K and p = 1 bar. The partial charges on each site were taken from the OPLS/2016 model with the only difference of putting the negative charge on the fourth site ( M) instead of on the O atom, as done in four-site water models.

A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence.

Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the C-O interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for σC,O, good agreement was found, except when the correction to the UA model was applied.

Direct Coexistence Methods to Determine the Solubility of Salts in Water from Numerical Simulations. Test Case NaCl.

The solubility of NaCl, an equilibrium between a saturated solution of ions and a solid with a crystalline structure, was obtained from molecular dynamics simulations using the SPC/E and TIP4P-Ew water models. Four initial setups on supersaturated systems were tested on sodium chloride (NaCl) solutions to determine the equilibrium conditions and computational performance: (1) an ionic solution confined between two crystal plates of periodic NaCl, (2) a solution with all the ions initially distributed randomly, (3) a nanocrystal immersed in pure water, and (4) a nanocrystal immersed in an ionic solution. In some cases, the equilibration of the system can take several microseconds.

A non-polarizable model of water that yields the dielectric constant and the density anomalies of the liquid: TIP4Q.

A four-site rigid water model is presented, whose parameters are fitted to reproduce the experimental static dielectric constant at 298 K, the maximum density of liquid water and the equation of state at low pressures. The model has a positive charge on each of the three atomic nuclei and a negative charge located at the bisector of the HOH bending angle. This charge distribution allows increasing the molecular dipole moment relative to four-site models with only three charges and improves the liquid dielectric constant at different temperatures.

Dynamics and structure of Ln(III)-aqua ions: a comparative molecular dynamics study using ab initio based flexible and polarizable model potentials.

Aqueous solutions of a light (Nd3+), a middle (Gd3+), and a heavy (Yb3+) lanthanide ion were studied using ab initio based flexible and polarizable analytical potentials in classical molecular dynamics simulations to describe their thermodynamic, structural, and dynamic features. To avoid the spurious demise of O-H bonds, it was necessary to reparametrize an existing water model, which resulted in an improved description of pure water. The good agreement of the results from the simulations with the experimental hydration enthalpies, the Ln(III)-water radial distribution functions, and the water-exchange rates validated the potentials, though the r(Ln-Ow) distances were 6% longer than the experimentally determined values.

Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation.

Monte Carlo simulations of liquid methanol were performed using a refined ab initio derived potential which includes polarizability, nonadditivity, and intramolecular relaxation. The results present good agreement between the energetic and structural properties predicted by the model and those predicted by ab initio calculations of methanol clusters and experimental values of gas and condensed phases. The molecular level picture of methanol shows the existence of both rings and linear polymers in the methanol liquid phase.

A molecular dynamics calculations of hole transfer rates in DNA strands.

A computational model, which includes both tunneling and thermal hopping mechanisms, has been applied to study the charge transfer in DNA (GC)n and (AT)n strands. The calculations revealed the crucial role played by the A or G NH2-group vibrations in the hole transfer in both types of strands. Charge-transfer rates in the two strands have been determined based on the molecular dynamics calculations.

Ion hydration in nanopores and the molecular basis of selectivity.

Using a simple model, it is shown that the cost of constraining a hydrated potassium ion inside a narrow pore is smaller than the cost of constraining hydrated sodium or lithium ions in pores of radius around 1.5 A. The opposite is true for pores of radius around 2.

Water liquid-vapor equilibria predicted by refined ab initio derived potentials.

Coexistence properties for water near the critical point using several ab initio models were calculated using grand canonical Monte Carlo simulations with multiple histogram reweighting techniques. These models, that have proved to yield a good reproduction of the water properties at ambient conditions, perform rather well, improving the performance of a previous ab initio model. It is also shown that bulk geometry and dipole values, predicted by the simulation, can be used and a good approximation obtained with a polarizable rigid water model but not when polarization is excluded.

Water models based on a single potential energy surface and different molecular degrees of freedom.

Up to now it has not been possible to neatly assess whether a deficient performance of a model is due to poor parametrization of the force field or the lack of inclusion of enough molecular properties. This work compares several molecular models in the framework of the same force field, which was designed to include many-body nonadditive effects: (a) a polarizable and flexible molecule with constraints that account for the quantal nature of the vibration [B. Hess, H.

Minimalist molecular model for nanopore selectivity.

Using a simple model it is shown that the cost of constraining a hydrated potassium ion inside a narrow nanopore is smaller than the cost of constraining the smaller hydrated sodium ion. The former allows for a greater distortion of its hydration shell and can therefore maintain a better coordination. We propose that in this way the larger ion can go through narrow pores more easily.

An application of flexible constraints in Monte Carlo simulations of the isobaric--isothermal ensemble of liquid water and ice Ih with the polarizable and flexible mobile charge densities in harmonic oscillators model.

The method of flexible constraints was implemented in a Monte Carlo code to perform numerical simulations of liquid water and ice Ih in the constant number of molecules, volume, and temperature and constant pressure, instead of volume ensembles, using the polarizable and flexible mobile charge densities in harmonic oscillators (MCDHO) model. The structural and energetic results for the liquid at T=298 K and rho=997 kg m(-3) were in good agreement with those obtained from molecular dynamics. The density obtained at P=1 atm with flexible constraints, rho=1008 kg m(-3), was slightly lower than with the classical sampling of the intramolecular vibrations, rho=1010 kg m(-3).