Daniel Laria

Reference interaction site model polaron theory of the hydrated electron

We have extended the reference interaction site model (RISM)‐polaron theory of Chandler et al. [J. Chem. Phys. 81, 1975 (1984)] to treat self‐trapping and localized states of excess electrons in polar fluids. The extension is based on a new closure of the RISM equation presented herein. The theory is applied to the hydrated electron employing a simple class of electron‐water pseudopotentials. Included in this class are models coinciding with those already examined by others using computer simulations. In those cases, the results for both structural and energetic properties compare well with those of simulation. The work function, or equivalently, the excess chemical potential of the hydrated electron are also computed; the theoretical result agrees with experiment to about 1%. Most interesting, however, is that as the parameter characterizing the pseudopotentials is varied, a critical parameter is found where the electron behavior changes essentially discontinuously from a trapped state to a ‘‘super’’‐tra...

Solvation response of polar liquid mixtures: Water-dimethylsulfoxide

The solvation dynamics following the instantaneous creation of a positive or negative electronic charge in a previously neutral solute immersed in different water-dimethyl sulfoxide (DMSO) mixtures, spanning the entire composition range, is analyzed by molecular dynamics simulations. The solvation responses are strongly dependent on the sign of the solute charge, being considerably faster in the presence of cations for all mixtures considered. In terms of the composition dependence, the mixtures’ solvation response to the creation of the anion departs substantially from the pure solvents’, whereas for the cation, the mixtures’ responses are close to those exhibited by pure DMSO. In the case of anions, the mixture overall solvation time, defined as the time integral of the nonequilibrium response, can be as large as ten times the solvation time in pure DMSO, the slowest of the two cosolvents. The DMSO contribution to the mixtures’ solvation response may present an intriguing negative branch in the rotation...

Comparative study of theory and simulation calculations for excess electrons in simple fluids

RISM‐polaron theory and simulation results of the primitive hard sphere model for an excess electron in simple fluids are used to interpret the recent path integral quantum Monte Carlo studies of an electron in supercritical helium and in xenon by Coker, Berne, and Thirumalai. It is shown that the different behaviors of the excess electron in these two different fluids are due primarily to differences in excluded volume effects. For xenon, due to the nature of the electron–solvent pseudopotential, this volume is relatively small and the excess electron remains extended for all fluid densities. In contrast, for helium, the random excluded volume is high leading to self‐trapping or localization of the electron.

Dielectric relaxation of supercritical water: Computer simulations

Dielectric relaxation times of supercritical SPC/E water from molecular dynamics simulations are found to be in good agreement with recent experimental data for densities ρ⩾0.4 g/cm3, but the sharp increase in the experimental Debye time as ρ decreases is not reproduced. Large discrepancies between experimental and simulation data in the dilute regime strongly suggest the need for additional measurements and/or theoretical work.

Solvation dynamics of coumarin 153 in dimethylsulfoxide-water mixtures: Molecular dynamics simulations

We present results of molecular dynamics simulations of solvation dynamics of coumarin 153 in dimethylsulfoxide (DMSO)–water mixtures of different compositions (xD=0.00, 0.25, 0.32, 0.50, 0.75, and 1.00) using an all-atom model for the solute probe. Results are reported for the global solvation responses of the simulated systems, as well as for the separate contributions from each cosolvent and the individual solute–site couplings to water and DMSO. The solvation dynamics is predominantly given by DMSO’s contribution, even at low (25%) DMSO content, because of the preferential solvation of the probe. We find that the water molecules are only mildly coupled to the charge transfer in the coumarin, resulting in a small, largely diffusive, water relaxation component. Simulation results, including solvation responses, characteristic times, and Stokes shifts are compared with recent fluorescence upconversion experimental measurements showing good agreement for the relaxation but significant differences for the ...

Confined polar mixtures within cylindrical nanocavities.

Using molecular dynamics experiments, we have extended our previous analysis of equimolar mixtures of water and acetonitrile confined between silica walls [J. Phys. Chem. B 2009, 113, 12744] to examine similar solutions trapped within carbon nanotubes and cylindrical silica pores. Two different carbon tube sizes were investigated, (8,8) tubes, with radius R(cnt) = 0.55 nm, and (16,16) ones, with R(cnt) = 1.1 nm. In the narrowest tubes, we found that the cylindrical cavity is filled exclusively by acetonitrile; as the radius of the tube reaches approximately 1 nm, water begins to get incorporated within the inner cavities. In (16,16) tubes, the analysis of global and local concentration fluctuations shows a net increment of the global acetonitrile concentration; in addition, the aprotic solvent is also the prevailing species at the vicinity of the tube walls. Mixtures confined within silica nanopores of radius approximately 1.5 nm were also investigated. Three pores, differing in the effective wall/solvent interactions, were analyzed, (i) a first class, in which dispersive forces prevail (hydrophobic cavities), (ii) a second type, where oxygen sites at the pore walls are transformed into polar silanol groups (hydrophilic cavities), and (iii) finally, an intermediate scenario, in which 60% of the OH groups are replaced by mobile trimethylsilyl groups. Within the different pores, we found clear distinctions between the solvent layers that lie in close contact with the silica substrate and those with more central locations. Dynamical modes of the confined liquid phases were investigated in terms of diffusive and rotational time correlation functions. Compared to bulk results, the characteristic time scales describing different solvent motions exhibit significant increments. In carbon nanotubes, the most prominent modifications operate in the narrower tubes, where translations and rotations become severely hindered. In silica nanopores, the manifestations of the overall retardations are more dramatic for solvent species lying at the vicinity of trimethylsilyl groups.

Molecular dynamics study of water clusters containing ion pairs: From contact to dissociation

We have studied the potential of mean force between pairs of monovalent ions immersed in water clusters composed of up to 64 molecules at 200 K using constrained molecular dynamics techniques. Two different Hamiltonians for the water particles were investigated: one has fixed‐point charges while the other has induced atomic dipoles which explicitly introduce effects due to fluctuations in the electronic density of the molecules. The qualitative behaviors of both models present similarities. For the case of pairs of equally charged ions, the solvent reactive field introduces a net attraction between the ions that prevents the dissociation of the clusters over a wide range of interionic distances. Similar binding effects are found for neutral ion pairs where the solvent reinforces the ionic attraction when the interionic distance attains values comparable to the cluster size. The correct thermodynamic interpretation of the calculated averages is restricted to small interionic distances; beyond this range pr...

Polar Mixtures under Nanoconfinement

We present results from molecular dynamics simulations describing structural and dynamical characteristics of equimolar mixtures of water and acetonitrile, confined between two silica walls separated at interplate distances of d=0.6, 1, and 1.5 nm. Two different environments were investigated: a first one where wall-solvent dispersion forces prevail (hydrophobic confinement) and a second one in which the terminal O atoms at the silica surface are transformed into silanol groups (hydrophilic confinement). For the former case, we found that, at the shortest interplate distance examined, the confined region is devoid of water molecules. At an interplate distance of the order of 1 nm, water moves into the confined region, although, in all cases, there is a clear enhancement of the local concentration of acetonitrile in detriment of that of water. Within hydrophilic environments, we found clear distinctions between a layer of bound water lying in close contact with the silica substrates and a minority of confined water that occupies the inner liquid slab. The bound aqueous layer is fully coordinated to the silanol groups and exhibits minimal hydrogen bonding with the second solvation layer, which exclusively includes acetonitrile molecules. Dynamical characteristics of the solvent mixture are analyzed in terms of diffusive and rotational motions in both environments. Compared to bulk mixtures, we found significant retardations in all dynamical modes, with those ascribed to water molecules bound to the hydrophilic plates being the most dramatic.

Lithium solvation in dimethyl sulfoxide-acetonitrile mixtures

We present molecular dynamics simulation results pertaining to the solvation of Li(+) in dimethyl sulfoxide-acetonitrile binary mixtures. The results are potentially relevant in the design of Li-air batteries that rely on aprotic mixtures as solvent media. To analyze effects derived from differences in ionic size and charge sign, the solvation of Li(+) is compared to the ones observed for infinitely diluted K(+) and Cl(-) species, in similar solutions. At all compositions, the cations are preferentially solvated by dimethyl sulfoxide. Contrasting, the first solvation shell of Cl(-) shows a gradual modification in its composition, which varies linearly with the global concentrations of the two solvents in the mixtures. Moreover, the energetics of the solvation, described in terms of the corresponding solute-solvent coupling, presents a clear non-ideal concentration dependence. Similar nonlinear trends were found for the stabilization of different ionic species in solution, compared to the ones exhibited by their electrically neutral counterparts. These tendencies account for the characteristics of the free energy associated to the stabilization of Li(+)Cl(-), contact-ion-pairs in these solutions. Ionic transport is also analyzed. Dynamical results show concentration trends similar to those recently obtained from direct experimental measurements.

SOLVATION EFFECTS ON A MODEL SN2 REACTION IN WATER CLUSTERS

We present a series of molecular dynamics experiments for the nucleophilic substitution reaction Cl−+CH3Cl→ClCH3+Cl− taking place in liquid simple point charge water nanoclusters containing 6, 16, and 32 solvent molecules at temperatures close to 200 K. A three‐dimensional potential energy for the reagent interatomic interactions is employed. Equilibrium and dynamical aspects of the reactive process are investigated. Solvation effects lead to significant enhancements of the computed free energy barriers even in aggregates containing only six water molecules. The equilibrium spatial and orientational correlations describing the changes in the solvation structure along the reaction path are also presented. The reactive/product states are characterized by a fully solvated Cl− ion embedded within the cluster while the CH3Cl remains on the surface; at the transition state, the complex lies at the cluster surface adopting a linear geometry tangential to the cluster boundary. We have also monitored the time rela...