Search

We present results from molecular dynamics simulations that describe structural and dynamical characteristics of the solvation pertinent to the Li+ + e– → Li reduction reaction at the vicinity of t...

We present path integral molecular dynamics results that describe the effects of nuclear quantum fluctuations on equilibrium and dynamical characteristics pertaining to bifurcation pathways in hydrogen bonded dimers combining water and ammonia, at cryogenic temperatures of the order of 20 K. Along these isomerizations, the hydrogen atoms in the molecules acting as hydrogen-bond donors interchange their original dangling/connective characters. Our results reveal that the resulting quantum transition paths comprise three stages: the initial and final ones involve overall rotations during which the two protons retain their classical-like characteristics. Effects from quantum fluctuation are clearly manifested in the changes operated at the intermediate passages over transition states, as the spatial extents of the protons stretch over typical lengths comparable to the distances between connective and dangling basins of attractions. Consequently, the classical over-the-hill path is replaced by a tunneling controlled mechanism which, within the path integral perspective, can be cast in terms of concerted inter-basin migrations of polymer beads from dangling-to-connective and from connective-to-dangling, at practically no energy costs. We also estimated the characteristic timescales describing such interconversions within the approximate ring polymer rate theory. Effects derived from full and partial deuteration are also discussed.

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid’s structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed. Graphic abstract: [Figure not available: see fulltext.]

We present results from path integral molecular dynamics simulations that describe effects from the explicit incorporation of nuclear quantum fluctuations on the topology of the free energy associated with the geared exchange of hydrogen bonds in the water–water dimer. Compared to the classical treatment, our results reveal important reductions in the free energy barriers and changes at a qualitative level in the overall profile. Most notable are those manifested by a plateau behavior, ascribed to nuclear tunneling, which bridges reactant and product states, contrasting with the usual symmetric double-well profile. The characteristics of the proton localizations along the pathway are examined. An imaginary time analysis of the rotational degrees of freedom of the partners in the dimer at the vicinities of transition states shows a clear “anticorrelation” between intermolecular interactions coupling beads localized in connective and dangling basins of attractions. As such, the transfer is operated by gradual concerted inter-basin migrations in opposite directions, at practically no energy costs. Modifications operated by partial deuteration and by the asymmetries in the hydrogen bonding characteristics prevailing in water–methanol heterodimers are also examined. Fil: Méndez, Emilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina

Using path-integral molecular dynamics simulations, we examine isomerization paths involving collective proton transfers in [H2O]5 and [H2O]8 clusters under cryogenic conditions. We focused attention on combined effects derived from solvation and nuclear quantum fluctuations on the characteristics of free energy barriers and relative stabilities of reactants and products. In particular, we analyzed two different processes: the first one involves the exchange of donor-acceptor hydrogen bond roles along cyclic moieties, whereas the second one corresponds to charge separation leading to stable [H3O]+[OH]- ion pairs. In the first case, the explicit incorporation of quantum tunneling introduces important modifications in the classical free energy profile. The resulting quantum profile presents two main contributions, one corresponding to compressions of O-O distances and a second one ascribed to nuclear tunneling of the light protons. Solvation effects promote a moderate polarization of the cyclic structures and a partial loss of concertedness in the collective modes, most notably, at the onset of tunneling. Still, the latter effects are also sufficiently strong to promote the stabilization of ion pairs along the classical trajectories. In contrast, the explicit incorporation of nuclear quantum fluctuations leads to charge separated configurations that are marginally stable. As such, the latter states could also be regarded as short-lived intermediate states along the reactive exchange path. Fil: Turi, Laszlo. Eötvös Loránd University; Hungría Fil: Rodriguez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Constituyentes | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Constituyentes; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina

We present results from computer simulations that shed light on structural and dynamic characteristics of hydrogen bonding of aqueous phases at ambient conditions, at the close vicinity of electrified metal interfaces. Our simulation strategy relied on the consideration of a Hamiltonian that explicitly incorporates effects from polarization fluctuations at the metal surface, induced by the instantaneous local electric field promoted by the partial charges at the solvent molecules. Compared to bulk environments, our results reveal important modifications in the hydrogen bond architectures that critically depend on the atomic arrangements of the interfaces exposed to the liquid phases and the net charges allocated at the metal plates. These modifications have equally important consequences on the characteristic time scales describing the ruptures of hydrogen bonds which are operated by mechanisms which are absent in descriptions that omit atomic detail and polarization fluctuations at the metal plates. We also analyze how the latter modifications are translated into spectral shifts in the stretching bands of infrared spectra of water adlayers. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Ansourian, Lisandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Comisión Nacional de Energía Atómica; Argentina

We present results from ring polymer molecular dynamics experiments that provide microscopic insights into the characteristics of the isotopic stabilizations of H and D aqueous species in the first solvation shell of a halide I- ion in water nanoclusters at low temperatures. The analysis of the simplest I-·(HOD) dimer shows a clear propensity for the light isotope to lie at the non-hydrogen-bonded dangling position. Our results predict that, at T ∼ 50 K, I-·(DOH) isomers are three times more abundant than I-·(HOD) ones. The reasons for such stabilization can be traced back to differences in the nuclear kinetic energy projected along directions perpendicular to the plane of the water molecule. Dynamical implications of these imbalances are shown to be reflected in the characteristics of the corresponding bands of the infrared spectroscopic signals. A similar analysis performed in larger aggregates containing ∼20 water molecules reveals, in contrast, a stabilization of the light isotope along I-⋯HO hydrogen bonds. Effects derived from the consideration of smaller halide anions with larger electric fields at the surface are also examined. (Figure Presented). Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Rossky, Peter J.. Rice University; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

By combining path-integrals molecular dynamics simulations with the accurate MB-pol potential energy surface, we investigate the role of alternative potential models on isotopic fractionation ratios between H and D atoms at dangling positions in water clusters at low temperatures. Our results show clear stabilizations of the lighter isotope at dangling sites, characterized by free energy differences ΔG that become comparable to or larger than kBT for temperatures below ∼75 K. The comparison between these results to those previously reported using the empirical q-TIP4P/F water model [P. E. Videla et al., J. Phys. Chem. Lett. 5, 2375 (2014)] reveals that the latter Hamiltonian overestimates the H stabilization by ∼25%. Moreover, predictions from the MB-pol model are in much better agreement with measured results reported for similar isotope equilibria at ice surfaces. The dissection of the quantum kinetic energies into orthogonal directions shows that the dominant differences between the two models are to be found in the anharmonic characteristics of the potential energy surfaces along OH bond directions involved in hydrogen bonds. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Rossky, Peter J.. Rice University; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

Using ring polymer molecular dynamics simulations, we examine equilibrium and dynamical characteristics of solid-like, aqueous clusters that combine isotopic mixtures of HDO dilute in H2O, at temperatures intermediate between 50 and 175 K. In particular, we focus attention on the relative thermodynamic stabilities of the two isotopes at dangling hydrogen bond sites. The water octamer is analyzed as a reference system. For this aggregate, decreasing temperature yields a gradual stabilization of the light isotope at dangling sites in molecules acting as single-donor-double-acceptors of hydrogen bonds. At T ∼ 50 K, the imbalance between the corresponding quantum kinetic energies leads to a free energy difference between dangling and hydrogen bonded sites of the order of ∼2kBT. Similar free energy differences were found at dangling sites in Nw = 50 water clusters. The extent of the H/D segregation can be adequately monitored by modifications in the peak intensity of the high frequency shoulder of the stretching band of the infrared spectrum. These signals, in turn, represent a potential experimental signature of the elusive temperature of clusters in molecular beams. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Rossky, Peter J.. University of Texas at Austin; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

We present results from molecular dynamics simulations that examine microscopic characteristics of mixtures combining acetonitrile (ACN) and dimethyl sulfoxide (DMSO) at the vicinity of liquid/air and liquid/graphene interfaces. In the former interfaces, our simulations reveal a clear propensity of ACN to lie adjacent to the vapor phase at all concentrations. A simple model based on the consideration of a chemical equilibrium between bulk and surface states was found to be adequate to reproduce simulation results. Orientational correlations at the interface showed a mild tendency for dipolar aligments pointing toward the vapor phase in ACN-rich solutions; contrasting, in DMSO-rich mixtures, the preferential orientations looked mostly parallel to the interface. Close to graphene plates, the local scenarios reverse and local concentrations of DMSO are larger than the one observed in the bulk. Dynamical results reveal that the characteristic time scales describing orientational relaxations and residence times at the interfaces stretch as the concentration of ACN diminishes. For liquid/air interfaces residence times for ACN were found to be larger than those for DMSO. A classical treatment for the predictions of the C-H stretching band of the IR peaks in the bulk and at the interfaces reveals shifts that agree with experimental measurements. Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín; Argentina Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica; Argentina Fil: Martí, Jordi. Universidad Politécnica de Catalunya; España Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

We present molecular dynamics simulation results describing proton/deuteron exchange equilibria along hydrogen bonds at the vicinity of HX acids (X = F, I) in aqueous clusters at low temperatures. To allow for an adequate description of proton transfer processes, our simulation scheme resorted on the implementation of a multistate empirical valence bond hamiltonian coupled to a path integral scheme to account for effects derived from nuclear quantum fluctuations. We focused attention on clusters comprising a number of water molecules close to the threshold values necessary to stabilize contact-ion-pairs. For X = F, our results reveal a clear propensity of the heavy isotope to lie at the bond bridging the halide to the nearest water molecule. Contrasting, for X = I, the thermodynamic stability is reversed and the former connectivity is preferentially articulated via the light isotope. These trends remain valid for undissociated and ionic descriptions of the stable valence bond states. The preferences are rationalized in terms of differences in the quantum kinetic energies of the isotopes which, in turn, reflect the extent of the local spatial confinements prevailing along the different hydrogen bonds in the clusters. In most cases, these features are also clearly reflected in the characteristics of the corresponding stretching bands of the simulated infrared spectra. This opens interesting possibilities to gauge the extent of the isotopic thermodynamic stabilizations and the strengths of the different hydrogen bonds by following the magnitudes and shifts of the spectral signals in temperature-controlled experiments, performed on mixed clusters combining H2O and HOD. Fil: Litman, Yair Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Comisión Nacional de Energía Atómica; Argentina

Equilibrium and dynamical characteristics pertaining to the solvation of the fluorescent probe coumarin 153 in liquid methanol confined within cylindrical silica pores are investigated using molecular dynamics techniques. Three kinds of pores are examined: (i) Soft hydrophobic cavities, in which wall-solvent interactions were exclusively of the Lennard-Jones type; (ii) Hydrophilic cavities, in which unsaturated oxygen sites at the wall were transformed into hydroxyl groups; (iii) Rugged pores, in which 60% of the polar groups were transformed into bulkier and mobile trimethylsilyl moieties. Equilibrium solvation structures in the three pores differ considerably: In hydrophobic environments, the solute remains adsorbed to the pore wall, with its molecular plane mostly parallel to the interface. Upon hydroxylation, the solid interface becomes preferentially coated by methanol, leading to a bistable solvation state of the probe, with alternation of "wall-like" and "bulk-like" events. An increment in the interface roughness promotes a solvation structure characterized by the embedding of the probe within a wall domain surrounded by trimethylsilyl groups. In hydrophobic environments, the relevant dynamical modes of the probe can be cast in terms of in-the-wall rotations, whereas in hydrophilic pores, out-of-the-wall evolutions are also present. The embedding of the probe at wall domains in more rugged pores, leads to restrained angular motions, with maximum amplitudes of the order of 20°. Results of early stages of the solvation response of the environment following a vertical excitation of the probe are also presented. During the initial 30 ps, we found no evidence of modifications in the spatial localizations of the probe. The overall responses are found to be between 2 and 4.5 times slower than the one observed in the bulk, being the fastest relaxation the one associated to rugged pores whereas the slowest one corresponds to hydrophilic cavities. These features are rationalized in terms of the composition of the first solvation shells and the local dynamical inhomogeneities prevailing within the different regions of the pores.

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.

We present a path-integral-molecular-dynamics study of the thermodynamic stabilities of DOH⋯ X- and HOD⋯ X- (X = F, Cl, Br, I) coordination in aqueous solutions at ambient conditions. In agreement with experimental evidence, our results for the F- case reveal a clear stabilization of the latter motif, whereas, in the rest of the halogen series, the former articulation prevails. The DOH⋯ X- preference becomes more marked the larger the size of the ionic solute. A physical interpretation of these tendencies is provided in terms of an analysis of the global quantum kinetic energies of the light atoms and their geometrical decomposition. The stabilization of the alternative ionic coordination geometries is the result of a delicate balance arising from quantum spatial dispersions along parallel and perpendicular directions with respect to the relevant O-H⋯X- axis, as the strength of the water-halide H-bond varies. This interpretation is corroborated by a complementary analysis performed on the different spectroscopic signals of the corresponding IR spectra. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Rossky, Peter J.. Rice University; Estados Unidos Fil: Laria, Daniel Hector. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina

We present results from nonequilibrium molecular dynamics experiments describing the relaxation of local concentrations at two reservoirs, initially filled with water (W) and acetonitrile (ACN), as they become connected through a membrane composed of (16,16) carbon nanotubes. Within the hydrophobic nanotube cavities, the equilibrium concentrations contrast sharply to those observed at the reservoirs, with a clear enhancement of ACN, in detriment of W. From the dynamical side, the relaxation involves three well-differentiated stages; the first one corresponds to the equilibration of individual concentrations within the nanotubes. An intermediate interval with Fickian characteristics follows, during which the overall transport can be cast in terms of coaxial opposite fluxes, with a central water domain segregated from an external ACN shell, in close contact with the tube walls. We also found evidence of a third, much slower, mechanism to reach equilibration, which involves structural modifications of tightly bound solvation shells, in close contact with the nanotube rims. Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Universidad Nacional de San Martín; Argentina Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Universidad Nacional de San Martín; Argentina Fil: Laria, Daniel Hector. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina

Using molecular dynamics experiments, we analyze the association of "Janus" 6-amino-6-deoxy-2O-carboxymethyl-beta-cyclodextrins (JCD) in aqueous solutions. In JCD dimers, the free energy associated with the primary-rim-secondary-rim docking shows a stable minimum of approximately -45 kcal mol(-1). Trimers in solution are also remarkably stable, exhibiting minimal distortions in their spatial and orientational distributions. The resulting geometrical docking shows the incipient characteristics of flexible nanotubes in solution, with eventual water interchange between the central channel and the bulk at the junctions between monomers. Structural and dynamical properties of the trapped water filling the nanotube are dictated to a large extent by the charge density at the rims.

Using molecular dynamics techniques, we examined equilibrium and dynamical characteristics pertaining to the solvation of a single β-cyclodextrin (CD) in water and in dimethylsulfoxide (DMSO). Compared to its global minimum structure, the overall shape of the solute in solution is reasonably well preserved. While in aqueous solutions, the average number of solvent molecules retained within the central cavity of the oligosaccharide is close to 5, for DMSO, that number reduces to ∼1. No evidence of significant orientational correlations of the trapped molecules were found in either solvent. The main contributions to the hydrogen-bond (HB) connectivity between the solute and the bulk phases are due to the more distal HO6-O6 hydroxyl groups, acting as HB donors and acceptors. The average residence time for retained DMSO was found to be in the nanosecond range, and it is, at least, 1 order of magnitude longer that the one observed for water. We also analyzed the characteristics of the solvation of the β-CD in an equimolar water-DMSO mixture. In this environment, we found a preferential localization of a single DMSO molecule in the interior of the CD and a very minor retention of water. In the mixture, the characteristic time of residence of the trapped DMSO molecule increases by a factor of ∼2. The observed difference was rationalized in terms of the fluctuations of the local concentrations of the two species in the vicinity of the CD top and bottom rims. Fil: Rodriguez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Rico, Daniel Hernán. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Domenianni, Luis Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

We present results of ring polymer molecular dynamics simulations that shed light on the effects of nuclear quantum fluctuations on tunneling motions in cyclic [H2O]3 and [D2O]3, at the representative temperature of T = 75 K. In particular, we focus attention on free energies associated with two key isomerization processes: The first one corresponds to flipping transitions of dangling OH bonds, between up and down positions with respect to the O-O-O plane of the cluster; the second involves the interchange between connecting and dangling hydrogen bond character of the H-atoms in a tagged water molecule. Zero point energy and tunneling effects lead to sensible reductions of the free energy barriers. Due to the lighter nature of the H nuclei, these modifications are more marked in [H2O]3 than in [D2O]3. Estimates of the characteristic time scales describing the flipping transitions are consistent with those predicted based on standard transition-state-approximation arguments. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina Fil: Rossky, Peter J.. Rice University; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Comisión Nacional de Energía Atómica; Argentina

Using molecular dynamics techniques, we investigate surface states of the surfactant N-dodecylimidazole (DIm) in its basic and acid forms adsorbed at the water/air interface. Two different surface coverages were examined: an infinitely diluted detergent and a saturated monolayer. Spatial and orientational correlations of the surfactants and the aqueous substrate are presented. At large surface coverages, Dim presents two solvation states with well differentiated structural and dynamical characteristics. Solvation of the protonated surfactant becomes unstable at large concentrations, while the relative stability of the surface states of N-dodecylimidazolium (DImH + ) with respect to bulk states increases at infinite dilution. The surface acidic behavior of DImH + was investigated using a multistate empirical valence bond Hamiltonian model. Our simulation results suggest that the acidic characteristics of DIm are enhanced at the surface. The differences are rationalized in terms of the distinctive features in the overall solvation structure of the reactive complex.

We present results of a computer simulation study on the reorientational dynamics of water molecules in aqueous ionic solutions at supercritical conditions. We carried out molecular dynamics simulations along the isotherm T = 650 K and densities ranging from ρ = 0.7 g cm − 3 down to ρ = 0.05 g cm − 3 . We adopted the SPC/E water model. The ionic species included Na + and Cl − ions. Our attention has been focused on the reorientational dynamics of the solvation shell molecules. Reorientational correlation times are reported and compared with those corresponding to pure water at the same thermodynamic conditions. We find that the influence of the ions goes beyond the first solvation shell and significantly affects the motion of the molecules lying in the second solvation shell. This differs with earlier studies of aqueous ionic solutions at room temperature.

Hydrogen bond (HB) connectivity in aqueous electrolyte solutions at ambient and supercritical conditions has been investigated by molecular dynamics techniques. Alkali metal and halides with different sizes have been considered. Modifications in the water HB architecture are more noticeable in the first ionic solvation shells and do not persist beyond the second shells. The coordination pattern is established between partners located in the first and second solvation shells. High-temperature results show dramatic reductions in the coordination number of water; at liquidlike densities the number of HBs is close to 2, while in steamlike environments water monomers are predominant. The addition of ions does not bring important modifications in the original HB structure for pure water. From the dynamical side, the lifetime of HBs shows minor modifications due to the simultaneous competing effects from a weaker HB structure combined with a slower reorientational dynamics of water induced by the Coulomb coupling with solute. At supercritical conditions, the overall dynamics of HB is roughly 1 order of magnitude faster than that at ambient conditions, regardless of the particular density considered.

Through the use of molecular dynamics techniques, we analyze equilibrium and dynamical aspects of the solvation of Coumarin 314 adsorbed at water/air interfaces in the presence of sodium dodecyl sulfate surfactant molecules. Three different coverages in the submonolayer regime were considered, 500, 250, and 100 A(2)/SDS molecule. The surfactant promotes two well-differentiated solvation environments, which can be clearly distinguished in terms of their structures for the largest surfactant coverage considered. The first one is characterized by the probe lying adjacent or exterior to two-dimensional spatial domains formed by clustered surfactant molecules. A second type of solvation environment is found in which the coumarin appears embedded within compact surfactant domains. Equilibrium and dynamical aspects of the interfacial orientation of the probe are investigated. Our results show a gradual transition from parallel to perpendicular dipolar alignment of the probe with respect to the interface as the concentration of surfactant rho(s) increases. The presence of the surfactant leads to an increase in the roughness and in the characteristic width of the water/air interface. These modifications are also manifested by the decorrelation times for the probe reorientational dynamics, which become progressively slower with rho(s) in both solvation states, although much more pronounced for the embedded ones. The dynamical characteristics of the solvation responses of the charged interfaces are also analyzed, and the implications of our findings to the interpretation of available experimental measurements are discussed.

A series of molecular dynamics (MD) simulations have been carried out to study hydrogen bonding in aqueous ionic solutions at ambient conditions. Alkali metal and halides with different sizes have been considered. A separate study of the hydrogen bond (HB) populations in the first and second hydration shells has been performed and characteristic lifetimes of the hydrogen bonds have been determined in each case. The influence of ion size and charge is discussed.

Molecular dynamics simulations have been performed to study equilibrium and dynamical aspects of solvation of excess electrons in supercritical ammonia along the T=450 K isotherm. The interval of supercritical densities investigated spans from typically dense liquid down to dilute vapor ambients. Equilibrium aspects of solvation were analyzed using combined path integral-molecular dynamics techniques. The transition from localized to quasifree states, described in terms of the isomorphic electron-polymer spatial extent, was observed at approximately one fourth of the triple point density, a value somewhat higher than that recently reported for supercritical water [D. Laria and M. Skaf, J. Phys. Chem. A 106, 8066 (2002)]. The density of electronic eigenstates shows typically one s- and three p-like bound states that gradually lose their symmetry characteristics as the density lowers. The computed ground state absorption spectrum exhibits redshifts in the absorption bands as the density decreases; these shifts are much larger than those reported by pulse radiolysis experiments. By performing adiabatic dynamics, we also investigate mechanisms for solvent relaxation at high and intermediate supercritical densities following a vertical excitation of the electron.

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 ...

Microscopic aspects of electron solvation in aqueous reverse micelles are investigated using molecular dynamics simulation techniques. Two micelle sizes, with water/surfactant ratios of 3 and 7.5, are examined. The electron is treated quantum mechanically using Feynman path integral methods while the water, surfactant head groups, and counter ions are treated classically. Through computations of the free energy as a function of the radial distance, the electron is found to be preferentially solvated in the interior of the micelle in the “bulk” water pool. For small micelles, the presence of the electron leads to a depletion of water in the central region of the micelle and thus strongly disrupts the water equilibrium structure. Contact and solvent-separated ion pairs between the electron and Na+ counter ions are found to play an important role in the equilibrium structure. For the two micelle sizes investigated, the most stable solvation structures correspond to contact ion pairs. The localization of the ...

We have carried out path integral-molecular dynamics simulations to describe microscopic details of excess electrons in supercritical water over a wide range of solvent densities, ρ w , along the T = 645 K isotherm. The well-tested simple-point charge model for water was used. The transition from localized to quasifree states described in terms of the electron spatial extent is observed in the vicinity of ρ w = 0.15 g cm - 3 . For smaller densities, the electron undergoes quantum tunneling through nearest neighboring water molecules. The ground-state absorption spectrum exhibits significant red shifts in the absorption maxima with decreasing density, showing reasonable agreement with recent pulse radiolysis measurements.

Equilibrium and dynamical aspects of the earliest stages of solvation following the electronic excitation of I− dissolved in aqueous clusters were investigated, using a mixed quantum-classical molecular dynamics algorithm. The systems were modeled as composed by an excess quantum charge coupled to a classical bath that includes a neutral iodine and Nw=6, 50, and 100 water molecules. In small clusters, the equilibrium solvation structures of the iodine are characterized by surface states that gradually turn into interior states as one considers larger aggregates. Electronic properties of the ground and first excited states are described. In small aggregates, both states are characterized by highly diffuse density profiles that extend well beyond the spatial extent of the aggregates. In larger clusters, the confining potential provided by the classical bath localizes the electron within the aggregates. Dynamical aspects of the solvation relaxation following a vertical excitation of the solute were also exam...

Dynamical pathways for the interconversion between two stable (H 2 O) 9 derived from the D 2d octamer have been investigated using the transition path sampling formalism. Instantaneous normal mode analysis reveals that the dynamics over the ensemble of transition states is characterized by the concerted translation of three molecules. At a total energy E =−76 kcal/mol, the characteristic time for the interconversion is of the order of 0.5 ms. This estimate compares well with predictions from classical RRKM theory.

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. Fil: Semino, Rocio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Zaldivar, Gervasio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Calvo, Ernesto Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comisión Nacional de Energía Atómica; Argentina

Using molecular dynamics techniques, we examine structural and dynamical characteristics of liquid-like, Imidazole(Im) monolayers physisorbed onto a planar graphite sheet, at T = 384K. Our simulations reveal that molecular orientations in the saturated monolayer exhibit a bistable distribution, characterized by an inner parallel arrangement of the molecules in close contact with the substrate and a slanted alignment, in those lying in adjacent, outer locations. Compared to the results found in three dimensional, bulk phases, the analysis of the spatial correlations between sites participating in hydrogen bonding shows a clear enhancement of the intermolecular interactions, which also leads to stronger dipolar correlations. As a result, the gross structural features of the monolayer can be cast in terms of mesoscopic domains, comprising units articulated via winding hydrogen bonds, that persist along typical time intervals of a few tens of picoseconds. On the dynamical side, a similar comparison of the characteristic decorrelation time for orientational motions shows a 4-fold increment. Contrasting, the reduction of the system dimensionality leads to a larger diffusion constant. Possible substrate-induced anisotropies in the diffusive motions are also investigated. Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Núcleo de Investigación en Educacion Ciencia y Tecnologia; Argentina Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica; Argentina Fil: Laria, Daniel Hector. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina

Molecular dynamics experiments have been performed to study equilibrium aspects of the solvation of Coumarin-151 in polar nanoclusters containing N s = 5, 10, and 50 water and methanol molecules. In small aggregates, bothsolvents show preferential solvation of the amino group of the solute. Whereas in aqueous aggregates the dye molecule shows a propensity to reside on the cluster surface for all sizes investigated, the 50 methanol cluster exhibits clear signs of more uniform, bulklike solvation. Using nonequilibrium simulations, we also studied the solvation dynamics upon an electronic excitation of the probe. At temperatures close to T = 200 K, small clusters exhibit two well differentiated equilibrium solvation structures for the excited states of the solute. Interconversions between these structures take place in a time scale much longer than the one characterizing the solvation relaxation. In N s = 5 clusters, the nonequilibrium responses are exclusively dominated by a fast inertial relaxation lasting less that 0.1 ps; for N, = 50, the relaxations exhibit a slow diffusional regime that dominates the long time behavior as well. The overall response of the largest clusters is also analyzed in terms of linear response theories.

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.

Hybrid quantum-classical molecular-dynamics simulations were performed for the dissociation process of HNO 3 in aqueous clusters containing up to n =124 water molecules. By circumscribing the HNO 3 and one water molecule within the quantum region, no dissociation was observed, even for the largest cluster considered, in which the process is expected to be spontaneous. Incorporation of two additional water molecules into the quantum sub-system was necessary to observe the proton transfer. Ionic dissociation takes place for n =124 and n =10, but not for the n =3 case, confirming that the threshold cluster size for dissociation is intermediate between the last two values.

The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the trans...

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 rotational-diffusion regime which persists for times much longer than the time scales typically found in other polar liquids and mixtures. This negative portion is nearly cancelled by an equally long-lasting positive contribution from water, resulting in a fast-decaying, total response curve which is typical of many polar liquid environments. This behavior is rationalized in terms of the time evolution of the first solvation shell around each type of solute.

A hybrid quantum classical computational algorithm, which couples a density functional Hamiltonian to a classical bath, is applied to investigate the proton-transfer reaction OH- + HBr → H2O + Br- in aqueous clusters. The reagent was modeled using density functional theory with a Gaussian basis set; two different force fields for the classical bath were investigated: the TIP4P-FQ fluctuating charge and the TIP4P mean field potentials. Basis sets, functionals, and force field parameters have been validated by performing calculations on [HO-](H2O), [Br-](H2O), [HBr](H2O), and [H2O](H2O) isolated dimers at 0 K. Molecular dynamics simulations of the system [HOHBr]-(H2O)n, with n = 2 and 6, show that the reaction is spontaneous and rather exothermic, leading to the full detachment of the bromide ion from the halide and the generation of a water molecule within a few femtoseconds. In addition, our experiments show that the process involves a fast damping of the potential energy concomitant with a sudden increa...

Structural and dynamical properties of liquid-state, binary, water-acetonitrile nanoclusters are studied. When acetonitrile is the solute species, it exhibits a propensity to reside on the cluster surface, although one may identify regions interior to the cluster that are favorable for solvation. The dynamics of the interchange of acetonitrile between surface and interior solvation regions and structural aspects of surface solvation states are studied. When water is the solute it tends to be solvated in the interior of the cluster and form aggregates. The nature and dynamics of contact and solvent separated water pairs in these clusters is investigated, and the recombination dynamics of larger water aggregates in concentrated water-acetonitrile clusters is examined.

Dynamical aspects of the dielectric response of supercritical water following an instantaneous charge jump on an initially neutral Lennard-Jones solute are investigated using molecular dynamics. The SPC model was used to describe solvent−solvent interactions. The simulation experiments were performed over the density interval spanning from 0.3 up to 1 g cm-3 along the 645 K isotherm. Compared to room temperature results, the overall solvation process at high densities is an order of magnitude faster and becomes progressively slower as we move toward lower densities. In all cases, the nonequilibrium solvent responses present a bimodal behavior characterized by a fast inertial regime lasting a few femtoseconds followed by a much slower diffusional regime that dominates the long time behavior. This last portion of the response, which contributes to a small extent at high densities, accounts for the major contribution at lower densities. Predictions from linear response theory are quite accurate at high densi...

Monte Carlo simulations using a hybrid quantum and classical mechanical potential were performed for crystal and amorphous-like HCl-water(n) clusters The subsystem composed by HCl and one water molecule was treated within Density Functional Theory, and a classical force field was used for the rest of the system. Simulations performed at 200 K suggest that the energetic feasibility of HCl dissociation strongly depends on its initial placement within the cluster. An important degree of ionization occurs only if HCl is incorporated into the surface. We observe that local melting does not play a crucial role in the ionization process., Comment: 14 Latex pages with 4 postscript figures, to appear in Chem. Phys. Lett

Reactions in mesoscopic, molecular clusters may proceed by mechanisms and with rates that differ from those in bulk solvents. Two examples of reactions in large, liquid-state, molecular clusters are described to illustrate the distinctive features of these reactions: acid dissociation and proton transfer in aprotic, polar solvents. Both of these reactions involve proton dynamics so methods for dealing with mixed quantum–classical systems must be utilized to investigate the reaction dynamics. Surface versus bulk solvation effects play an important role in determining the reaction mechanisms as do the strong cluster fluctuations. Mechanisms for proton transfer within clusters that have no bulk analogs will be described. Keywords: proton reactions, mesoscopic clusters.

Ring polymer molecular dynamics experiments have been carried out to examine effects derived from nuclear quantum fluctuations at ambient conditions on equilibrium and non-equilibrium dynamical characteristics of charge solvation by a popular simple, rigid, water model, SPC/E , and for a more recent, and flexible, q-TIP4P/F model, to examine the generality of conclusions. In particular, we have recorded the relaxation of the solvent energy gap following instantaneous, ±e charge jumps in an initially uncharged Lennard-Jones-like solute. In both charge cases, quantum effects are reflected in sharper decays at the initial stages of the relaxation, which produce up to a ∼20% reduction in the characteristic timescales describing the solvation processes. For anionic solvation, the magnitude of polarization fluctuations controlling the extent of the water proton localization in the first solvation shell is somewhat more marked than for cations, bringing the quantum solvation process closer to the classical case. Effects on the solvation response from the explicit incorporation of flexibility in the water Hamiltonian are also examined. Predictions from linear response theories for the overall relaxation profile and for the corresponding characteristic timescales are reasonably accurate for the solvation of cations, whereas we find that they are much less satisfactory for the anionic case. Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; Argentina Fil: Rossky, Peter J.. University of Texas at Austin; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; Argentina. Comision Nacional de Energia Atomica. Gerencia Quimica. CAC; Argentina

We use ring-polymer-molecular-dynamics (RPMD) techniques and the semi-empirical q-TIP4P/F water model to investigate the relationship between hydrogen bond connectivity and the charac- teristics of nuclear position fluctuations, including explicit incorporation of quantum effects, for the energetically low lying isomers of the prototype cluster [H2O]8 at T = 50 K and at 150 K. Our results reveal that tunneling and zero-point energy effects lead to sensible increments in the magnitudes of the fluctuations of intra and intermolecular distances. The degree of proton spatial delocalization is found to map logically with the hydrogen-bond connectivity pattern of the cluster. Dangling hydro- gen bonds exhibit the largest extent of spatial delocalization and participate in shorter intramolecular O-H bonds. Combined effects from quantum and polarization fluctuations on the resulting individ- ual dipole moments are also examined. From the dynamical side, we analyze the characteristics of the infrared absorption spectrum. The incorporation of nuclear quantum fluctuations promotes red shifts and sensible broadening relative to the classical profile, bringing the simulation results in much more satisfactory agreement with direct experimental information in the mid and high fre- quency range of the stretching band. While RPMD predictions overestimate the peak position of the low frequency shoulder, the overall agreement with that reported using an accurate, parame- terized, many-body potential is reasonable, and far superior to that one obtains by implementing a partially adiabatic centroid molecular dynamics approach. Quantum effects on the collective dynam- ics, as reported by instantaneous normal modes, are also discussed Fil: Videla, Pablo Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; Argentina Fil: Rossky, Peter J.. University of Texas at Austin; Estados Unidos Fil: Laria, Daniel Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Comision Nacional de Energia Atomica. Gerencia Quimica. CAC; Argentina

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...

In view of recent experimental and simulation studies which show that solvent reorientation does not play a significant role in the dissolution of small apolar solutes in water, the performance of first order perturbation theories (FOPT) has been reassessed. A contribution which cannot be obtained from classical FOPT is the many-body effect of the instantaneous solvent electric field which induces a dipole on the apolar solute and contributes an attractive term. The average square electric field was determined by molecular dynamics for solutes of different size and its effect upon the solute's chemical potential was calculated from it with an appropriate extension of the test-particle method. The contribution of this term to the thermodynamic quantities of dissolution is discussed.

We have examined structural features of aqueous microenvironments in the neighborhood of an apolar solute (Xe atom) using molecular dynamics techniques and the simple point charge model to represent the solvent. A detailed comparative study of equilibrium spatial correlations of the solvent structures in bulk and in the neighborhood of the Xe atom was performed. We have also obtained structural information for water surrounding a cavity spontaneously generated in liquid water. Our results indicate the absence of any significant solvent reorientation following the introduction of an apolar solute in a cavity existing in the bulk liquid; this agrees with results from recent neutron scattering experiments.

Molecular-dynamics simulations have been performed for ionic aqueous aggregates in the vapour phase using the simple point-charge model to represent H2O–H2O interactions. To obtain equilibrium configurations efficiently by molecular-dynamics simulation, model water molecules from bulk steam were allowed to collide against the surface of the simulation sphere at the average collision rate. In this way we have obtained, for the first time, information about the configuration of ionic aggregates in model steam for two thermodynamic state points. Water molecules surrounding the ionic aggregates show liquid-like density confirming the existence of strong microheterogeneity in steam when it contains ions. The results show that the detailed distribution of H2O in the aggregates must be known for the formulation of plausible models for ions in steam. Evidence indicates that in steam, charged aggregates are relatively stable compared with neutral ones by their greater capacity to attract water molecules towards them.