Quasi-Chemical Theory for the Statistical Thermodynamics of the Hard-Sphere Fluid

Pratt, Lawrence R.; LaViolette, Randall A.; Gomez, Maria A.; Gentile, Mary E.

doi:10.1021/jp011525w

Cited by 48 publications

(92 citation statements)

References 42 publications

(143 reference statements)

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“…Structural and thermodynamic issues are connected, of course, and particularly directly by the quasichemical theory (9)(10)(11)(12)(13)(14)(15)(16)(17). A primitive quasichemical approximation has been developed for the hydration free energy of H ϩ (aq) (4, 7).…”

mentioning

confidence: 99%

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Asthagiri

Pratt

Kress

2005

Proc. Natl. Acad. Sci. U.S.A.

100

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The excess proton in water, H ؉ (aq), plays a fundamental role in aqueous solution chemistry. Its solution thermodynamic properties are essential to molecular descriptions of that chemistry and for validation of dynamical calculations. Within the quasichemical theory of solutions those thermodynamic properties are conditional on recognizing underlying solution structures. The quasichemical treatment identifies H 3O ؉ and H2O5 ؉ as natural innershell complexes, corresponding to the cases of n ‫؍‬ 1, 2 water molecule ligands, respectively, of a distinguished H ؉ ion. A quantum-mechanical treatment of the inner-shell complex with both a dielectric continuum and a classical molecular dynamics treatment of the outer-shell contribution identifies the latter case (the Zundel complex) as the more numerous species. Ab initio molecular dynamics simulations, with two different electron density functionals, suggest a preponderance of Zundel-like structures, but a symmetrical ideal Zundel cation is not observed.Eigen cation ͉ Zundel cation T he hydrated proton, H ϩ (aq), plays a fundamental role in aqueous phase chemistry. Here, we address two related issues of the molecular theory of H ϩ (aq) that haven't been resolved: the structural categorization of the local environment of H ϩ (aq) and hydration free energy of this ion under standard conditions.The difficulty of treating H ϩ (aq) on the basis of molecular theory is principally that the interactions are fundamentally chemical, and therefore complicated when viewed on a thermal energy scale. This is also true of other aqueous ions (1, 2), but H ϩ (aq) is an extreme example. In addressing this complexity, computations of thermodynamic properties of these solutions and comparisons with experimental results give the clearest assessment of theory and simulation work (3-8).Structural and thermodynamic issues are connected, of course, and particularly directly by the quasichemical theory (9-17). A primitive quasichemical approximation has been developed for the hydration free energy of H ϩ (aq) (4, 7). The conclusion of that work was that the H-centered Zundel complex ion H 5 O 2 ϩ provided a simple and valid structural description of the local environment of H ϩ (aq); that theory identifies the Eigen complex ion H 9 O 4 ϩ (i.e., hydrated oxonium) as a structural specification of outer-shell hydration of a distinguished H ϩ , and therefore assigns the Eigen cation less significance.A distinguishing feature of the Zundel complex is an OOO distance of 2.4 Å, shorter than OOO distances between water molecules, or within the hydrated oxonium ion. The latter case (the Eigen structure) might be taken as an O-center basis of a quasichemical model of the solution thermodynamics. But a subsequent observation obtained from ab initio molecular dynamics (AIMD) simulation of HO Ϫ (aq) then becomes relevant (1). AIMD results showed that the nearest coordinating O atom to a HO Ϫ oxygen has the 2.45-Å displacement distinctive of the H 3 O 2 Ϫ complex. This separation is analogous to the case o...

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mentioning

confidence: 99%

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Asthagiri

Pratt

Kress

2005

Proc. Natl. Acad. Sci. U.S.A.

100

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“…As was discussed previously [8] that a hit-or-miss Monte Carlo [18,37] approach to calculate P (n) proves inaccurate for large values of n (n > 8). But since…”

Section: Spherical Codementioning

confidence: 92%

“…Further, for systems with large size asymmetries even the pair-correlation information obtained using the Boublik-Mansoori-Carnahan-Starling-Leland equation is inadequate [16]. Our approach of including multi-body correlations rests on using the occupancy statistics [7,[17][18][19] of the hard-sphere solvent around the hard-sphere solute. We find that representing multi-body correlation functions in terms of occupancy statistics in physically reasonable observation volumes accurately captures the multi-bonding effects in asymmetric mixtures.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics of mixtures of patchy and spherical colloids of different sizes: A multi-body association theory with complete reference fluid information

Bansal

Parambathu

Asthagiri

et al. 2017

The Journal of Chemical Physics

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We present a theory to predict the structure and thermodynamics of mixtures of colloids of different diameters, building on our earlier work [J. Chem. Phys. 145, 074904 (2016)] that considered mixtures with all particles constrained to have the same size. The patchy, solvent particles have short-range directional interactions, while the solute particles have short-range isotropic interactions. The hard-sphere mixture without any association site forms the reference fluid. An important ingredient within the multi-body association theory is the description of clustering of the reference solvent around the reference solute. Here we account for the physical, multi-body clusters of the reference solvent around the reference solute in terms of occupancy statistics in a defined observation volume. These occupancy probabilities are obtained from enhanced sampling simulations, but we also present statistical mechanical models to estimate these probabilities with limited simulation data. Relative to an approach that describes only up to three-body correlations in the reference, incorporating the complete reference information better predicts the bonding state and thermodynamics of the physical solute for a wide range of system conditions. Importantly, analysis of the residual chemical potential of the infinitely dilute solute from molecular simulation and theory shows that whereas the chemical potential is somewhat insensitive to the description of the structure of the reference fluid the energetic and entropic contributions are not, with the results from the complete reference approach being in better agreement with particle simulations.

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“…K n is given by the ratio of partition functions of the cluster to individual molecules and can be challenging to calculate with full account of the surrounding medium 22,23,42,[44][45][46] . But the equilibrium constant K (0) n of the clustering reaction…”

Section: Theory a Quasichemical Theorymentioning

confidence: 99%

Ion-water clusters, bulk medium effects, and ion hydration

Merchant

Dixit

Dean

et al. 2011

The Journal of Chemical Physics

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Thermochemistry of gas-phase ion-water clusters together with estimates of the hydration free energy of the clusters and the water ligands are used to calculate the hydration free energy of the ion. Often the hydration calculations use a continuum model of the solvent. The primitive quasichemical approximation to the quasichemical theory provides a transparent framework to anchor such efforts. Here we evaluate the approximations inherent in the primitive quasichemical approach and elucidate the different roles of the bulk medium. We find that the bulk medium can stabilize configurations of the cluster that are usually not observed in the gas phase, while also simultaneously lowering the excess chemical potential of the ion. This effect is more pronounced for soft ions. Since the coordination number that minimizes the excess chemical potential of the ion is identified as the optimal or most probable coordination number, for such soft ions, the optimum cluster size and the hydration thermodynamics obtained without account of the bulk medium on the ion-water clustering reaction can be different from those observed in simulations of the aqueous ion. The ideas presented in this work are expected to be relevant to experimental studies that translate thermochemistry of ion-water clusters to the thermodynamics of the hydrated ion and to evolving theoretical approaches that combine high-level calculations on clusters with coarse-grained models of the medium.

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Quasi-Chemical Theory for the Statistical Thermodynamics of the Hard-Sphere Fluid

Cited by 48 publications

References 42 publications

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Thermodynamics of mixtures of patchy and spherical colloids of different sizes: A multi-body association theory with complete reference fluid information

Ion-water clusters, bulk medium effects, and ion hydration

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Quasi-Chemical Theory for the Statistical Thermodynamics of the Hard-Sphere Fluid

Cited by 48 publications

References 42 publications

Ab initio molecular dynamics and quasichemical study of H + (aq)

Ab initio molecular dynamics and quasichemical study of H + (aq)

Thermodynamics of mixtures of patchy and spherical colloids of different sizes: A multi-body association theory with complete reference fluid information

Ion-water clusters, bulk medium effects, and ion hydration

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Product

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Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)