VBSM: a solvation model based on valence bond theory

A new solvation model, called VBSM, is presented. The model combines valence bond (VB) theory with parameters determined for the SM6 solvation model (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theo. Comp. 2005, 1, 1133-1152). VBSM, like SM6, is based on the generalized Born (GB) approximation for bulk electrostatics and atomic surface tensions to account for cavitation, dispersion, and solvent structure (CDS). The solvation free energy of VBSM includes (i) a self-consistent polarization term obtained by using VB atomic charges in a GB reaction field with a VB self-consistent field procedure that minimizes the total energy of the system with respect to the valence bond orbitals and (ii) a geometry-dependent CDS term to account for deviations from bulk-electrostatic solvation. Test calculations for a few systems show that the liquid-phase partial atomic charges obtained by VBSM are in good agreement with liquid-phase charges obtained by charge model CM4 (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theo. Comp. 2005, 1, 1133-1152). Free energies of solvation are calculated for two prototype test cases, namely, for the degenerate S(N)2 reaction of Cl(-) with CH(3)Cl in water and for a Menshutkin reaction in water. These calculations show that the VBSM method provides a practical alternative to single-configuration self-consistent field theory for solvent effects in molecules and chemical reactions.     

Predicting aqueous free energies of solvation as functions of temperature

This work introduces a model, solvation model 6 with temperature dependence (SM6T), to predict the temperature dependence of aqueous free energies of solvation for compounds containing H, C, and O in the range 273-373 K. In particular, we extend solvation model 6 (SM6), which was previously developed (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theory Comput. 2005, 1, 1133) for predicting aqueous free energies of solvation at 298 K, to predict the variation of the free energy of solvation relative to 298 K. Also, we describe the database of experimental aqueous free energies of solvation for compounds containing H, C, and O that was used to parametrize and test the new model. SM6T partitions the temperature dependence of the free energy of solvation into two components: the temperature dependence of the bulk electrostatic contribution to the free energy of solvation, which is computed using the generalized Born equation, and the temperature dependence of first-solvation-shell effects which is modeled using a parametrized solvent-exposed surface-area-dependent term. We found that SM6T predicts the temperature dependence of aqueous free energies of solvation with a mean unsigned error of 0.08 kcal/mol over our entire database, whereas using the experimental value at 298 K produces a mean unsigned error of 0.53 kcal/mol.     

Aqueous solvation free energies of ions and ion-water clusters based on an accurate value for the absolute aqueous solvation free energy of the proton

Thermochemical cycles that involve pKa, gas-phase acidities, aqueous solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approximation to determine the absolute aqueous solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V. J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approximation to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the absolute aqueous solvation free energy of the proton. Considering the uncertainties associated with the experimental gas-phase free energies of ions that are required to use the cluster pair approximation as well as analyses of various subsets of data, we estimate an uncertainty for the absolute aqueous solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the absolute aqueous solvation free energy of the proton, we expand and update our previous compilation of absolute aqueous solvation free energies; this new data set contains conventional and absolute aqueous solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and absolute aqueous solvation free energies for 51 clustered ions containing from 1 to 6 water molecules. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously determined high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate reference data.     

New analytic approximation to the standard molecular volume definition and its application to generalized Born calculations

In a recent article (Lee, M. S.; Salsbury, F. R. Jr.; Brooks, C. L., III. J Chem Phys 2002, 116, 10606), we demonstrated that generalized Born (GB) theory provides a good approximation to Poisson electrostatic solvation energy calculations if one uses the same definitions of molecular volume for each. In this work, we present a new and improved analytic method for reproducing the Lee-Richards molecular volume, which is the most common volume definition for Poisson calculations. Overall, 1% errors are achieved for absolute solvation energies of a large set of proteins and relative solvation energies of protein conformations. We also introduce an accurate SASA approximation that uses the same machinery employed by our GB method and requires a small addition of computational cost. The combined methodology is shown to yield an efficient and accurate implicit solvent representation for simulations of biopolymers.     

Design of surface active soluble peptide molecules at the air/water interface

Surface active molecules collect at interfaces and have the potential to be used for water evaporation reduction. The objective of this work is to design surface active soluble peptides that collect at the air/water interface using molecular simulations. Rotational isomeric state Monte Carlo (RISMC) sampling together with a solvation model that we recently invented, the AAD solvation model [Gu, C.; Lustig, S.; Trout, B. J. Phys. Chem. B 2006, 110 (3), 1476-1484] was applied to calculate the adsorption free energy of the peptide molecule at the air/water interface. The results were validated by both molecular dynamics simulations with an explicit solvent model and surface tension measurements on synthesized peptides. It was demonstrated that this approach is able to give a reasonable prediction of surface activity with an approximately 50% hit rate in terms of designed surface active molecules actually being surface active. The relationship between the chemical composition and the surface morphology is also discussed.     

Optimizing the performance of the multiconfiguration molecular mechanics method

Multiconfiguration molecular mechanics (MCMM) is a general algorithm for constructing potential energy surfaces for reactive systems (Kim, Y.; Corchado, J. C.; Villà, J.; Xing, J.; Truhlar, D. G. J. Chem. Phys. 2000, 112, 2718). This paper illustrates how the performance of the MCMM method can be improved by adopting improved molecular mechanics parameters. We carry out calculations of reaction rate constants using variational transition state theory with optimized multidimensional tunneling on the MCMM PESs for three hydrogen transfer reactions, and we compare the results to direct dynamics. We find that the MCMM method with as little as one electronic structure Hessian can describe the dynamically important regions of the ground-electronic state PES, including the corner-cutting-tunneling region of the reaction swath, with practical accuracy.     

Existence of a size-dependent diffusivity maximum for uncharged solutes in water and its implications

Recent studies suggest that there exists a size-dependent diffusivity maximum in binary mixtures interacting via Lennard-Jones potential when the size of one of the two components is varied (Ghorai, P. K.; Yashonath, S. J. Phys. Chem., 2005, 109, 5824). We discuss in the present paper the importance of the existence of a size-dependent maximum for an uncharged solute in liquid or amorphous solid water and its relation to the ionic conductivity maximum in water. We report molecular dynamics investigations into the size dependence of the self-diffusivity, D, of the uncharged solutes in water at low temperatures (30 K) with immobile as well as mobile water. We find that a maximum in self-diffusivity exists as a function of the size of solute diffusing within water at low temperatures but not at high temperatures. This is due to the relatively weak interactions between the solute and the water compared to the kinetic energy at room temperature. Previously, we have shown that a similar maximum exists for guests sorbed in zeolites and is known as the levitation effect (LE). Thus, it appears that the existence of a size-dependent maximum is universal and extends from zeolites to simple liquids to solvents of polyatomic species. We examine the implications of this for the size-dependent maximum in ionic conductivity in polar solvents known for over a hundred years. These results support the view that the size-dependent maximum seen for ions in water has its origin in the LE (see Ghorai, P. Kr.; Yashonath, S.; Lynden-Bell, R. M. J. Phys. Chem. 2005, 109, 8120).     

Ionic Solvation in Aqueous and Nonaqueous Solutions

Summary.  The history of studies on ionic solvation is briefly reviewed, and structural and dynamic properties of solvated ions in aqueous and nonaqueous solutions are discussed. An emphasis is placed on ionic solvation in nonaqueous mixed solvents in which preferential solvation of ions takes place. A new parameter for expressing the degree of preferential solvation of an ion is proposed. Received January 16, 2001. Accepted January 31, 2001     

Dipolar solvation dynamics in room temperature ionic liquids: an effective medium calculation using dielectric relaxation data

Solvation dynamics in four imidazolium cation based room temperature ionic liquids (RTIL) have been calculated by using the recently measured dielectric relaxation data [ J. Phys. Chem. B 2008, 112, 4854 ] as an input in a molecular hydrodynamic theory developed earlier for studying solvation energy relaxation in polar solvents. Coumarin 153 (C153), 4-aminophthalimide (4-AP), and trans-4-dimethylamino-4'-cyanostilbene (DCS) have been used as probe molecules for this purpose. The medium response to a laser-excited probe molecule in an ionic liquid is approximated by that in an effective dipolar medium. The calculated decays of the solvent response function for these RTILs have been found to be biphasic and the decay time constants agree well with the available experimental and computer simulation results. Also, no probe dependence has been found for the average solvation times in these ionic liquids. In addition, dipolar solvation dynamics have been predicted for two other RTILs for which experimental results are not available yet. These predictions should be tested against experiments and/or simulation studies.     

Heterolytic bond dissociation in water: why is it so easy for C4H9Cl but not for C3H9SiCl?

The recently developed (Song, L.; Wu, W.; Zhang, Q.; Shaik, S. J. Phys. Chem. A 2004, 108, 6017-6024) valence bond method coupled to a polarized continuum model (VBPCM) is used to address the long standing conundrum of the heterolytic dissociation of the C-Cl and Si-Cl bonds, respectively, in tertiary-butyl chloride and trimethylsilyl chloride in condensed phases. The method is used here to compare the bond dissociation in the gas phase and in aqueous solution. In addition to the ground state reaction profile, VB theory also provides the energies of the purely covalent and purely ionic VB structures as a function of the reaction coordinate. Accordingly, the C-Cl and Si-Cl bonds are shown to be of different natures. In the gas phase, the resonance energy arising from covalent-ionic mixing at equilibrium geometry amounts to 42 kcal/mol for tertiary-butyl chloride, whereas the same quantity for trimethylsilyl chloride is significantly higher at 62 kcal/mol. With such a high value, the root cause of the Si-Cl bonding is the covalent-ionic resonance energy, and this bond belongs to the category of charge-shift bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D.; Hiberty, P. C. Chem.- Eur. J. 2005, 11, 6358). This difference between the C-Cl and Si-Cl bonds carries over to the solvated phase and impacts the heterolytic cleavages of the two bonds. For both molecules, solvation lowers the ionic curve below the covalent one, and hence the bond dissociation in the solvent generates the two ions, Me3E+ Cl- (E = C, Si). In both cases, the root cause of the barrier is the loss of the covalent-ionic resonance energy. In the heterolysis reaction of Si-Cl, the covalent-ionic resonance energy remains large and fully contributes to the dissociation energy, thereby leading to a high barrier for heterolytic cleavage, and thus prohibiting the generation of ions. By contrast, the covalent-ionic resonance energy is smaller for the C-Cl bond and only partially contributes to the barrier for heterolysis, which is consequently small, leading readily to ions that are commonly observed in the classical SN1 mechanism. Thus, the reluctance of R3Si-X molecules to undergo heterolysis in condensed phases and more generally the rarity of free silicenium ions under these conditions are experimental manifestations of the charge-shift character of the Si-Cl bond.     

Nature of the water molecules in the palisade layer of a triton X-100 micelle in the presence of added salts: A solvation dynamics study

The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.     

Computation of methodology-independent single-ion solvation properties from molecular simulations. IV. Optimized Lennard-Jones interaction parameter sets for the alkali and halide ions in water

The raw single-ion solvation free energies computed from atomistic (explicit-solvent) simulations are extremely sensitive to the boundary conditions and treatment of electrostatic interactions used during these simulations. However, as shown recently [M. A. Kastenholz and P. H. Hu?nenberger, J. Chem. Phys. 124, 224501 (2006); M. M. Reif and P. H. Hu?nenberger, J. Chem. Phys. 134, 144103 (2010)], the application of appropriate correction terms permits to obtain methodology-independent results. The corrected values are then exclusively characteristic of the underlying molecular model including in particular the ion-solvent van der Waals interaction parameters, determining the effective ion size and the magnitude of its dispersion interactions. In the present study, the comparison of calculated (corrected) hydration free energies with experimental data (along with the consideration of ionic polarizabilities) is used to calibrate new sets of ion-solvent van der Waals (Lennard-Jones) interaction parameters for the alkali (Li(+), Na(+), K(+), Rb(+), Cs(+)) and halide (F(-), Cl(-), Br(-), I(-)) ions along with either the SPC or the SPC/E water models. The experimental dataset is defined by conventional single-ion hydration free energies [Tissandier et al., J. Phys. Chem. A 102, 7787 (1998); Fawcett, J. Phys. Chem. B 103, 11181] along with three plausible choices for the (experimentally elusive) value of the absolute (intrinsic) hydration free energy of the proton, namely, ΔG(hyd)(?)[H(+)] = -1100, -1075 or -1050 kJ mol(-1), resulting in three sets L, M, and H for the SPC water model and three sets L(E), M(E), and H(E) for the SPC/E water model (alternative sets can easily be interpolated to intermediate ΔG(hyd)(?)[H(+)] values). The residual sensitivity of the calculated (corrected) hydration free energies on the volume-pressure boundary conditions and on the effective ionic radius entering into the calculation of the correction terms is also evaluated and found to be very limited. Ultimately, it is expected that comparison with other experimental ionic properties (e.g., derivative single-ion solvation properties, as well as data concerning ionic crystals, melts, solutions at finite concentrations, or nonaqueous solutions) will permit to validate one specific set and thus, the associated ΔG(hyd)(?)[H(+)] value (atomistic consistency assumption). Preliminary results (first-peak positions in the ion-water radial distribution functions, partial molar volumes of ionic salts in water, and structural properties of ionic crystals) support a value of ΔG(hyd)(?)[H(+)] close to -1100 kJ·mol(-1).     

Experimental and theoretical investigations of solvation dynamics of ionic fluids: appropriateness of dielectric theory and the role of DC conductivity

An analysis is provided of the subnanosecond dynamic solvation of ionic liquids in particular and ionic solutions in general. It is our hypothesis that solvation relaxation in ionic fluids, in the nonglassy and nonsupercooled regimes, can be understood rather simply in terms of the dielectric spectra of the solvent. This idea is suggested by the comparison of imidazolium ionic liquids with their pure organic counterpart, butylimidazole (J. Phys. Chem. B 2004, 108, 10245-10255). It is borne out by a calculation of the solvation correlation time from frequency dependent dielectric data for the ionic liquid, ethylammonium nitrate, and for the electrolyte solution of methanol and sodium perchlorate. Very good agreement is obtained between these theoretically calculated solvation relaxation functions and those obtained from fluorescence upconversion spectroscopy. Our comparisons suggest that translational motion of ions may not be the predominant factor in short-time solvation of ionic fluids and that many tools and ideas about solvation dynamics in polar solvents can be adapted to ionic fluids.     

Variation of thermodynamic parameters of crown ether-metal complex formation and reactant solvation in binary nonaqueous solvent mixtures

General trends in the variation of thermodynamic parameters of complex formation of crown ethers with d-metal ions in binary nonaqueous solvent mixtures were determined. An equation was proposed for predicting variation of the stability of coordination compounds upon replacement of one nonaqueous solvent by another on the basis of the change in the Gibbs energy of solvation of the central ion. Calculation of the Gibbs energies for the formation of the [Ag18C6]⁺ ion in acetonitrile and a number of nonaqueous solvents confirmed the predictive ability of the proposed equation.     

Molecular dynamics simulation of the energetic room-temperature ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN)

Molecular dynamics (MD) simulations have been performed to investigate the structure and dynamics of an energetic ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN). The generalized amber force field (GAFF) was used, and an electronically polarizable model was further developed in the spirit of our previous work (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877). In the process of simulated annealing from a liquid state at 475 K down to a glassy state at 175 K, the MD simulations identify a glass-transition temperature region at around 250-275 K, in agreement with experiment. The self-intermediate scattering functions show vanishing boson peaks in the supercooled region, indicating that HEATN may be a fragile glass former. The coupling/decoupling of translational and reorientational ion motion is also discussed, and various other physical properties of the liquid state are intensively studied at 400 K. A complex hydrogen bond network was revealed with the calculation of partial radial distribution functions. When compared to the similarly sized 1-ethyl-4-methyl-1,4-imidazolium nitrate ionic liquid, EMIM+/NO3-, a hydrogen bond network directly resulting in the poorer packing efficiency of ions is observed, which is responsible for the lower melting/glass-transition point. The structural properties of the liquid/vacuum interface shows that there is vanishing layering at the interface, in accordance with the poor ion packing. The effects of electronic polarization on the self-diffusion, viscosity, and surface tension of HEATN are found to be significant, in agreement with an earlier study on EMIM+/NO3- (Yan, T.; Burnham, C. J.; Del Popolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877).     

Compensation Between the Entropic and Enthalpic Changes Arising from Changes in Solvent–Solvent Interactions in Mixed Solvents

A theorem presented by Professor Ben-Naim (J Phys Chem 82:874–885, 1978) states that the standard state enthalpy and entropy changes arising from changes in the solvent structure that are induced by solvation of a solute cancel exactly in the standard state Gibbs energy. In this paper this is explored by consideration of the thermodynamics of transfer of electrolytes in mixed solvents, using previously developed models of the solvation process. Two cases are considered. One is random solvation, where curvatures in plots of the transfer enthalpies and entropies, which arise from changes in solvent–solvent interactions, exactly compensate in the transfer Gibbs (free) energies, which are sensibly linear with solvent composition. The second type of system are those with strong preferential solvation where it is found that the transfer Gibbs energies can be accounted for quantitatively in terms of changes in the solute–solvent interactions, with no contribution from changes in solvent–solvent interactions. The results are entirely consistent with the Ben-Naim theorem.     

Equation of state for the phase coexistence region of insoluble monolayers under consideration of the entropy nonideality

The equation of state for the monolayer with a fluid (G, LE)/condensed (LC) phase transition derived earlier (Fainerman, V.B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145) in the framework of a quasichemical approach is generalized. A term is added that takes into account the entropy nonideality of mixing of the monomers and clusters of amphiphilic molecules. The results calculated from the proposed equations agree well with the experimental Pi-A isotherms obtained for various types of amphiphilic monolayers. The values of molecular areas of the amphiphilic molecules estimated from the fitting of experimental data to the proposed equation are quite similar to the real values. Another equation of state capable of describing the fluid state of insoluble monolayers and based on equations for the chemical potential of the solvent in the bulk phase and in the surface layer (Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 2006, 110, 10436) is also generalized to be extended to the fluid/condensed phase transition region (A < A(c)), taking into account entropy nonideality for mixing solvent molecules, monomers, and clusters of amphiphilic molecules. The values calculated on this basis agree also well with the experimental data.     

Mechanism and kinetics of the water-assisted formic acid + OH reaction under tropospheric conditions

In this work, we have revisited the mechanism of the formic acid + OH radical reaction assisted by a single water molecule. Density functional methods are employed in conjunction with large basis sets to explore the potential energy surface of this radical-molecule reaction. Computational kinetics calculations in a pseudo-second-order mechanism have been performed, taking into account average atmospheric water concentrations and temperatures. We have used this method recently to study the single water molecule assisted H-abstraction by OH radicals (Iuga, C.; Alvarez-Idaboy, J. R.; Reyes, L.; Vivier-Bunge, A. J. Phys. Chem. Lett. 2010, 1, 3112; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Chem. Phys. Lett. 2010, 501, 11; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Theor. Chem. Acc. 2011, 129, 209), and we showed that the initial water complexation step is essential in the rate constant calculation. In the formic acid reaction with OH radicals, we find that the water-acid complex concentration is small but relevant under atmospheric conditions, and it could in principle be large enough to produce a measurable increase in the overall rate constant. However, the water-assisted process occurs according to a formyl hydrogen abstraction, rather than abstraction of carboxylic hydrogen as in the water-free case. As a result, the overall reaction rate constant is considerably smaller. Products are different in the water-free and water-assisted processes.     

A universal approach for continuum solvent pK a calculations: are we there yet?

This paper reviews several pK _a calculation strategies that are commonly used in aqueous acidity predictions. Among those investigated were the direct or absolute method, the proton exchange scheme, and the hybrid cluster–continuum (Pliego and Riveros) and implicit–explicit (Kelly, Cramer and Truhlar) models. Additionally, these protocols are applied in the pK _a calculation of 55 neutral organic and inorganic acids in conjunction with various solvent models, including the CPCM-UAKS/UAHF, IPCM, SM6 and COSMO-RS, with a view to identifying a universal approach for accurate pK _a predictions. The results indicate that the direct method is unsuitable for general pK _a calculations, although moderately accurate results (MAD <3 units) are possible for certain classes of acids, depending on the choice of solvent model. The proton exchange scheme generally delivers good results (MAD <2 units), with CPCM-UAKS giving the best performance. Furthermore, the sensitivity of this approach to the choice of reference acid can be substantially lessened if the solvation energies for ionic species are calculated via the IPCM cluster–continuum approach. Reference-independent hybrid approaches that include explicit water molecules can potentially give reasonably accurate values (MAD generally ~2 units) depending on the solvent model and the number of explicit water molecules added.