Molecular dynamics simulations of liquid methanol and methanol-water mixtures with polarizable models

A polarizable model for simulation of liquid methanol, compatible with the COS/G2 water model, has been developed using the Charge-on-Spring (COS) technique. The model consists of three point charges, with one polarizable center on the oxygen atom. The Lennard-Jones parameters on the oxygen atom together with the molecular polarizability were varied to reproduce the experimental heat of vaporization and density of liquid methanol at ambient conditions. We examined the energies of various methanol dimers in the gas phase and compared them with values obtained from ab initio calculations. The model was then used to study the thermodynamic, dynamic, structural, and dielectric properties of liquid methanol as well as of a methanol-water mixture. A microscopic picture of the structure of pure liquid methanol and of the methanol-water mixture is provided. Good agreement was found between the results from our model simulations and available experimental and ab initio calculation data. In particular, the experimental dielectric permittivity of 32 could be reproduced, which had been shown to be difficult when using nonpolarizable models.     

Polarization energy gradients in combined quantum mechanics, effective fragment potential, and polarizable continuum model calculations

A method that combines quantum mechanics (QM), typically a solute, the effective fragment potential (EFP) discrete solvent model, and the polarizable continuum model is described. The EFP induced dipoles and polarizable continuum model (PCM) induced surface charges are determined in a self-consistent fashion. The gradients of these two energies with respect to molecular coordinate changes are derived and implemented. In general, the gradients can be formulated as simple electrostatic forces and torques among the QM nuclei, electrons, EFP static multipoles, induced dipoles, and PCM induced charges. Molecular geometry optimizations can be performed efficiently with these gradients. The formulas derived for EFPPCM can be generally applied to other combined molecular mechanics and continuum methods that employ induced dipoles and charges.     

Solution chemistry of cobalt in liquid hydrogen fluoride

Cobalt(II) fluoride dissolves in HF in the presence of excess BF₃ or AsF₅: solid adducts are isolable from these solutions. Insoluble CoF₃ decomposes to cobalt(II) under the same conditions. The CoF^3?₆ is solvolysed by HF except in the presence of excess F^?. Cs₂CoF₆ is soluble in HF but the solutions decompose above 10°C yielding F₂; such solutions are powerful fluorinating agents and convert MnF₃ to MnF^2?₆ and Xe to XeF₂.     

Importance of structural motifs in liquid hydrogen fluoride

The investigation of liquid phases by means of accurate electronic structure methods is a demanding task due to the high computational effort. We applied second-order M?ller-Plesset perturbation theory and high-level quantum chemical calculations using the coupled-cluster method with single, double and perturbative triple excitations in combination with Dunnings correlation-consistent basis sets up to quintuple ζ quality. Based on these calculations, we extrapolated the correlation energy to the basis set limit in order to improve the results even further. For comparison to the correlated electronic structure methods, density functional calculations employing different functionals are presented as well. The investigated species are a cyclic pentamer as well as a set of branched structures. The quantum cluster equilibrium method is employed for the investigation of the liquid-phase structure of hydrogen fluoride. The pentamer is found to be present to a high extent and in the case of the MP2/QZVP data, its presence improves the results significantly. Accounting for branched structures slightly improves results, so that they are found to be present but not to dominate in liquid hydrogen fluoride. Concerning both the interaction energy and the result of the quantum cluster equilibrium calculation the basis set has a major influence, whereas the difference between M?ller-Plesset perturbation theory and coupled-cluster calculations is less pronounced.     

Isothermal compressibility maxima of hydrogen fluoride in the supercritical and superheated vapor regions

The highly nonideal behavior of hydrogen fluoride (HF) vapor has been considered to be the origin of its numerous vapor phase anomalies. In this work, we report one such potential vapor phase anomaly for HF. For a nonassociating substance like propane, the response functions go through a maximum only once in the supercritical region. However, for HF, when an association model is used to predict the isothermal compressibility (KT), it exhibits a maximum in the supercritical region more than once, and this peak extends well in to the superheated vapor region upon decompression. This theoretical prediction is also supported by two other models recently developed for HF. Note that experimental values of KT for HF have not been reported in the literature so far. Preliminary investigations on this KT maximum for HF have suggested no reentrant spinodal, singularity-free scenario, or any additional first-order phase transition, unlike water, and, also, no lambda (or higher-order phase) transitions, unlike liquid helium. However, this KT peak is similar to the experimentally supported heat capacity (CP) peak of HF which extends into the supercritical and superheated vapor regions. Similar to the CP peak, which is understood based on vapor-phase clustering in HF, we relate KT to the derivatives of enthalpy and entropy of the system. Also, we analyze some of the P-v-T experimental data that are available to provide an overview of the KT behavior in the region of interest, and compare them with the model results. Finally, to explore the effect of including a distribution pattern for the oligomers, we report the results on a model that only includes association. Using this approach, we report KT results with and without a Poisson-type oligomer distribution and show that the KT appears once this distribution scheme is specified.     

Stable dialkyl ether/poly(hydrogen fluoride) complexes: dimethyl ether/poly(hydrogen fluoride), a new,convenient, and effective fluorinating agent

The preparation, (1)H, (13)C, and (19)F NMR structural characterization as well as with DFT-based theoretical calculations of stable dialkyl ether/poly(hydrogen fluoride) complexes are reported. Dimethyl ether/poly(hydrogen fluoride) (DMEPHF), are stable complexes of particular interest and use. The DFT calculations, that are in agreement with NMR data, suggest a cyclic poly(hydrogen fluoride) bridged structure for DMEPHF. The complex, DME-5 HF was found to be a convenient and effective new fluorinating agent with the ease of workup and applied to several fluorination reactions, such as the hydrofluorination and bromofluorination of alkenes, and fluorination of alcohols giving good to excellent yield with high selectivity. Homologous dialkyl ether/poly(hydrogen fluoride) (R(2)O/[HF](n,), R = Et, nPr) systems are also stable and suitable for fluorination reactions.     

Oxidative fluorination of aromatic compounds in liquid hydrogen fluoride

Toluene derivatives bearing electronegative substituents are cleanly fluorinated on the methyl group by reaction with lead dioxide or nickel dioxide in liquid HF. For example, 4-nitrotoluene gives in high yield a mixture of 4-nitrobenzyl fluoride and 4-nitrobenzal fluoride. Aromatic cation radicals are proposed as intermediates.     

Assessment of the potential models of acetone/CO2 and ethanol/CO2 mixtures by computer simulation and thermodynamic integration in liquid and supercritical states

Binary mixtures of CO(2) with ethanol and with acetone are studied by computer simulation, including extensive free energy calculations done by the method of thermodynamic integration, at 313 K, i.e., above the critical point of CO(2) in the entire composition range. The calculations are repeated with three different models of acetone and ethanol, and two models of CO(2). Comparisons of the molar volume of the different systems as well as of the change of their molar volume accompanying the mixing of the two components with experimental data reveal that, among the model pairs tested, the best results are obtained if both components are described by the Transferable Potentials for Phase Equilibria (TraPPE) force field. Around the ethanol/acetone mole fraction of 0.05 all ethanol/CO(2) and almost all acetone/CO(2) model pairs considered predict the existence of a sharp maximum of the molar volume. Due to the lack of experimental data in this composition range, however, these predictions cannot be verified/falsified yet. Most of the model pairs considered also predict limited miscibility of these compounds, as seen from the positive values of the free energy change accompanying their mixing, and the miscibility gap is located at the same composition range as the aforementioned molar volume maximum.     

Toward effective and reliable fluorescence energies in solution by a new state specific polarizable continuum model time dependent density functional theory approach

A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra in solution, and it can be used for studying different relaxation time regimes. SS and conventional linear response (LR) models have been compared by computing the emission energies for different benchmark systems (formaldehyde in water and three coumarin derivatives in ethanol). Special attention is given to the influence of dynamical solvation effects on LR geometry optimizations in solution. The results on formaldehyde point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents and/or weak transitions. The computed emission energies for coumarin derivatives are very close to their experimental counterparts, pointing out the importance of a proper treatment of nonequilibrium solvent effects on both the excited and the ground state energies. The availability of SS-PCM/TD-DFT models for the study of absorption and emission processes allows for a consistent treatment of a number of different spectroscopic properties in solution.     

Molecular dynamics simulation of sub-and supercritical water with a new interaction potential

A molecular dynamics experiment was performed for water under sub-and supercritical conditions with a new interaction potential including the nonelectrostatic H-bond component. The internal energy, self-diffusion coefficient, mean number of H-bonds per water molecule, and distributions of molecules according to the number of H-bonds were calculated over a wide temperature range at pressures of 50 and 100 MPa. The temperature dependences of these properties were analyzed.     

The B 1sigma+ and X 1sigma+ electronic states of hydrogen fluoride: a direct potential fit analysis

All literature pure rotational and vibration-rotational spectroscopic data on the ground X (1)Sigma(+) electronic state of HF and DF, together with the entire set of spectroscopic line positions from analyses of the B (1)Sigma(+) --> X (1)Sigma(+) emission band systems of HF and DF, have been used in a global least-squares fit to the radial Hamiltonian operators, in compact analytic form, for both electronic states. With a data set consisting of 6157 spectroscopic line positions, the reduced standard deviation of the fit was sigma = 1.028. Sets of quantum mechanically significant rotational and centrifugal distortion constants were calculated for both electronic states using Rayleigh-Schr?dinger perturbation theory.     

Electrostatic properties of aqueous salt solution interfaces: a comparison of polarizable and nonpolarizable ion models

The effects of ion force field polarizability on the interfacial electrostatic properties of approximately 1 M aqueous solutions of NaCl, CsCl, and NaI are investigated using molecular dynamics simulations employing both nonpolarizable and Drude-polarizable ion sets. Differences in computed depth-dependent orientational distributions, "permanent" and induced dipole and quadrupole moment profiles, and interfacial potentials are obtained for both ion sets to further elucidate how ion polarizability affects interfacial electrostatic properties among the various salts relative to pure water. We observe that the orientations and induced dipoles of water molecules are more strongly perturbed in the presence of polarizable ions via a stronger ionic double layer effect arising from greater charge separation. Both anions and cations exhibit enhanced induced dipole moments and strong z alignment in the vicinity of the Gibbs dividing surface (GDS) with the magnitude of the anion induced dipoles being nearly an order of magnitude larger than those of the cations and directed into the vapor phase. Depth-dependent profiles for the trace and z z components of the water molecular quadrupole moment tensors reveal 40% larger quadrupole moments in the bulk phase relative to the vapor which mimics a similar observed 40% increase in the average water dipole moment. Across the GDS, the water molecular quadrupole moments increase nonmonotonically (in contrast to the water dipoles) and exhibit a locally reduced contribution just below the surface due to both orientational and polarization effects. Computed interfacial potentials for the nonpolarizable salts yield values 20-60 mV more positive than pure water and increase by an additional 30-100 mV when ion polarizability is included. A rigorous decomposition of the total interfacial potential into ion monopole, water and ion dipole, and water quadrupole components reveals that a very strong, positive ion monopole contribution is offset by negative contributions from all other potential sources. Water quadrupole components modulated by the water density contribute significantly to the observed interfacial potential increments and almost entirely explain observed differences in the interfacial potentials for the two chloride salts. By lumping all remaining nonquadrupole interfacial potential contributions into a single "effective" dipole potential, we observe that the ratio of quadrupole to "effective" dipole contributions range from 2:1 in CsCl to 1:1.5 in NaI, suggesting that both contributions are comparably important in determining the interfacial potential increments. We also find that oscillations in the quadrupole potential in the double layer region are opposite in sign and partially cancel those of the "effective" dipole potential.     

Formation and relaxation of excited states in solution: a new time dependent polarizable continuum model based on time dependent density functional theory

In this paper a novel approach to study the formation and relaxation of excited states in solution is presented within the integral equation formalism version of the polarizable continuum model. Such an approach uses the excited state relaxed density matrix to correct the time dependent density functional theory excitation energies and it introduces a state-specific solvent response, which can be further generalized within a time dependent formalism. This generalization is based on the use of a complex dielectric permittivity as a function of the frequency, epsilonomega. The approach is here presented in its theoretical formulation and applied to the various steps involved in the formation and relaxation of electronic excited states in solvated molecules. In particular, vertical excitations (and emissions), as well as time dependent Stokes shift and complete relaxation from vertical excited states back to ground state, can be obtained as different applications of the same theory. Numerical results on two molecular systems are reported to better illustrate the features of the model.     

Regio - and stereospecific halogenation of 7alogenoalkenes in liquid hydrogen fluoride

The products of bromo and chlorofluorination of E and Z-1,2-dichloroethylenes, 1, 3-dichloro-1-propenes, 1, 1-dichloro- ethylene and 1, 3-dichloro-2-fluoro-1-propene by N-bromosuccinimide and hexachloromelamine in anhydrous hydrogen fluoride have been studied. It was found that the reaction was in all cases 100% regio and 93–100% trans-stereospecific with the exception of E-1, 2-dichloro-ethylene, its trans-stereospecificity being 85%.Threo and erithro-1-bromo-1, 2-dichloro-2-fluoroethanes, 2-bromo-1, 3-dichloro-1-fluoropropanes and 1, 2, 3-trichloro-1-fluoro-propanes as well 1, 1, 2-trichloro-2-fluoroethane, 1-bromo-2, 2-dichloro-2-fluoroethane, 1, 2, 2-trichloro-2 fluoroethane, 1-bromo-1, 3-dichloro-2, 2-difluoropropane, and 1, 1, 3-trichloro-2,2-difluoropropane were obtained in 50–70% yield.The bromination of E and Z-1, 3-dichloro-1-propenes with molecular bromine in carbon tetrachloride in the dark is non-stereospecific and gives a mixture of erithro and threo-1, 2-dibromo-1, 3-dichloropropanes in the ratio about 1:1. However, the bromination reaction in anhydrous hydrogen fluoride solution proceeds with a high degree of stereospecificity (94–95%) and gives threo-1, 2-dibromo-1, 3-dichloropropane from Z and erithro-1, 2-dibromo-1, 3-dichloropropane from E-1, 3-dichloro-1-propene.The data obtained are considered in terms of an electrophilic mechanism of halogenoalkene halogenation in anhydrous hydrogen fluoride and a free-radical mechanism in carbon tetrachloride.     

Electron mobility in liquid and supercritical helium measured using corona discharges: a new semi-empirical model for cavity formation

Electron mobilities in supercritical and liquid helium were investigated as a function of the density. The mobilities were derived from I(V) curves measured in a high-pressure cryogenic cell using a corona discharge in point-plane electrode geometry for charge generation. The presented data spans a wide pressure and temperature range due to the versatility of our experimental set-up. Where data from previous investigations is available for comparison, very good agreement is found. We present a semi-empirical model to calculate electron mobilities both in the liquid and supercritical phase. This model requires the electron-helium scattering length and thermodynamic state equations as the only input and circumvents any need to consider surface tension. Our semi-empirical model reproduces experimental data very well, in particular towards lower densities where transitions from localised to delocalised electron states were observed.     

The acidity of tert-butyl alcohol in near- and supercritical water: a polarizable continuum approach

We use a polarizable continuum approach to study the acidity of tert-butyl alcohol in water at ambient, near-critical, and supercritical conditions. In the most straightforward calculation, the bare ionic species (the tert-butoxy anion and the hydronium cation) are placed in cavities surrounded by a dielectric continuum, using a dielectric constant corresponding to the state point. A second method is first to solvate these ions with a small number of explicit water molecules and then surround this cluster with the dielectric. This is the cluster-continuum approach. We discuss the advantages and disadvantages of these two schemes and we also discuss various ways in which the second method can be implemented. No method showed quantitative agreement with all available experimental results but the first, straightforward method was the most successful in predicting the correct trends. From both a numerical and a theoretical point of view, we believe this is the method of choice.     

Molecular dynamic simulation of sub- and supercritical water with new interaction potential

Molecular dynamic simulation of water under sub- and supercritical conditions was performed with a new form of O···H pairwise potential with an additional non-electrostatic term corresponding hydrogen bond formation. Some structural properties of water and hydrogen bonds characteristics were calculated in a wide temperature range and pressure 50 MPa. Dependences of these values on temperature were analyzed.