Some examples of the use of molecular dynamics simulation to study solutions of small molecules i... more Some examples of the use of molecular dynamics simulation to study solutions of small molecules in ionic liquids are discussed. It is shown that electrostatic forces, while not the dominant solute–solvent interaction, determine the local solute environment. The solubility of aromatics and the changes in the spectra of low-frequency intermolecular vibrations in two related ionic liquids (one dicationic and one monocationic) are compared with experimental results and related to the local environment.
The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an e... more The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an equimolar mixture of benzene and dimethylimidazolium triflimide [C1C1im][NTf2] is that in the former the benzene moieties are tied to the imidazolium ring, while in the latter they move independently. We use femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and molecular simulations to explore some properties of these two systems. The Kerr spectra show small differences in the spectral densities; the simulations also show very similar environments for both the imidazolium rings and the phenyl or benzene parts of the molecules. The low frequency vibrational densities of states are also similar in the model systems. In order to perform the simulations we developed a model for the [BzC1im](+) cation and found that the barriers to rotation of the two parts of the molecule are low.
ABSTRACT Constrained molecular dynamics simulations are carried out to compare the adsorption/inc... more ABSTRACT Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl.
Phys. Chem. Chem. Phys., 2002, 4, 5281-5288 DOI:10.1039/B203429H (Paper). Solvation effects on eq... more Phys. Chem. Chem. Phys., 2002, 4, 5281-5288 DOI:10.1039/B203429H (Paper). Solvation effects on equilibria: Triazoles and N-methyl piperidinol. Stuart E. Murdocka, RM Lynden-Bella, J. Kohanoffa, CJ Margulisb and Graham ...
ABSTRACT The diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 °C... more ABSTRACT The diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 °C and at infinite dilution are studied by computer simulation using the SPC/E model for water and solute−water potentials employed in previous work (Koneshan, S.; et al. J. Phys. Chem. 1998, 102, 4193−4204). The mobilities of the ions calculated from the diffusion coefficients pass through a maximum as a function of ion size, with distinct curves and maximums for positive and negative ions in qualitative agreement with experiment. We aim to understand this at a microscopic level in terms of theoretical studies of the friction coefficient ζ, which is related to diffusion coefficient by the Stokes−Einstein relation. This provides one method of calculating ζ, but it can also be obtained in principle from the random force autocorrelation function which is the starting point of molecular theories of the friction. Molecular and continuum theories divide the friction into hydrodynamic and dielectric components calculated in different ways on the basis of certain approximations that are tested in this paper. The two methods of determining ζ give consistent results in simulations of large ions or uncharged solutes but differ by a factor of nearly 1.5 for smaller ions. This is attributed to the assumption, in our simulations and in some molecular theories, that the random force autocorrelation function of a moving ion can be approximated by the total force autocorrelation function of a fixed ion but the observed trends in ζ with ion size are unchanged. Three different separations of the force autocorrelation function are studied; namely, partitioning this into (a) electrostatic and Lennard-Jones forces (b) hard and soft forces, and (c) forces arising from the first shell and more distant forces. The cross-terms are found to be significant in all cases, and the contributions of the sum of the soft term and the cross-terms, which are of opposite sign, to the total friction in the hard−soft separation, is small for all the ions (large and small). This suggests that dielectric friction calculated using this separation, with the neglect of cross-terms, is less accurate than previously supposed and the success of these theories is due to a cancellation of errors in the approximations. This is supported by recent a theoretical study of Chong and Hirata (Chong, C.; Hirata, F. J. Chem. Phys. 1998, 108, 739) which evaluates the cross-terms. A phenomenological theory due to Chen and Adelman (Chen, J. H.; Adelman, S. A. J. Chem. Phys. 1980, 72, 2819) calculates the friction in terms of effective hydrodynamic and dielectric radii for the ions (cf. solventberg picture) and predicts low dielectric friction for large ions and small strongly solvated ions. This also agrees qualitatively with our simulations but a complete molecular theory, applicable to positive and negative ions in hydrogen-bonded solvents such as water, has yet to be developed.
We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous ... more We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous solution at 25 °C using a simple point charge model (SPC/E) for water and charged spherical Lennard-Jones solutes. We use a novel method with an extended Lagrangian or Hamiltonian in which the charge and the size of the ions are considered as dynamical variables. This enables us to determine thermodynamic properties as continuous functions of solute size and charge and to move smoothly from hydrophilic to hydrophobic solvation conditions. On passing between these extremes, the entropy of solvation goes through maxima. For example it shows a double maximum as a function of charge at constant size and a single maximum as a function of size at constant (non-zero) charge. These maxima correspond to extremes of structure-breaking and are associated with the disappearance of the second solvation shell in the radial distribution function; no anomalies are seen in the first shell. We also presen...
A model for the description of the electronic ground state of the triiodide ion in solution is de... more A model for the description of the electronic ground state of the triiodide ion in solution is developed. It is based on the ``diatomics in molecules'' technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent-ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix
Phys. Chem. Chem. Phys., 2002, 4, 3016-3021 DOI:10.1039/B200991A (Paper). Determining the electro... more Phys. Chem. Chem. Phys., 2002, 4, 3016-3021 DOI:10.1039/B200991A (Paper). Determining the electronic structure and chemical potentials of molecules in solution. Stuart E. Murdocka, RM Lynden-Bell*a, J. Kohanoffa and ...
Some examples of the use of molecular dynamics simulation to study solutions of small molecules i... more Some examples of the use of molecular dynamics simulation to study solutions of small molecules in ionic liquids are discussed. It is shown that electrostatic forces, while not the dominant solute–solvent interaction, determine the local solute environment. The solubility of aromatics and the changes in the spectra of low-frequency intermolecular vibrations in two related ionic liquids (one dicationic and one monocationic) are compared with experimental results and related to the local environment.
The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an e... more The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an equimolar mixture of benzene and dimethylimidazolium triflimide [C1C1im][NTf2] is that in the former the benzene moieties are tied to the imidazolium ring, while in the latter they move independently. We use femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and molecular simulations to explore some properties of these two systems. The Kerr spectra show small differences in the spectral densities; the simulations also show very similar environments for both the imidazolium rings and the phenyl or benzene parts of the molecules. The low frequency vibrational densities of states are also similar in the model systems. In order to perform the simulations we developed a model for the [BzC1im](+) cation and found that the barriers to rotation of the two parts of the molecule are low.
ABSTRACT Constrained molecular dynamics simulations are carried out to compare the adsorption/inc... more ABSTRACT Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl.
Phys. Chem. Chem. Phys., 2002, 4, 5281-5288 DOI:10.1039/B203429H (Paper). Solvation effects on eq... more Phys. Chem. Chem. Phys., 2002, 4, 5281-5288 DOI:10.1039/B203429H (Paper). Solvation effects on equilibria: Triazoles and N-methyl piperidinol. Stuart E. Murdocka, RM Lynden-Bella, J. Kohanoffa, CJ Margulisb and Graham ...
ABSTRACT The diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 °C... more ABSTRACT The diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 °C and at infinite dilution are studied by computer simulation using the SPC/E model for water and solute−water potentials employed in previous work (Koneshan, S.; et al. J. Phys. Chem. 1998, 102, 4193−4204). The mobilities of the ions calculated from the diffusion coefficients pass through a maximum as a function of ion size, with distinct curves and maximums for positive and negative ions in qualitative agreement with experiment. We aim to understand this at a microscopic level in terms of theoretical studies of the friction coefficient ζ, which is related to diffusion coefficient by the Stokes−Einstein relation. This provides one method of calculating ζ, but it can also be obtained in principle from the random force autocorrelation function which is the starting point of molecular theories of the friction. Molecular and continuum theories divide the friction into hydrodynamic and dielectric components calculated in different ways on the basis of certain approximations that are tested in this paper. The two methods of determining ζ give consistent results in simulations of large ions or uncharged solutes but differ by a factor of nearly 1.5 for smaller ions. This is attributed to the assumption, in our simulations and in some molecular theories, that the random force autocorrelation function of a moving ion can be approximated by the total force autocorrelation function of a fixed ion but the observed trends in ζ with ion size are unchanged. Three different separations of the force autocorrelation function are studied; namely, partitioning this into (a) electrostatic and Lennard-Jones forces (b) hard and soft forces, and (c) forces arising from the first shell and more distant forces. The cross-terms are found to be significant in all cases, and the contributions of the sum of the soft term and the cross-terms, which are of opposite sign, to the total friction in the hard−soft separation, is small for all the ions (large and small). This suggests that dielectric friction calculated using this separation, with the neglect of cross-terms, is less accurate than previously supposed and the success of these theories is due to a cancellation of errors in the approximations. This is supported by recent a theoretical study of Chong and Hirata (Chong, C.; Hirata, F. J. Chem. Phys. 1998, 108, 739) which evaluates the cross-terms. A phenomenological theory due to Chen and Adelman (Chen, J. H.; Adelman, S. A. J. Chem. Phys. 1980, 72, 2819) calculates the friction in terms of effective hydrodynamic and dielectric radii for the ions (cf. solventberg picture) and predicts low dielectric friction for large ions and small strongly solvated ions. This also agrees qualitatively with our simulations but a complete molecular theory, applicable to positive and negative ions in hydrogen-bonded solvents such as water, has yet to be developed.
We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous ... more We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous solution at 25 °C using a simple point charge model (SPC/E) for water and charged spherical Lennard-Jones solutes. We use a novel method with an extended Lagrangian or Hamiltonian in which the charge and the size of the ions are considered as dynamical variables. This enables us to determine thermodynamic properties as continuous functions of solute size and charge and to move smoothly from hydrophilic to hydrophobic solvation conditions. On passing between these extremes, the entropy of solvation goes through maxima. For example it shows a double maximum as a function of charge at constant size and a single maximum as a function of size at constant (non-zero) charge. These maxima correspond to extremes of structure-breaking and are associated with the disappearance of the second solvation shell in the radial distribution function; no anomalies are seen in the first shell. We also presen...
A model for the description of the electronic ground state of the triiodide ion in solution is de... more A model for the description of the electronic ground state of the triiodide ion in solution is developed. It is based on the ``diatomics in molecules'' technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent-ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix
Phys. Chem. Chem. Phys., 2002, 4, 3016-3021 DOI:10.1039/B200991A (Paper). Determining the electro... more Phys. Chem. Chem. Phys., 2002, 4, 3016-3021 DOI:10.1039/B200991A (Paper). Determining the electronic structure and chemical potentials of molecules in solution. Stuart E. Murdocka, RM Lynden-Bell*a, J. Kohanoffa and ...
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