An improved intermolecular potential function for simulations of liquid hydrogen fluoride

Quantification of Anisotropy in Exchange and Dispersion Interactions: A Simple Model for Physics-Based Force Fields

In some compounds, exchange repulsion is orientation dependent. However, in contrast to quantum chemical methods that treat exchange explicitly, empirical models assume exchange to be spherically symmetric, yielding an average description only. Here we quantify the anisotropy of exchange and dispersion energy for hydrogen halides and water by probing these compounds with a helium atom using the symmetry-adapted perturbation theory (SAPT). The exchange interaction is reduced by up to 33% due to the σ-hole in hydrogen iodide, depending on the location of the probe. We demonstrate how this anisotropy can be modeled in empirical force fields either using an angle-dependent potential or by introducing virtual sites, reducing the error in the empirical model by a factor of 5 compared to isotropic atoms. Lone-pairs on water, positioned close to perpendicular to the plane of the molecule, on a line with the oxygen atom, and, surprisingly, σ-holes on water both modulate the exchange interaction strongly. Both lone-pairs and σ-holes can be modeled by virtual sites, leading to an 80% reduced error.

Percolation transition and bimodal density distribution in hydrogen fluoride

Hydrogen-bond networks in associating fluids can be extremely robust and characterize the topological properties of the liquid phase, as in the case of water, over its whole domain of stability and beyond. Here, we report on molecular dynamics simulations of hydrogen fluoride (HF), one of the strongest hydrogen-bonding molecules. HF has more limited connectivity than water but can still create long, dynamic chains, setting it apart from most other small molecular liquids. Our simulation results provide robust evidence of a second-order percolation transition of HF's hydrogen bond network occurring below the critical point. This behavior is remarkable as it underlines the presence of two different cohesive mechanisms in liquid HF, one at low temperatures characterized by a spanning network of long, entangled hydrogen-bonded polymers, as opposed to short oligomers bound by the dispersion interaction above the percolation threshold. This second-order phase transition underlines the presence of marked structural heterogeneity in the fluid, which we found in the form of two liquid populations with distinct local densities.

Stabilization of hydrogen-bonded molecular chains by carbon nanotubes

We study numerically nonlinear dynamics of several types of molecular systems composed of hydrogen-bonded chains placed inside carbon nanotubes with open edges. We demonstrate that carbon nanotubes provide a stabilization mechanism for quasi-one-dimensional molecular chains via the formation of their secondary structures. In particular, a polypeptide chain (Gly)N placed inside a carbon nanotube can form a stable helical chain (310-, α-, π-, and β-helix) with parallel chains of hydrogen-bonded peptide groups. A chain of hydrogen fluoride molecules ⋯FH⋯FH⋯FH can form a hydrogen-bonded zigzag chain. Remarkably, we demonstrate that for molecular complexes (Gly)N∈CNT and (FH)N∈CNT, the hydrogen-bonded chains will remain stable even at T=500 K. Thus, our results suggest that the use of carbon nanotubes with encapsulated hydrogen fluoride molecules may be important for the realization of high proton conductivity at high temperatures.

Hidden Halogen-Bonding Ability of Fluorine Manifesting in the Hydrogen-Bond Configurations of Hydrogen Fluoride

Elucidating how the intermolecular interactions of a covalently bonded fluorine atom are similar to and different from those of the other halogen atoms will be helpful for a better unified understanding of them. In the present study, the case of hydrogen fluoride is theoretically studied from this viewpoint by using the techniques of electron density analysis, molecular dynamics of liquid, and others. It is shown that the extra-point model, which locates an additional charge site on the line extended from (not within) the covalent bond and has been adopted for halogen-bonding systems as a key to the generation of proper stability and directionality, works well also in this case. A significantly bent hydrogen-bond configuration, which is characteristic of the intermolecular interactions of hydrogen fluoride, is reasonably well reproduced, meaning that it is a manifestation of the latent halogen-bonding ability, which is hidden by the strongly electronegative nature.

New Molecular-Mechanics Model for Simulations of Hydrogen Fluoride in Chemistry and Biology

Hydrogen fluoride (HF) is the most polar diatomic molecule and one of the simplest molecules capable of hydrogen-bonding. HF deviates from ideality both in the gas phase and in solution and is thus of great interest from a fundamental standpoint. Pure and aqueous HF solutions are broadly used in chemical and industrial processes, despite their high toxicity. HF is a stable species also in some biological conditions, because it does not readily dissociate in water unlike other hydrogen halides; yet, little is known about how HF interacts with biomolecules. Here, we set out to develop a molecular-mechanics model to enable computer simulations of HF in chemical and biological applications. This model is based on a comprehensive high-level ab initio quantum chemical investigation of the structure and energetics of the HF monomer and dimer; (HF)_n clusters, for n = 3-7; various clusters of HF and H₂O; and complexes of HF with analogs of all 20 amino acids and of several commonly occurring lipids, both neutral and ionized. This systematic analysis explains the unique properties of this molecule: for example, that interacting HF molecules favor nonlinear geometries despite being diatomic and that HF is a strong H-bond donor but a poor acceptor. The ab initio data also enables us to calibrate a three-site molecular-mechanics model, with which we investigate the structure and thermodynamic properties of gaseous, liquid, and supercritical HF in a wide range of temperatures and pressures; the solvation structure of HF in water and of H₂O in liquid HF; and the free diffusion of HF across a lipid bilayer, a key process underlying the high cytotoxicity of HF. Despite its inherent simplifications, the model presented significantly improves upon previous efforts to capture the properties of pure and aqueous HF fluids by molecular-mechanics methods and to our knowledge constitutes the first parameter set calibrated for biomolecular simulations.

Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs

Characteristic hydrogen bonded motifs, including solvent separated anion pairs, in concentrated aqueous LiCl solutions.

Quantum mechanical force field for hydrogen fluoride with explicit electronic polarization

The explicit polarization (X-Pol) theory is a fragment-based quantum chemical method that explicitly models the internal electronic polarization and intermolecular interactions of a chemical system. X-Pol theory provides a framework to construct a quantum mechanical force field, which we have extended to liquid hydrogen fluoride (HF) in this work. The parameterization, called XPHF, is built upon the same formalism introduced for the XP3P model of liquid water, which is based on the polarized molecular orbital (PMO) semiempirical quantum chemistry method and the dipole-preserving polarization consistent point charge model. We introduce a fluorine parameter set for PMO, and find good agreement for various gas-phase results of small HF clusters compared to experiments and ab initio calculations at the M06-2X/MG3S level of theory. In addition, the XPHF model shows reasonable agreement with experiments for a variety of structural and thermodynamic properties in the liquid state, including radial distribution functions, interaction energies, diffusion coefficients, and densities at various state points.

Lattice stretching bistability and dynamic heterogeneity

A simple one-dimensional lattice model is suggested to describe the experimentally observed plateau in force-stretching diagrams for some macromolecules. This chain model involves the nearest-neighbor interaction of a Morse-like potential (required to have a saturation branch) and a harmonic second-neighbor coupling. Under an external stretching applied to the chain ends, the intersite Morse-like potential results in the appearance of a double-well potential within each chain monomer, whereas the interaction between the second neighbors provides a homogeneous bistable (degenerate) ground state, at least within a certain part of the chain. As a result, different conformational changes occur in the chain under the external forcing. The transition regions between these conformations are described as topological solitons. With a strong second-neighbor interaction, the solitons describe the transition between the bistable ground states. However, the key point of the model is the appearance of a heterogenous structure, when the second-neighbor coupling is sufficiently weak. In this case, a part of the chain has short bonds with a single-well potential, whereas the complementary part admits strongly stretched bonds with a double-well potential. This case allows us to explain the existence of a plateau in the force-extension diagram for DNA and α-helix protein. Finally, the soliton dynamics are studied in detail.

TOWARDS A MOLECULAR THEORY FOR THE VAN DER WAALS–MAXWELL DESCRIPTION OF FLUID PHASE TRANSITIONS

We briefly review developments of theories for phase transitions of molecular fluids and mixtures, from semi-phenomenological approaches providing equations of state with adjustable parameters to first-principles microscopic methods qualitatively correct for a variety of molecular models with realistic interaction potentials. We further present the generalization of the van der Waals–Maxwell description of fluid phase diagrams to account for chemical specificities of polar molecular fluids, such as hydrogen bonding. Our theory uses the reference interaction site model (RISM) integral equation approach complemented with the new closure we have proposed (KH approximation), successful over a wide range of density from gas to liquid. The RISM/KH theory is applied to the known three-site models of water, methanol, and hydrogen fluoride. It qualitatively reproduces their vapor-liquid phase diagrams and the structure in the gas as well as liquid phases, including hydrogen bonding. Furthermore, phase transitions of water and methanol sorbed in nanoporous carbon aerogel are described by means of the replica generalization of the RISM approach we have developed. The changes as compared to the bulk fluids are in agreement with simulations and experiment. The RISM/KH theory is promising for description of phase transitions in various associating fluids, in particular, electrolyte as well as non-electrolyte solutions and ionic liquids.

Effective force field for liquid hydrogen fluoride from ab initio molecular dynamics simulation using the force-matching method

A recently developed force-matching method for obtaining effective force fields for condensed matter systems from ab initio molecular dynamics (MD) simulations has been applied to fit a simple nonpolarizable two-site pairwise force field for liquid hydrogen fluoride. The ab initio MD in this case was a Car-Parrinello (CP) MD simulation of 64 HF molecules at nearly ambient conditions within the Becke-Lee-Yang-Parr approximation to the electronic density functional theory. The force-matching procedure included a fit of short-ranged nonbonded forces, bonded forces, and atomic partial charges. The performance of the force-match potential was examined for the gas-phase dimer and for the liquid phase at various temperatures. The model was able to reproduce correctly the bent structure and energetics of the gas-phase dimer, while the results for the structural properties, self-diffusion, vibrational spectra, density, and thermodynamic properties of liquid HF were compared to both experiment and the CP MD simulation. The force-matching model performs well in reproducing nearly all of the liquid properties as well as the aggregation behavior at different temperatures. The model is computationally cheap and compares favorably to many more computationally expensive potential energy functions for liquid HF.

Adsorption of Octyl Cyanide at the Free Water Surface as Studied by Monte Carlo Simulation

Monte Carlo simulations of the adsorption layer of octyl cyanide have been performed on the canonical (N, V, T) ensemble at 300 K. The systems simulated cover the range of octyl cyanide surface densities from 0.27 to 7.83 mumol/m2. The surface density value at which the saturation of the adsorption layer occurs is estimated to be 1.7 mumol/m2. At low surface densities, the main driving force of the adsorption is found to be the formation of hydrogen bonds between the water and octyl cyanide molecules, whereas at higher surface concentrations, the dipole-dipole attraction between the neighboring adsorbed octyl cyanide molecules becomes more important. At low surface concentrations, the water-octyl cyanide hydrogen bonds prefer tilted alignments relative to the interface; however, in the case of the saturated adsorption layer, the number of such hydrogen bonds is maximized, leading to the preference of these bonds for the orientation perpendicular to the interface. Contrary to nonionic surfactants of multiple hydrogen bonding abilities (e.g., 1-octanol, C8E3), the increasing surface concentration of octyl cyanide was not found to lead to considerable competition of the molecules for positions of optimal arrangement. As a consequence, the energy and geometry of the water-octyl cyanide hydrogen bonds are found to be insensitive to the octyl cyanide surface concentration.

On the variation of the structure of liquid deuterium fluoride with temperature

The structure of liquid deuterium fluoride has been measured using pulsed neutron diffraction and high energy x-ray diffraction techniques as a function of temperature. The neutron experiments were performed at T=296+/-2 K, 246+/-2 K, and 193+/-2 K and the x-ray measurements carried out at 296+/-2 K and 195+/-2 K. The x-ray pair correlation functions, which are dominated by fluorine-fluorine interactions, show the first peak at approximately 2.53+/-0.05 A remains very nearly invariant with decreasing temperature. Peaks around 4.5 and 5.0 A also appear at both temperatures in the x-ray data. In contrast, the intermolecular peaks in the total neutron pair correlation function show that significant systematic local structural changes occur as the temperature is lowered. The first intermolecular peak position shortens from 1.64+/-0.05 A at 296 K to 1.56+/-0.05 A at 195 K. Although there are overlapping contributions from the intermolecular hydrogen-fluorine and hydrogen-hydrogen correlations, it is clear that the temperature dependent structural changes are largely due to a rearrangement of the deuterium atom positions in the fluid. By comparison with partial structure factor data the hydrogen bonds appear to become more linear at lower temperatures.

Development of a new polarizable potential model of hydrogen fluoride and comparison with other effective models in liquid and supercritical states

Development of a new polarizable potential of hydrogen fluoride through the reparametrization of the JV-P model is presented: The length of the H-F bond has been shortened and the other parameters of the model have been readjusted accordingly. The structural, thermodynamic, and liquid-vapor equilibrium properties of the new model are compared with those of other effective potential models of HF as well as with experimental data in a broad range of thermodynamic states, from near-freezing to supercritical conditions. It is found that although the reparametrization does not change the structural properties of the HF model noticeably at the level of the pair correlations, it improves the reproduction of the thermodynamic properties of hydrogen fluoride over the entire range of existence of a thermodynamically stable liquid phase and also that of the vapor-liquid coexistence curve. However, the new model, which still overestimates the close-contact separation of the HF molecules, underestimates the density of the coexisting liquid phase and overestimates the saturation pressure, probably due to the too steep repulsion of the potential function.

The (4,0) mode of HF dimer at 14700cm−1

The deltaK = 0 and 1 subbands of the (4,0) <-- (0,0) transition of (HF)2, near 14,700 cm(-1), have been measured by molecular-beam intracavity laser-induced fluorescence. The hydrogen interchange tunneling is basically quenched in (4, 0) for both K = 0 and 1 levels, consistent with the early suggestion from a phenomenological model [H.-C. Chang and W. Klemperer, J. Chem. Phys. 104, 7830 (1996)]. The band origin upsilon0 = 14,700.458(7) cm(-1) and rotational constant (B + C)/2 = 0.22278(31) cm(-1) are determined for K = 0 of the (4, 0) mode. From the observed deltaK = 1 <-- 0 spectrum, we determined that A = 24.3 cm(-1), (B + C)/2 = 0.22296(20) cm(-1), and (B-C) = 4.5(2) x 10(-3) cm(-1). The predissociation linewidths of both K = 0 and 1 levels are 470(30) MHz with no apparent rotational dependence.

Effect of the Range of Interactions on the Properties of Fluids. 2. Structure and Phase Behavior of Acetonitrile, Hydrogen Fluoride, and Formic Acid

To complete the study on the effect of the long-range part of Coulombic interactions on properties of complex polar and associating fluids, we have investigated in detail three compounds with extreme features: acetonitrile for its unusually large dipole moment, hydrogen fluoride with very strong hydrogen bonding, and formic acid for its potential formation of different n-mers in liquid and gaseous phases. The effect of the long-range Coulombic interactions on both the structure and thermodynamics of the homogeneous phase, and on the vapor-liquid equilibria has been examined using the same decomposition of realistic potential models into a short-range part and a residual part as in the previous paper [Kettler, M.; et al. J. Phys. Chem. B 2002, 106, 7537-7546]. The present results fully confirm the previous findings that the properties of polar and associating systems are determined primarily by the short-range interactions regardless of their nature, i.e., contributions arising from the long-range interactions constitute only a small portion of the total properties, and thus that the short-range potential counterpart of full realistic models can be used as a convenient reference for a successful perturbation expansion.

Trimer Based Polarization as a Multibody Molecular Model. Application to Hydrogen Fluoride

A molecular modeling approach is introduced as a way to treat multibody (more than two molecules) contributions to the intermolecular potential. There are two key features to the method. First, it employs polarizable electrostatics on the molecules, but converges the charges and fields for only three molecules at a time, taken separately for all trimers (three molecules falling within a cutoff distance) in the system. This feature introduces significant computational savings when applied in Monte Carlo simulation (in comparison to a full N-body polarization treatment), as movement of a single molecule does not require re-converging of the polarization of all molecules, and it achieves this without approximations that cause the value of the energy to depend on the history of the simulation. Second, the approach defines the polarization energy in excess of the pairwise contribution, meaning that the trimer energy has subtracted from it the sum of the energies obtained by converging the polarization of each molecule pair in the trimer. This feature is advantageous because it removes the need (often found in polarizable models) to stiffen inappropriately the repulsive part of the pair potential. The polarization contribution is thus a purely three-body potential. The approach is applied to model hydrogen fluoride, which in experiments exhibits unusual properties that have proven difficult to capture well by molecular models. The new HF model is shown to be much more successful than previous modeling efforts in obtaining agreement with a broad range of experimental data (volumetric properties, heat effects, molecular structure, and vapor-liquid equilibria).