Methane Hydration Research Articles

Calculations of the relative free energies of aqueous solvation of several fluorocarbons: A test of the bond potential of mean force correction

The relative free energies of aqueous solvation of several fluorinated derivatives of methane were calculated using the free energy perturbation (FEP) method. The calculations in general duplicated the experimental free energies with relatively good accuracy, but the calculation of the bond potential of mean force (bond-PMF) contribution [D. A. Pearlman and P. A. Kollman, J. Chem. Phys. 94, 4532 (1991)] was necessary in order to get the most satisfactory agreement with experiment. In particular, it was necessary to use this contribution to obtain even qualitatively correct results for the relative free energies of hydration of methane and tetrafluoromethane. The reasons for this are discussed in terms of the accurate calculation of the effect of the size of the solute. In addition, it is noted that the bond-PMF contribution is important even for FEP calculations not involving large changes in size, such as the ethanol to ethane perturbation, if the length of a bond to a disappearing atom is changed during the perturbation. The relative free energy of aqueous solvation for ethanol and ethane was calculated to demonstrate that if the bond between the oxygen and the hydroxyl hydrogen being removed is ‘‘shrunken’’ during the perturbation without including the bond-PMF correction, the calculated free energy is too low by ∼3 kcal/mol.

Molecular dynamics study of methane hydration and methane association in a polarizable water phase

Molecular dynamics simulations are used to investigate the influence of adding an explicit polarization term to the water potential on the structural and dynamic properties of aqueous methane solutions and on methane-methane association in water. Calculations are performed using respectively two different water models: the three-center polarizable water model (PSPC), where the many-body effects are treated explicitly using the extended Lagrangian method, and the mean-field classical SPC model. Comparing the results obtained from the two sets of calculations the authors find that electronic polarization of the water molecules has a subtle though significant influence on the structure and dynamic of water molecules surrounding the hydrophobic solute. But its most remarkable influence by far is on the methane-methane potential of mean force: addition of the polarization term to the water potential effectively abolishes the much questioned solvent-separated minimum found in many previous studies with nonpolarizable water models. Polarizable water models thus appear to yield an improved physical picture of the system and should be well suited for investigating the process of hydrophobic association. 45 refs., 5 figs., 1 tab.

A computer-simulation study of hydrophobic hydration of rare gases and of methane. I. Thermodynamic and structural properties

A theory is proposed to study the hydrophobic hydration of rare gases and methane in water. The Ostwald absorption coefficient γ, the hydration energy ΔE, and entropy ΔS are calculated by combining large-scale molecular-dynamics simulations and test-particle methods. The convergence of calculations is checked with particular care. The structure of the first two hydration shells is analyzed in terms of appropriate pair distribution functions. The picture conveyed by this theory is compared to that provided by the early work.

Free energy of TIP4P water and the free energies of hydration of CH 4 and Cl - from statistical perturbation theory

Statistical perturbation theory has been applied in Monte Carlo simulations to compute the free energy of TIP4P water and the absolute free energies of hydration of methane and chloride ion. The calculations entailed the mutation of a water molecule and of a chloride ion to methane and the subsequent disappearance of the methane in the presence of 216 TIP4P solvent molecules. The NPT ensemble was used at 25°C and 1 atm, so Gibbs free energies were obtained. The mutations to methane proceeded with high precision, while the removal of the methane yielded an uncertainty of about ±0.3 kcal/mol in Δ G. The accord with experimental results is good in all cases; in particular, the computed free energy of TIP4P water is −6.1±0.3 kcal/mol, while the experimental value is −6.3 kcal/mol. The present results demonstrate the utility of statistical perturbation theory for computing absolute free energies of solution and the quality of the underlying potential functions.

Acceleration of convergence in Monte Carlo simulations of aqueous solutions using the metropolis algorithm. Hydrophobic hydration of methane

AbstractConvergence problems encountered in the computer simulations of aqueous solutions are discussed. Solute–solvent radial distribution functions are shown to converge very poorly when the standard Metropolis Monte Carlo procedure is applied. To overcome this difficulty, several modifications are made in the Metropolis method. Optimum maximum step sizes are determined for simulations of liquid water. A scheme is employed for preferential sampling of both the solvent and the solute molecules. To test these modifications, a simulation is carried out for pure liquid water, treating a single water molecule as a “solute.” The convergence of the radial distribution functions is found to be accelerated significantly. A further test is made by simulating an aqueous solution of methane, consisting of one methane molecule (using the EPEN/2 potential for methane–water interactions) and 124 water molecules (using the MCY potential for water–water interactions). Again, the convergence of solute–solvent radial distribution functions is found to be accelerated. The computation of partial molar thermodynamic quantities, however, still suffers from convergence difficulties. This problem is discussed in detail. The EPEN/2 potential is found to yield structural and thermodynamic features of hydrophobic hydration that are consistent with available experimental and theoretical results for aqueous solutions of methane.