The Structure of Dilute Clusters of Methane and Water by ab Initio Quantum Mechanical Calculations

Abstract

Ab initio quantum mechanical methods have been used to examine clusters formed of molecules of methane and water. The clusters contained one molecule of one component (methane or water) and several (10, 8, 6, 4, and 1) molecules of the other component. The Møller−Plesset perturbation theory (MP2 method) was used in the calculations. The cluster geometries were obtained via optimization and the interaction energies between the nearest neighbors were calculated for the geometries obtained in the first step. It is shown that the interaction energies and intermolecular distances between the molecules of methane and water are quite different in the clusters CH4···(H2O)10 and H2O···(CH4)10. They are also different from those in the water/methane dimer. The structure of the cluster CH4···(H2O)10 is highly affected by the hydrogen bonding among the water molecules, and the methane molecule is located inside a cage formed of water molecules. In contrast, the molecules of methane and water are randomly distributed in the cluster H2O···(CH4)10. The average methane/water intermolecular distance in the cluster CH4···(H2O)10 provided by the quantum mechanical calculations is in agreement with the experimental and simulation results regarding the position of the first maximum in the radial distribution function g oc = g oc(r oc) in dilute mixtures of methane in water, where r oc is the distance between the C atom of methane and the O atom of water. It is shown that the water molecules in the vicinity of a central methane molecule can be subdivided into two groups, A and B. Molecules of type A are touching nearest neighbors of the central methane molecule. They are located on a sphere with a radius corresponding to the first maximum in the radial distribution function g oc = g oc(r oc) and are tangentially oriented toward the central methane molecule. The layer of A water molecules is somewhat denser than bulk water. The molecules of type B are also located in the first hydration layer of a central methane molecule (up to a distance given by the position of the first minimum of the radial distribution function g oc = g oc(r oc)), but are not touching nearest neighbors. They are distributed more randomly than the molecules of type A, because they are less affected by the hydrophobic core of the solute.

STRUCTURE AND DYNAMICS OF COLD WATER SUPER-EARTHS: THE CASE OF OCCLUDED CH4AND ITS OUTGASSING

“…In the classical theory of solutions, a hydrophobic solute molecule becomes encapsulated in a hydration cell (i.e., cavity) formed of water molecules. The hydration cell around the hydrophobic solute requires both restructuring and some loss of hydrogen bonding on the water molecules' part (see Ruckenstein et al 2003 and references therein). As water molecules are mobile, particularly in the liquid phase, forming a hydration cell demands that a solute molecule enter immediately or the cell will collapse.…”

“…We take Z = 20 (Jorgensen et al 1985). For the spherical cavity radius, we assume a cavity = 3.74 × 10 −8 cm based on the carbon-oxygen distance found via molecular cluster simulations (Ruckenstein et al 2003). The cavity radius is also adjusted to pressure using the multiplication factor of Equation (C11).…”

DFT study of possible lattice defects in methane-hydrate and their appearance in 13C NMR spectra

“…The structure of the solid hydrate and different methane-water clusters was also subject of extensive theoretical [4][5][6][7][8][9][10][11], and spectroscopic studies [12][13][14][15][16][17][18][19]. A main concern for the computational studies was to create a proper potential for molecular dynamics calculations [4,5,7,8], and to determine the hydratation number of methane. Monte Carlo simulations revealed that a distorted dodecahedral structure has a high statistic probability; furthermore it was shown that the interaction between solvent molecules around the methane is stronger than that in the bulk liquid [4].…”

“…While the interaction energy at the HF/6-31+G ** level was somewhat more than 2 kcal/mol, using a composite scheme [11] to extrapolate for larger basis sets and electron correlation (MP2) the interaction energy increases above 5 kcal/mol. In a recent MP2/6-31+G ** ab initio study, a single dodecahedral cage occupied with methane was investigated [7]. It has been concluded that the insertion of a methane molecule in this cage results in minor structural changes of the water network.…”

A computational study of microsolvation effect on ethylene glycol by density functional method

“…The basis set that includes both diffuse and polarization functions has already been used for several hydrogen bonded systems. [42][43][44][45][46][47][48] The energies are corrected for BSSE using counterpoise method and generalized counterpoise method. [49][50][51] …”