A theoretical study of the confinement of methane in water clusters

Abstract

A search for stable local minima CH 4 -(H 2 O) n closed methane clathrates was done at the xB97X-D/aug-cc-pVDZ level of the theory, for n up to 20, starting from various different configurations for each size. The reliability of the method was validated by comparison to the complete basis set limit (CBSL) values of the methane-water interactions, of the water-water interactions and of the binding energies of (H 2 O) 20 clusters. A potential model fitted to reproduce the CBSL interaction energy of the optimal CH 4 -(H 2 O) n pair was also used in Monte Carlo optimizations. Confinement was found to occur already at n ¼ 14. Optimizations with two empirical models for numerical simulations resulted in the same configurations. The addition of more water molecules, though, favored an increase of the size of the cage, instead of making an external hydrogen bond with the other water molecules in the cluster. An analysis was made at the xB97X-D/aug-cc-pVDZ level of the interactions of methane with the confining clusters. In all cases, the interaction energy was negative, and for the dodecahedral cavity, the confinement of methane resulted in a significant stabilization relative to the unperturbed empty (H 2 O) 20 cluster and an external methane molecule. Another analysis was made of the energetic cost of rotating the methane within the dodecahedral cavity. The corresponding barrier was lower than K B T at ambient temperature. Figure 7. Energetic cost of rotating the methane molecule within the 5 12 cage, relative to the optimal position. Left: around a CAH bond; right: around an axis in the bisector of an HACAH angle.

A Simple Correction for Nonadditive Dispersion within Extended Symmetry-Adapted Perturbation Theory (XSAPT)

“…There have been numerous theoretical studies of the interactions between host water cages and small guest molecules, ,− using functionals including revPBE, B3LYP, , B3LYP-D2, ,,, M05-2X, M06-L, M06-HF, M06-2X, B97-D2, ,, ωB97X-D, and vdW-DF . Small-basis Hartree–Fock and MP2 calculations have also been reported for these systems. ,,,− Despite this considerable computational effort, there has been no attempt to benchmark the computed interaction energies with respect to high-quality ab initio calculations.…”

Anion-tunable control of thermal Z→E isomerisation in basic azobenzene receptors

“…14 This functional proved to be robust and accurate in describing noncovalent supramolecular complexes. [15][16][17][18][19][20] Initial accuracy test for b97x-d, B3LYP, and EDF2 20,21 functionals in comparison to available X-ray structures of the Eand Zisomers of an azobenzene (AB) clearly demonstrate that b97x-d functional gave results very similar to X-ray data (at least for E-AB), whereas B3LYP and EDF2 functionals slightly overestimate the bond distance between azo nitrogen atoms (Table S2). Nevertheless, contrary to X-ray data all studied functionals predicted shorter distance of the azo linkage for Z-AB than for E-AB.…”

The water molecule arrangement over the side chain of some aliphatic amino acids: A quantum chemical and bottom‐up investigation

“…Monte Carlo large scale searches for the optimal configurations of water molecules around the methane molecule, CH 4 @(H 2 O) n for n up to 20, reported that the smallest value at which the (H 2 O) n cluster encapsulates the CH 4 molecule occurs at n = 14 . Furthermore, water molecules are tangentially oriented with respect to the methane surface in a clathrate‐like shape.…”