Ab Initio Studies on the Clathrate Hydrates of Some Nitrogen- and Sulfur-Containing Gases

Abstract

Ab initio calculations are performed to investigate the host-guest interactions and multiple occupancies of some sulfur- (HS, CS) and nitrogen-containing (N, NO, and NH) molecules in dodecahedral, tetrakaidecahedral, and hexakaidecahedral water cages in this work. Five functionals in the framework of density functional theory are compared, and the M06-2X method appears to be the best to predict the binding energies as well as the geometries. Results show that N and NO molecules are more stable in the 56 cage, while NH and HS prefer to stabilize in the 56 cage. This suggests that the sI hydrates of NH and HS exhibit higher stability than the sII structures and that sII NO hydrate is more stable than sI NO hydrate. N is found to be more stable in type II structure with single occupancy and to form type I hydrate with multiple occupancy, which is consistent with the experimental observations. As to the guest molecule CS, it may undergo severe structural deformation in the 5 and 56 cage. For multiple occupancies, the 5, 56, and 56 water cages can trap up to two N molecules, and the 56 water cage can accommodate two HS molecules. This work is expected to provide new insight into the formation mechanism of clathrate hydrates for atmospherically important molecules.

Distortion of the Host Water Cages of Structure I Gas Hydrates: Structural Analysis of C2H4 Hydrate by Powder X-ray Diffraction

“…It is consistent with the results of the structural analysis of sII hydrates; the shape and volume of the empty 5 12 cages adjust with 5 12 6 4 cages, which occupy guest molecules, such as tetrahydrofuran (THF: C 4 H 8 O) and cyclopentane (CP: C 5 H 10 ) to stabilize sII hydrate structure . The host–guest interaction energy or lattice distortion associated with guest species has been predicted based on theoretical calculations; − hence, further computational analyses may elucidate the correlation between the volumes of host cages and the stability of sI hydrate structures. In addition, these investigations may provide insight into guest-dependent physical properties, such as the mechanical properties, of gas hydrates.…”

What is the hydrophobic interaction contribution to the stabilization of micro-hydrated complexes of trimethylamine oxide (TMAO)? A joint DFT-D, QTAIM, and MESP study

“…26 The structures of the water cages used for the study of the complexes TMAO:(H 2 O) 20 and TMAO:(H 2 O) 24 were taken from the literature. 27,28 The main QTAIM topological features of the species herein discussed are presented in Supporting Information S1-S10, and the Cartesian Coordinates of all the complexes are given in Supporting Information S11.…”

“…For this study, two water cages were considered: 5 12 and 5 12 6 2 water cages constituting elementary assemblies of some water clathrates. 27 The energy of the encapsulation (E encapsulation ) was calculated as the difference between the sum of the energies of the isolated water cage and the TMAO on the one hand, and the energy of the TMAO@(H 2 O) 20,24 , on the other hand. The D0 was calculated from the most stable (H 2 O) 20,24 isomers.…”

Clathrate Hydrates

“…Mondal and Chattaraj used ab initio molecular dynamics (AIMD) via atom-centered density matrix propagation (ADMP) methodologies to study time-dependent structural behavior of noble gas hydrates on the timescale of ~500 fs [36]. Sun et al carried out DFT calculations using the M06-2X method to determine the relative stabilities of guest species in various clathrate hydrate host sites [37]. They predict that the guest molecules N 2 and NO are more stable in a 5 12 cage.…”