(356d) Estimation of Direct Transition Mechanism for Molecular Diffusion in Type I Gas Hydrates Using Density Functional Theory

Remarkable research efforts have been performed concerning the study of hydrates and organic clathrates using different theoretical approaches, such as molecular equations of state, molecular simulations including both Molecular Dynamics and Monte Carlo techniques, and different ab initio Quantum Mechanics approaches^1-3. In this work, CO₂ and CH₄ type I hydrates have been studied using electronic Density Functional Theory (DFT)⁴and the Quantum Theory of Atoms in Molecules (QTAIM). Several mechanisms for transport of gases inside hydrates have been proposed so far in literature. Previous results have pointed out the apparent imposibility of some guests passing directly through faces connecting adjacent cages without destroying the water structure. Both types of cells included in the structure of type I hydrate were modeled as isolated double semi-flexible atomic systems. Interaction potentials of guest molecules with the enclathrating cell, when moving between neighbour cells were calculated using B3LYP/6-311+g(d,p) DFT approximation. Our calculations show that direct transitions are feasible through hexagonal and pentagonal faces without compromising the overall structure integrity in opposition to other results previously reported in literature. This stability has been explored using the QTAIM theory and reveals that even in the case that some bond may break during the transition, all of them are recovered, because the face distorsion is absorbed locally by the hydrogen bond network. The validity of the theory level selected has been stated, and the high anisotropy of the guest-cell interaction potential for the molecules analysed is shown, which may be considered in the formulation of hydrate thermodynamic models as equations of state, and also for the description of transport properties.

References:

[1] A. K. Sum, C. A. Koh and E. D. Sloan, Ind. Eng. Chem. Res, 48, 7457, 2009.

[2] Á. Vidal-Vidal, M. Pérez-Rodríguez, J.-P. Torré and M. M. Piñeiro, Phys. Chem. Chem. Phys., 17, 6963, 2015.

[3] Á. Vidal-Vidal, M. Pérez-Rodríguez and M. M. Piñeiro, RSC Advances, 6, 1966, 2016.

[3] Gaussian 09 Revision D.01, Gaussian Inc. Wallingford CT, 2009.

Acknowledgements

The authors acknowledge CESGA (www.cesga.es) in Santiago de Compostela, Spain, for providing access to computing facilities, and Ministerio de Economía y Competitividad (grant n. FIS2015-68910-P), also in Spain, for financial support.