Structural Stability of the CO2@sI Hydrate: a Bottom‐Up Quantum Chemistry Approach on the Guest‐Cage and Inter‐Cage Interactions

Abstract

Through reliable first-principles computations, we have demon-strated the impact of CO2molecules enclathration on thestability of sI clathrate hydrates. Given the delicate balancebetween the interaction energy components (van der Waals,hydrogen bonds) present on such systems, we follow a system-atic bottom-up approach starting from the individual 512and51262sI cages, up to all existing combinations of two-adjacent sIcrystal cages to evaluate how such clathrate-like modelsperform on the evaluation of the guest-host and first-neighborsinter-cage effects, respectively. Interaction and binding energiesof the CO2occupation of the sI cages were computed using DF-MP2 and different DFT/DFT D electronic structure method-ologies. The performance of selected DFT functionals, togetherwith various semi-classical dispersion corrections schemes, werevalidated by comparison with reference ab initio DF-MP2 data,as well as experimental data from x-ray and neutron diffractionstudies available. Our investigation confirms that the inclusionof the CO2in the cage/s is an energetically favorable process,with the CO2molecule preferring to occupy the large 51262sIcages compared to the 512ones. Further, the present resultsconclude on the rigidity of the water cages arrangements,showing the importance of the inter-cage couplings in thecluster models under study. In particular, the guest-cageinteraction is the key factor for the preferential orientation ofthe captured CO2molecules in the sI cages, while the inter-cageinteractions seems to cause minor distortions with the CO2guest neighbors interactions do not extending beyond thelarge 51262sI cages. Such findings on these clathrate-like modelsystems are in accord with experimental observations, drawinga direct relevance to the structural stability of CO2@sI clathrates.