Confining CO₂ inside sI clathrate‐hydrates: The impact of the CO₂–water interaction on quantized dynamics

Abstract

We report new results on the translational‐rotational (T‐R) states of the CO2 molecule inside the sI clathrate‐hydrate cages. We adopted the multiconfiguration time‐dependent Hartree methodology to solve the nuclear molecular Hamiltonian, and to address issues on the T‐R couplings. Motivated by experimental X‐ray observations on the CO2 orientation in the D and T sI cages, we aim to evaluate the effect of the CO2–water interaction on quantum dynamics. Thus, we first compared semiempirical and ab initio‐based pair interaction model potentials against first‐principles DFT‐D calculations for ascertaining the importance of nonadditive many‐body effects on such guest–host interactions. Our results reveal that the rotational and translational excited states quantum dynamics is remarkably different, with the pattern and density of states clearly affected by the underlying potential model. By analyzing the corresponding the probability density distributions of the calculated T‐R eigenstates on both semiempirical and ab initio pair CO2–water nanocage potentials, we have extracted information on the altered CO2 guest local structure, and we discussed it in connection with experimental data on the orientation of the CO2 molecule in the D and T sI clathrate cages available from neutron diffraction and 13C solid‐state NMR studies, as well as in comparison with previous molecular dynamics simulations. Our calculations provide a very sensitive test of the potential quality by predicting the low‐lying T‐R states and corresponding transitions for the encapsulated CO2 molecule. As such spectroscopic observables have not been measured so far, our results could trigger further detailed experimental and theoretical investigations leading to a quantitative description of the present guest–host interactions.

Confining He Atoms in Diverse Ice-Phases: Examining the Stability of He Hydrate Crystals through DFT Approaches

“…The first part involves a benchmark study on the crystal structures of ice II and ice XVII. The objective is to evaluate the performance of different DFT-D functionals that have been recognized as effective in describing ices and clathrate hydrates in the literature [38,45,[48][49][50][51][52][53][54]. In addition, the benchmark study encompasses ice II and ice XVII structures containing one or multiple He atoms, respectively, alongside their respective empty ice II/ice XVII lattices, in order to consider the impact of an increasing number of He atoms occupying the interstitial spaces within the ice lattices.…”