DFT calculation of the potential energy landscape topology and Raman spectra of type I CH₄and CO₂hydrates

Abstract

CO2 and CH4 clathrate hydrates of type I were studied by means of DFT and QTAIM, in order to better understand their properties at the molecular level. Sub-cells of type I hydrates were modeled as independent rigid cages, both empty and containing guest molecules. Interaction potentials of guest molecules inside each cage, and moving from a cell to the adjacent one, were calculated using the DFT approximation B3LYP/6-311+g(d,p), considering the cases with and without long range Coulombic corrections. The selected theory level was validated by comparison of the simulated Raman spectra with the experimental ones, for the case of type I lattice at full occupation of CO2 and CH4, respectively. For this comparison, Fermi resonances of CO2 were taken into account by transforming experimental bands to the corresponding theoretical non-mixed states. On the one hand, our results confirm the validity of the theory level selected for the model. We have shown the high anisotropy of the guest-cell interaction potential of the molecules analyzed, which has implications in the formulation and use of equations of state, and in the study of transport properties as well. On the other hand, our results suggest that the concentration of guest species inside type I hydrates could be computed from the comparison of experimental and predicted Raman spectra, although there are non-trivial experimental limitations to get over for that purpose.

Computational study of the interplay between intermolecular interactions and CO2orientations in type I hydrates

“…Despite the cited limitations for this approach, which must be always kept in mind, type I hydrate properties can be described to a great extent by calculating separately the corresponding properties of isolated T and D cages. This applies for instance to infrared and Raman spectra, 16 and therefore, this approach will be used here as well as first approximation.…”

“…Nevertheless, the trend of the molecular axis orientation evolution allows us to guess lower bounds for the CO 2 molecular rotation period by extrapolation. In a previous work 16 we compared CO 2 experimental Raman spectra with the calculations obtained using the same setup used here. The anti-symmetric stretching vibration, denoted usually as n 3 , lies at approximately 2420 cm À1 , which corresponds to a period value of 13.78 fs.…”

A Systematic Protocol for Benchmarking Guest–Host Interactions by First‐Principles Computations: Capturing CO2 in Clathrate Hydrates

“…Although several ab initio studies have addressed the intermolecular interactions of H 2 O-H 2 O, CO 2 -H 2 O, and CO 2 -CO 2 complexes, [28][29][30][31][32][33][34] there is little ab initio data on the CO 2 encapsulated in water cages. [35][36][37][38] The lackofabinitio reference computations motivatesu st oc arry out such high quality benchmark data for CO 2 @H 2 Of rom first-principles approaches in sI, sII, and sH type cavities. Computer simulations are ak ey challenge for molecular-level understanding of clathrate behavior,e specially in terms of interpreting the underlying mechanisms of various physicochemical processes (see Ref.…”

“…[39] and references therein). Nowadays, most of the molecular dynamics (MD) simulations still rely on empirical or semiempirical force fields as an important tool for investigating processes, such as the nucleation, growth, structural organization, and cage occupancy of clathrate hydrates, as well as the dissolution of the guest gas in water, [40][41][42][43][44][45][46][47][48][49][50] whereas for ab initio simulations, issues such as computational efficiency,s ystem-size scaling, and accurate electronic structuret reatments are of importance,w ith density functional theory (DFT) approaches [36,38,[51][52][53][54][55] being, more recently,a lso valuable in studyings uch inclusion compounds. In this vein, energy benchmarksf rom accurate quantum-mechanical calculations are essential for testing both force fields and DFT methods.…”

Molecular dynamics simulation of CO2 hydrates: Prediction of three phase coexistence line

“…There is an important body of quantum ab initio calculation studies about hydrates. This alternative theoretical approach provides crucial information on many microscopic energetic and structural features not accessible to MS techniques, representing an alternative and complementary calculation level (see, for instance, Vidal-Vidal et al 30 and references therein). As another combination of both approaches, recently Velaga and Anderson 31 used preliminary quantum calculations to parameterize an intermolecular model and describe CO 2 hydrate phase equilibria.…”