Exploring Guest-Host Interactions in Gas Hydrates: Insights from Quantum Mechanics

Gas hydrates are comprised of guest molecules and host cages. The stability of a gas hydrate is determined by the host-guest interactions. In this work, the relationship between the characteristics of the guest molecules and the gas hydrate stability were systematically investigated using quantum mechanics (QM) calculations. The physical components (Delta E-exch, Delta E-ind, Delta E-elst, and Delta E-dis) of the host-guest interaction energy (Delta Ehost-guest) of 21 types of gas hydrate clusters were investigated by symmetry-adapted perturbation theory (SAPT). Correlation analyses revealed that Delta E-exch, Delta E-ind, Delta E-elst, and Delta E-dis had good linear relationships with Delta Ehost-guest. In addition, the molecules (CO2, H2S) with a high value for the electrostatic interaction (Delta E-elst) have a distinct characteristic in the host-guest interactions. The molecular electrostatic potential (ESP), van der Waals (vdW) volume, and vdW surface areas were calculated to explore those distinct characteristics. The results show that the molecules with higher maximum (V-s,V-max) points of the ESP values have a higher stabilization in the host-guest interactions. It was concluded that the vdW volumes or vdW surface areas and the V-s,V-max values of the guest molecules are the key factors for the stability of gas hydrates.