Guest–host interactions in gas clathrate hydrates under pressure

Abstract

First-principles calculations were performed to determine equilibrium geometries, static equation of state parameters, the energetics and orientation of the guest molecule inside the 512 and 51262 cages, and vibrational frequencies of methane clathrate hydrate. According to our results, the progressive inclusion of one CH4 molecule in each clathrate cavity is always a stabilizing process up to saturation. The released energy is very similar for both types of cages. In agreement with the experimental observation of rotovibrational spectra in this hydrate, we calculate an energy barrier of less than 0.5 kcal/mol, indicating free rotation of methane inside the cages. The stabilizing effect of applied pressure leads to a red shift of the O–H stretching frequencies of the water molecules of around 80 cm−1 in average at 1 GPa.