Effects of Empty Cages on Lattice Distortion of Tetrahydrofuran Clathrate Hydrate

Abstract

Tetrahydrofuran (THF) clathrate hydrate has been studied as a new storage system for H2. To investigate effects of empty cages on lattice distortion, molecular dynamics calculations of THF clathrate hydrate were performed. The results show that the hexakaidecahedral cages shrink due to absence of THF from the cages, whereas the surrounding dodecahedral cages expand. The vibrational density of states indicates that the O–H symmetric stretching mode shifts to lower frequency with decreasing cage occupancy. The results suggest that the average strength of the hydrogen bonds increases with decreasing cage occupancy. It is concluded that a small number of empty cages can affect not only the local structures, but also the macroscopic properties of the crystal. The effects of empty cage may be an important factor governing the efficiencies of storage and release of H2 in THF clathrate hydrate.