Structural Characterization of Structure H (sH) Clathrate Hydrates Enclosing Nitrogen and 2,2-Dimethylbutane

Abstract

In this study, we characterized structure H (sH) clathrate hydrates (hydrates) containing nitrogen (N2) and 2,2-dimethylbutane (neohexane, hereafter referred to as NH) molecules. On the basis of the powder X-ray diffraction profile, we estimated the unit cell dimensions of the sH hydrate of N2 + NH to be a = 1.22342(15) nm and c = 0.99906(17) nm at 153 K. The c axis of this hydrate was slightly shorter (i.e., 0.00584 nm) than that of CH4 + NH, whereas we observed no difference in the a axis between these two hydrates. We successfully observed a symmetric N–N stretching (N–N vibration) Raman peak with two bumps, and we determined that the N–N vibrational mode in the 512 and 435663 cages occurred at approximately 2323.8 and 2323.3 cm–1, respectively. We found the cage occupancy ratio of the 435663/512 cages (θMθS) of the sH hydrate of N2 + NH to be approximately 1.30. From a comparison of the N–N vibrational modes in the 512, 435663, 51262, and 51264 cages of the sI, sII, and sH hydrates, we determined that N2 molecules in the distorted 435663 cages experience more attractive guest–host interaction than those in spherical 51264 cages, whereas the guest/cage diameter ratio of 435663 cages is larger than that of 51264 cages. We determined the L1–L2–H–V four-phase equilibrium pressure–temperature conditions in the N2–NH–water system in the temperature range of 274.36–280.71 K. Using the Clausius–Clapeyron equation, we estimated the dissociation enthalpies of the sH hydrates of N2 + NH to be 388.4 and 395.9 kJ·mol–1 (per one molar of N2 molecules) in the experimental temperature range.