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 <i>a</i> = 1.22342(15) nm and <i>c</i> = 0.99906(17) nm at 153 K. The <i>c</i> axis of this hydrate was slightly shorter (i.e., 0.00584 nm) than that of CH₄ + NH, whereas we observed no difference in the <i>a</i> 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 (θ_Mθ_S) 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 <i>attractive</i> 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