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For the Raman measurements, a Renishaw MK III Raman spectrometer was used, utilizing a 30 mW argon laser emitting green light at a wavelength of 514.53 nm as an excitation source. The light was transported through a 50 μm optical fiber cable to the probe and was focused on the sample using a 20× objective lens.
Backscattered light was filtered using a 2400 grooves/mm grating. The spectrometer was calibrated using the emission lines from neon, assuring an accuracy of 0.3 cm1, while the spectral resolution was 4.5 cm1. Raman spectra were analyzed using GRAMS/AI software from Galactic Industries Corporation. The gas hydrate was placed in a stainless steel sample holder, which was immersed in liquid nitrogen in order to keep the gas hydrate sample at 77 K.
The outer surface of the gas hydrate samples was cleaved and placed in a cryostat maintained at 77 K under liquid nitrogen. Because Raman is a local technique (with the laser spot on the order of micrometers), various areas of the gas hydrate could be measured to detect any heterogeneity. The resulting spectra were deconvoluted in the same manner for all gas hydrate spectra with a mixed Gaussian and Lorentzian peak shape (Sum et al., 1997).
All 13C magic angle spinning (MAS) NMR spectra were recorded on a Chemagnetics Infinity 400 NMR spectrometer operating at 100.5 MHz for 13C. A single-pulse 13C MAS experiment with a pulse delay of 60 s was used to measure the sample. To verify this was sufficient for quantification, the same experiment was performed with a 120 s pulse delay, and the cage occupancies were in agreement with the 60 s pulse delay case. This verified the pulse rate was sufficiently slow to record quantitative spectra. Proton decoupling of ~50 kHz was used during acquisition of the 13C NMR signal.
The samples were cold-loaded into 7.5 mm ZrO2 sleeves as powder and placed in the NMR Chemagnetics variable temperature (VT) probe at ~248 K. The samples were then cooled in ~10 s intervals to ~165 K. MAS between 2 and 4 kHz was used. A 90° pulse of 5 μs was measured for the power settings used.
The gas hydrate cage occupancy ratio is defined as θL/θS, where θ is the absolute occupancy of a particular gas hydrate cage type and the subscripts L and S indicate the large and small cages, respectively (L = 51262 for sI, S = 512). This occupancy ratio can be determined from the Raman or NMR spectrum by using θL/θS = (AL/3)/AS, where A is the area of the peak corresponding to the given cage type. AL is divided by 3 to account for the sI cage distribution.